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Safety Recommendation Details

Safety Recommendation A-98-092
Details
Synopsis: On January 9, 1997, an Empresa Brasileira de Aeronautica, S/A (Embraer) EMB-120RT, operated by COMAIR Airlines, Inc., crashed during a rapid descent after an uncommanded roll excursion near Monroe, Michigan. The flight was a scheduled, domestic passenger flight from the Cincinnati/Northern Kentucky International Airport, Covington, Kentucky, to Detroit Metropolitan/Wayne County Airport, Detroit, Michigan. The flight departed Covington with 2 flightcrew, 1 flight attendant, and 26 passengers on board. There were no survivors. The airplane was destroyed by ground impact forces and a postaccident fire. IMC prevailed at the time of the accident, and the flight was operating on an IFR flight plan.The probable cause of this accident was the FAA's failure to establish adequate aircraft certification standardds for flight in icing conditions.
Recommendation: TO THE FEDERAL AVIATION ADMINISTRATION: With the NASA and other interested aviation organizations, conduct additional research to identify realistic ice accumulations, to include intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of the deicing boots, and to determine the effects and criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft certification requirements and pilot training programs at all levels.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Action
Mode: Aviation
Location: MONROE, MI, United States
Is Reiterated: Yes
Is Hazmat: No
Is NPRM: No
Accident #: DCA97MA017
Accident Reports: In-Flight Icing Encounter and Uncontrolled Collision with Terrain, Comair Flight 3272, Embraer EMB-120RT, N265CA
Report #: AAR-98-04
Accident Date: 1/9/1997
Issue Date: 11/30/1998
Date Closed: 10/16/2008
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s): Icing,

Safety Recommendation History
From: NTSB
To: FAA
Date: 8/27/2010
Response: Notation 6486F: The National Transportation Safety Board (NTSB) has reviewed the Federal Aviation Administration (FAA) Notice of Proposed Rulemaking (NPRM), “Airplane and Engine Certification Requirements in Supercooled Large Drop, Mixed Phase, and Ice Crystal Icing Conditions,” published at 75 Federal Register 37311 on June 29, 2010. The FAA proposes to amend the airworthiness standards applicable to certain transport-category airplanes certificated for flight in icing conditions and the icing airworthiness standards applicable to certain aircraft engines. The NTSB initially became concerned with supercooled large droplet (SLD) conditions, including freezing rain and freezing drizzle, in the early 1980s and issued a report on September 9, 1981, titled Aircraft Icing Avoidance and Protection. This report identified the need for the FAA to review and revise the icing criteria in 14 Code of Federal Regulations (CFR) Part 25, Appendix C, to include freezing rain and for that reason issued Safety Recommendation A-81-116. On October 31, 1994, American Eagle flight 4184, ATR-72, crashed at Roselawn, Indiana. The investigation found that freezing drizzle icing conditions led to the creation of an ice ridge aft of the deice boots on the upper surface of the wing. This ridge caused the airflow over the ailerons to separate, which resulted in an uncommanded roll of the airplane. Based on its findings, the NTSB issued Safety Recommendation A-96-54, which superseded recommendation A-81-116 and requested the following of the FAA: Revise the icing criteria published in 14 Code of Federal Regulations (CFR), Parts 23 and 25, in light of both recent research into aircraft ice accretion under varying conditions of liquid water content, drop size distribution, and temperature, and recent developments in both the design and use of aircraft. Also, expand the Appendix C icing certification envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary. This recommendation is one of the oldest recommendations on the NTSB’s Most Wanted List of Transportation Safety Improvements and is currently classified “Open—Unacceptable Response.” The status of this recommendation highlights the NTSB’s ongoing concern that current icing certification requirements do not ensure that airplanes are tested in the most severe icing conditions, as defined in the environmental icing envelope contained in Appendix C. Accordingly, the NTSB has repeatedly stressed the need to consider the full range of icing conditions in which an airplane is authorized to operate. Additionally, the NTSB has voiced its concern that flight crews must be provided a means to recognize when an aircraft encounters conditions beyond those for which the aircraft has a demonstrated safe flight capability. In this regard, and as a result of its investigation of the Roselawn accident, the NTSB issued Safety Recommendation A-96-56 to the FAA: Revise the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate, or are otherwise shown to be capable of safe flight into such conditions. If safe operations cannot be demonstrated by the manufacturer, operational limitations should be imposed to prohibit flight in such conditions and flightcrews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification. This recommendation is also on the NTSB’s Most Wanted List of Transportation Safety Improvements and is currently classified “Open—Unacceptable Response.” Since the issuance of these recommendations, the NTSB has investigated several accidents and incidents that have involved SLD conditions, including the 1997 Comair 3272 Embraer (EMB) 120 accident, which occurred in Monroe, Michigan; the 2001 Comair 5054 EMB-120 stall and loss of control event, which resulted in the flight diverting to West Palm Beach, Florida; the 2005 Circuit City Cessna 560 accident, which occurred on approach to Pueblo, Colorado; and the 2006 American Eagle flight 3008 Saab 340B in-flight upset and loss of control over Santa Maria, California. These accidents and incidents again highlighted the hazards of operating in SLD conditions, which were not required to be considered at the time of the original icing certification of these airplanes. As a result of these investigations, the NTSB stressed the need to include SLD conditions in the icing certification standards for all airplanes certificated for flight in icing conditions. These accidents and incidents demonstrated that SLD conditions can be more hazardous than those considered during current icing certification, that SLD conditions can cause ice accretions more aerodynamically detrimental than those that develop while flying within the Part 25, Appendix C, envelope, and that airplanes need to be evaluated over the full range of icing conditions for which they are authorized to operate, including a range of SLD conditions. Given the accident history described above, the NTSB is pleased that the NPRM proposes to add Section 25.1420, which would require evaluating the operation of airplanes in the SLD environment, and Appendix O, Part I, to Part 25, which expands the certification icing environment to include freezing rain and freezing drizzle by using four separate droplet size distributions and includes droplet sizes greater than 1,000 microns in one of the distributions. Additionally, the NTSB is pleased to see the inclusion of Appendix O, Part II, which states the following: The most critical ice accretion in terms of airplane performance and handling qualities for each flight phase must be used to show compliance with the applicable airplane performance and handling qualities requirements for icing conditions contained in subpart B of this part. Applicants must demonstrate that the full range of atmospheric icing conditions specified in part I of this appendix have been considered, including drop diameter distributions, liquid water content, and temperature appropriate to the flight conditions. The NTSB has been concerned for many years about the inadequacy of the existing certification regulations for flight in icing conditions, which have not required manufacturers to demonstrate an airplane’s flight handling, stall characteristics, minimum airspeeds, and stall margins under a sufficiently realistic range of adverse ice accretions, including SLD. Additionally, the regulations have not required airplanes to be tested with the thin, rough ice that can accrete on protected surfaces prior to the activation of ice protection systems or between activation cycles of the ice protection systems, especially in larger water droplet environments. The proposed rule addresses these problems with the addition of Section 25.1420 by requiring that airplane operations in icing conditions do not diminish the safety margins, handling qualities, and performance of the airplane and that the airplane should be as safe to operate in icing as in non-icing conditions. In addition, the proposed addition of Part 25, Appendix O, Part II, will provide an additional margin of safety for these airplanes when operating in icing conditions by defining the ice accretions and revising the performance and handling requirements for flight in icing conditions that must be satisfied to certify an airplane for operation in Appendix O icing conditions. These requirements include, for example, requirements for takeoff performance, stall warning, and landing climb. The NTSB has stated in past NPRM comments that the full range of atmospheric conditions must be considered when evaluating icing certification, and inclusion of this provision in Part II of Appendix O will ensure that all of these conditions are addressed during the icing certification process. The NTSB has also reviewed the FAA’s proposed changes for 14 CFR Section 33.68 “Induction System Icing,” Section 33.77 “Foreign Object Ingestion – Ice,” and the addition of Appendix D, Icing Envelope Limits. The NTSB investigated three incidents of dual-engine flameouts in Hawker Beechcraft (previously identified as Raytheon) Beechjet 400A airplanes that occurred between July 12, 2004, and June 14, 2006. In one of these incidents, the pilots had to accomplish a dead stick landing after they were unable to restart either of the engines. The NTSB’s investigations of these incidents showed that all three airplanes were operating in areas near convective weather activity with high altitude ice crystals, and the flight crews had retarded the power levers. The manufacturer of the engines installed on the Beechjet conducted a study that showed the possibility that high altitude ice crystals would partially melt due to the temperature rise in the air across the fan and ice could then adhere and refreeze on the compressor inlet stator in the engine’s core, causing the engine to flame out. Hawker-Beechcraft developed procedures for the Beechjet 400A airplane prescribing activation of the engine anti-icing system when operating within specified horizontal and vertical distances from convective weather activity to prevent ice from forming inside the engine. The FAA mandated incorporation of the revised procedures into the airplane flight manual with an airworthiness directive (AD). To date, there have been no further reports of dual-engine flameouts in Beechjet 400A airplanes. While pleased with the FAA’s issuance of the AD, the NTSB has been concerned that the FAA’s action addressed Beechjet 400A aircraft only without allowing for future engine designs that could be susceptible to the deleterious effects of high altitude ice crystals. The NTSB agrees with the FAA’s proposed changes in 14 CFR 33.68 and 33.77. In particular, the NTSB supports inclusion of proposed 14 CFR 33.68(e) and the Appendix D Inflight Icing Envelope, which address mixed phase and ice crystal conditions to ensure that future engine designs are not susceptible to high altitude ice crystals. The NTSB also agrees with the proposed turbine engine icing certification requirement, which would include the icing conditions of Appendixes C and O of Part 25, and the mixed phase and ice crystal icing envelope of Appendix D of Part 33. Despite the improvements noted in this letter, the NTSB is disappointed with the proposed rule and believes that it should be expanded to include all aircraft regardless of maximum takeoff weight (MTOW) or flight control design and that the requirements should apply to both newly manufactured and currently certificated aircraft under Part 25 (transport-category airplanes) and Part 23 (normal, utility, aerobatic, and commuter airplanes). If the requirements are not expanded to include currently certificated airplanes, the NTSB believes that flight crews need additional means to positively determine when they are in icing conditions that exceed the limits of the aircraft. As described in the NPRM, the proposed rule will require that only transport-category airplanes with an MTOW of less than 60,000 pounds, or those with reversible flight controls, meet the safety standards in the expanded certification icing environment defined by Part 25 Appendix O. As a result, the proposed rule excludes numerous aircraft models that would benefit from the new requirements. The NTSB does not agree with this position, nor does the Aviation Rulemaking Advisory Committee (ARAC) Ice Protection Harmonization Working Group (IPHWG), as noted in the NPRM: The IPHWG majority … did not accept the exclusion of airplanes with the three aforementioned design features because one cannot predict with confidence that the past service experience of airplanes with these specific design features will be applicable to future designs. The IPHWG majority recommended applying the new SLD airplane certification requirements proposed in the new § 25.1420 to all future transport category airplane type designs. The IPHWG majority opposed limiting the applicability of the rule based on airplane gross weight, in part, because the ratio of wing and control surface sizes to airplane weight varies between airplane designs. Therefore, airplane takeoff weight is not a consistent indicator of lifting and control surface size or chord, which are the important parameters affecting sensitivity to a given ice accretion. In addition to its reservations concerning weight limits, the NTSB believes that this rule should apply to all airplanes regardless of their flight control system design and that the effects of SLD accretions should be considered for all aircraft with either reversible or irreversible flight control systems. However, flight control system anomalies while operating in an SLD environment comprise only one aspect of the SLD threat. Another insidious feature of ice accreted while operating in SLD conditions is that the ice may significantly reduce the angle of attack required for aerodynamic stall to occur, so much so that stall can occur prior to the stall warning, as demonstrated in the Pueblo Cessna 560 accident and five ATR-42 incidents preceding the Roselawn accident. This problem is particularly critical when the stall protection system and warning margins have been determined using Part 25 Appendix C ice accretions, but the aircraft encounters SLD icing conditions, thereby causing much greater reductions of maximum lift than the system was designed for. The NTSB therefore believes that the requirements in the proposed rule should retroactively apply to all aircraft, regardless of their MTOW or the design of their flight control systems. An additional shortcoming in the proposed rule is its restriction to airplanes certificated under Part 25. As recommended in A-96-54, the NTSB believes that SLD should be considered for both Part 25 and Part 23 airplanes, which include several commuter type airplanes regularly used in passenger service. The NTSB has noted in past comments that these types of smaller aircraft typically operate at lower altitudes and are therefore more likely to encounter SLD conditions than the larger, turbine-engine aircraft certificated in Part 25, which spend less of their time operating at lower altitudes. Finally, because SLD is an atmospheric condition that can create dangerous flight conditions for the current fleet of aircraft, the NTSB believes that the proposed rule should be expanded beyond newly certificated airplanes to include all deice boot-equipped airplanes currently in service that are certificated for flight in icing conditions. The NTSB has noted that deice boot-equipped airplanes typically operate at lower altitudes more conducive to SLD encounters and must be properly tested for all icing conditions in which they are authorized to operate. The NTSB believes that the FAA should apply icing certification standards contained in this NPRM, including the provisions of Appendix O, to all deice boot-equipped airplanes currently certificated for flight in icing conditions. The NTSB stated this concern 12 years ago in Safety Recommendation A-98-100, issued to the FAA as a result of the Comair EMB-120 accident in Monroe, Michigan: When the revised icing certification standards and criteria are complete, review the icing certification of all turbopropeller-driven airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. This recommendation was classified “Closed?Unacceptable Action/Superseded” when Safety Recommendation A-07-16 was issued to the FAA on February 27, 2007, as a result of the February 16, 2005, Pueblo Circuit City accident. This new recommendation again stressed that revised certification requirements should apply to currently certificated deice boot-equipped aircraft, as well as new aircraft: When the revised icing certification standards (recommended in Safety Recommendations A-96-54 and A-98-92) and criteria are complete, review the icing certification of pneumatic deice boot-equipped airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. (A-07-16) (Open?Unacceptable Response) To summarize, the NTSB supports the Part 25 regulatory revisions proposed in this NPRM and believes that the new rule will be an essential step in improving the safety of flight for airplanes operating in icing conditions. However, the NTSB continues to believe that the proposed rule should be expanded to include all aircraft regardless of MTOW or flight control design and that the requirements should apply to both newly manufactured and currently certificated aircraft under both Parts 25 and 23. The NTSB appreciates the opportunity to comment on this NPRM.

From: NTSB
To: FAA
Date: 10/16/2008
Response: On October 26, 2005, the FAA informed the Safety Board that, based on research conducted during December 1999 and March 2000, it planned to issue a revision to AC 20-73 that included guidance on determining critical ice shapes for use during aircraft certification. The FAA also stated that, although the material in the revised AC would be appropriate for use by certification engineers and airframe manufacturers, the FAA believed that pilots did not require similar information to be able to safely operate in icing conditions. In lieu of providing the AC to pilots, the FAA, in collaboration with NASA, produced and distributed icing training materials for pilots. Previously, on September 21, 2001, the FAA indicated to the Board that, in September 2000, a working group co chaired by the FAA and NASA had produced a report, titled Report of the 12A Working Group on Determination of Critical Ice Shapes for the Certification of Aircraft, which concluded that sufficient information and methods were not available at that time to provide guidance concerning the determination of critical ice shapes in aircraft certification. The FAA indicated in its September 21, 2001, letter that it was sponsoring the additional research that the Board recommended in Safety Recommendation A-98-92. In its May 10, 2006, response to the FAA’s October 26, 2005, letter, the Safety Board stated that, although the FAA had previously indicated that additional research was needed to produce the needed guidance on icing certification, the FAA stated in its 2005 letter that it planned to issue the guidance based on the research done prior to 2000. The Board asked the FAA to clarify this situation. The Board indicated that the delay was unacceptable if the FAA had completed its research in 2000 but would not issue the resulting guidance material until 2006. The Board asked the FAA to confirm that the revised AC would include guidance to address certification issues identified by its working group for which the later research had produced additional information. Pending information regarding whether the FAA had conducted icing research since 2000 whose results would be included in the revised AC and issuance of the revision to AC 20-73 including this information, Safety Recommendation A-98-92 was classified Open Unacceptable Response. On August 16, 2006, the FAA issued AC 20-73A, the revision to AC 20-73. The Safety Board recently reviewed AC 20-73A and its relationship to Safety Recommendation A-98-92. The results of icing research sponsored by the FAA and NASA are included in Appendix R of AC 20-73A. Appendix R also includes guidance on determining critical ice shapes and their associated roughness, descriptions of ice accreted in the time prior to activation of an ice protection system, intercycle ice accretions, and the aerodynamic penalties associated with these ice shapes. The references in Appendix R include several based on research concluded after 2000. The revision to AC 20-73, including the information provided in Appendix R, provides useful information for better evaluating an airplane’s performance and handling capabilities in icing conditions. The Board is aware that more research concerning in-flight ice accretions and their effects on aircraft is currently being conducted by other organizations. The Board encourages the FAA to continue to incorporate into the advisory material additional information and results from these ongoing research programs as they become available. Issuance of AC 20-73A, and previously issued FAA- and NASA-developed pilot training materials, meet the intent of Safety Recommendation A-98-92; accordingly, the recommendation is classified Closed Acceptable Action.

From: NTSB
To: FAA
Date: 7/23/2007
Response: Notation 7903: The National Transportation Safety Board has reviewed the Federal Aviation Administration’s (FAA) Notice of Proposed Rulemaking (NPRM), “Activation of Ice Protection; Proposed Rule,” which was published at 72 Federal Register 20924 on April 26, 2007. The notice proposes to amend Federal Aviation Regulation (FAR) Part 25 to require a means to ensure timely activation of the airframe ice protection system (IPS) on newly certified transport category airplanes for flight in icing conditions. Specifically, the rule would require that airplane manufacturers provide one of the following methods to detect ice and to ensure that the airframe IPS is activated: • automatic activation, or • installation of an ice detection system that alerts the crew to activate the IPS, or • the identification and publication of environmental conditions conducive to icing for use by the crew to activate the IPS. The NPRM states that the proposed rule will partially address Safety Recommendations A-96-56 and A-98-91 issued by the Safety Board: Revise the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate, or are otherwise shown to be capable of safe flight into such conditions. If safe operations cannot be demonstrated by the manufacturer, operational limitation should be imposed to prohibit flight in such conditions and flightcrews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification. (A-96-56) Require manufacturers and operators of modern turbopropeller-driven airplanes in which ice bridging is not a concern to review and revise the guidance contained in their manuals and training programs to include updated icing information and to emphasize that leading edge deicing boots should be activated as soon as the airplane enters icing conditions. (A-98-91) In reviewing the proposed rule changes, the Safety Board is pleased that the FAA agrees that the regulations regarding flight in icing conditions need to be improved. The NPRM proposes an alerting system for icing conditions that exceed the limits of aircraft certification, as recommended in part by A-96-56, by adding section 25.1419(e). The NPRM proposes to address A-07-14 in part, which supersedes A-98-91 (discussed later in this response), by requiring IPS activation as soon as the airplane enters icing conditions. However, it is only an NPRM and regulatory changes may be years away. It has been almost 11 years since the Safety Board issued Safety Recommendation A-96-56 as a result of the October 31, 1994, in-flight icing encounter and loss of control of American Eagle Flight 4184, an ATR-72, over Roselawn, Indiana (Safety Board accident DCA95MA001), and 9 years have passed since the issuance of A-98-91, issued as a result of the January 9, 1997, Monroe, Michigan, Comair EMB-120 accident (Safety Board accident DCA97MA017). The Safety Board has investigated several more accidents involving in-flight icing since then, including the following: • March 14, 1997, Detroit, Michigan, Reno Air MD-87 (accident CHI97FA083) • March 19, 2001, West Palm Beach, Florida, Comair EMB-120 (accident DCA01MA031) • February 16, 2005, Pueblo, Colorado, Circuit City Citation 560 (accident DCA05MA037) • September 8, 2006, Santa Maria, California, American Eagle SF340B (accident LAX06IA076) Ice-related accidents like these are likely to continue unless the airplane certification regulations are updated to include the icing knowledge and research developed over the past decade. The Safety Board therefore encourages the FAA to act expeditiously on ice certification regulations and IPS activation requirements, including guidance materials and training programs. The Safety Board would like to see the following issues also addressed in the proposed rule. The first issue is the ongoing disconnect between industry’s current guidance on deice boot activation and what the FAA has learned and research has shown about ice bridging and deice boot effectiveness. Information gathered from the FAA’s 1997 Airplane Deice Boot Bridging Workshop, as well as subsequent icing tunnel and flight tests, revealed that ice bridging does not occur on modern airplanes, which is contrary to what has previously been thought and why some manufacturers’ guidance addresses delayed activation of deice boots. In addition, icing tunnel tests conducted by the National Aeronautics and Space Administration have shown that thin, rough ice accumulations on the wing leading edge deice boot surfaces (like the 1/4 inch or less prescribed by some manufacturers before boot activation) can, depending on distribution, be as aerodynamically detrimental to an airplane’s performance as larger ice accumulations. As a result, deice boots should be operated at the first sign of ice. Delays intended to prevent ice bridging are inappropriate, given the results of current research, and manufacturers’ guidance to operators must be revised accordingly. For example, Cessna operating procedures for the 208 Caravan (C-208) instruct crews to wait for 1/4 to 3/4 inch of ice to accrete before activating the C 208 pneumatic boots. The NPRM, on the other hand, prescribes turning on ice protection at the first sign of ice accretion (or even before, if the conditions are right). As a result of the Safety Board’s investigation of the February 16, 2005, Cessna Citation 560 accident at Pueblo, Colorado, the Safety Board issued Safety Recommendation A-07-14, which superseded A-98-91: Require manufacturers and operators of pneumatic deice boot-equipped airplanes to revise the guidance contained in their manuals and training programs to emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions. (A-07-14) The Safety Board encourages the FAA to address this recommendation in the final rule by addressing the ice bridging issue directly to ensure that manufacturers revise the guidance in their manuals and training programs so that operators receive clear, complete, and unambiguous guidance on deice boot activation, as set forth in Safety Recommendation A-07-14. The second issue is the need for more specific guidance with respect to the identification of environmental conditions conducive for icing, the third method of compliance offered in the NPRM. As stated in Safety Recommendation A-07-14, the Safety Board believes that the IPS should be activated “as soon as the airplane enters icing conditions,” which may include visible moisture and the requisite outside air temperature. The FAA, however, has been reluctant to support IPS activation without the presence of ice, citing concerns of reduced IPS life and airplane performance. Most recently, on April 3, 2007, the FAA issued a letter of interpretation for comment in the Federal Register (Docket No. FAA-2007-27758) seeking clarification from industry on the subject of “known icing.” In it, the FAA stated that “Permutations on the type, combination, and strength of meteorological elements that signify or negate the presence of known icing conditions are too numerous to describe….” It is the Safety Board’s position that industry cannot realistically be expected to implement the third method in the proposed rule until the FAA provides a more specific definition of “environmental conditions conducive to icing.” Understandably, industry has concerns about decreased system life and airplane performance and therefore hesitates to direct operators to activate an IPS based solely on icing potential. The FAA should use the research it has commissioned through the Aviation Rulemaking Advisory Committee to provide a more detailed description of conditions conducive to icing. Until the FAA provides specific environmental parameters and prescribed levels, the third method proposed to address activation of the IPS, despite being the most proactive solution, will not be practical. The third issue is that the proposed rule does not address operations when certain functions of the IPS are known to be inoperable. Since the proposed rule would alert crews to icing conditions, the Safety Board believes that the proposed rule should prohibit crews from operating when certain functions of the IPS are inoperable. This situation occurred on January 2, 2006, when American Eagle flight 3008, a SAAB SF340B, departed San Luis County Regional Airport, San Luis Obispo, California, with a scheduled destination of Los Angeles International Airport. The airplane encountered icing conditions during the en route climb and departed controlled flight at an altitude of about 11,500 feet mean sea level (msl), descending to an altitude of about 6,500 feet msl. During the previous inbound flight and subsequent incident flight, the airplane’s automatic deice control was inoperable due to a deicer timer light failure. According to American Eagle’s Minimum Equipment List, the crew could dispatch the airplane into known or forecast icing conditions with the automatic function of the IPS inoperable. The proposed rule does not address such operations—that is, operations with an inoperable automatic IPS. The Safety Board believes that the proposed rule should prohibit flight into known icing if certain functions of the IPS are inoperable. Finally, because ice is a major safety issue that affects all airplanes, regardless of age, the Safety Board believes that the proposed rule should be expanded beyond newly certificated airplanes to include deice boot-equipped airplanes currently certified for flight in icing conditions. The Board hopes that the FAA will apply the new certification standards to ensure timely IPS activation, developed as a result of this NPRM, to earlier recommendations related to icing certification criteria (see Safety Recommendations A-96-54 and A-98-92), so that accidents like the EMB-120 accident in Monroe, Michigan, will not reoccur. The Safety Board stated this concern 9 years ago by issuing Safety Recommendation A-98-100, as a result of the accident in Monroe, Michigan: When the revised icing certification standards and criteria are complete, review the icing certification of all turbopropeller-driven airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. (A-98-100) This recommendation was classified “Closed–Unacceptable Action/Superseded” with the issuance of Safety Recommendation A-07-16 to the FAA on February 27, 2007, issued as a result of the February 16, 2005, Pueblo, Colorado, Cessna Citation 560 accident. The prior recommendation addressed turbopropeller airplanes; however, the Citation 560 involved in the Pueblo accident was a deice boot-equipped turbojet. The circumstances of the Pueblo accident demonstrated that pneumatic deice boot-equipped turbojet airplanes also require review and testing to meet the expanded icing certification standards. Hence, this new recommendation, A 07-16, reiterates the point that certification requirements should apply to currently certified deice boot-equipped aircraft, as well as new aircraft: When the revised icing certification standards (recommended in Safety Recommendations A-96-54 and A-98-92) and criteria are complete, review the icing certification of pneumatic deice boot-equipped airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. (A-07-16) The Safety Board supports issuance of the regulatory revisions proposed in this NPRM. Ice is detrimental to airplane performance and handling qualities, and it represents a serious threat to aviation safety. The Board has investigated many accidents caused by in-flight icing. Operators need to be fully informed before entering icing conditions, and the proposed rule should help do this. However, the Board continues to believe that the issues outlined above—bridging, clearer guidance on deice activation, operation with a known inoperative IPS, and applying revised certification standards to previously certified airplanes—should be addressed in the proposed rule. The Safety Board appreciates the opportunity to comment on this NPRM.

From: NTSB
To: FAA
Date: 2/27/2007
Response: Accident report crash during approach to landing Cessna Citation 560 in Pueblo, CO (adopted 1/23/2007): The Safety Board has previously identified concerns about inadequate flight test certification requirements. For example, it was revealed during the investigation for the October 31, 1994, accident involving American Eagle flight 4184 in which the airplane crashed during a rapid descent after an uncommanded roll excursion during icing conditions16 that SLD conditions can cause ice accretions that are more aerodynamically detrimental than those accretions that fall within the Part 25, Appendix C envelope.17 As a result, the Board issued Safety Recommendation A-96-54, which asked the FAA to do the following: Revise the icing criteria published in 14 Code of Federal Regulations Parts 23 and 25, in light of both recent research into aircraft ice accretion under varying conditions of liquid water content, drop size distribution, and temperature, and recent developments in both the design and use of aircraft. Also, expand the Appendix C icing certification envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary. Further, icing tunnel tests conducted as part of the Comair flight 3272 accident investigation indicated that the effects of ice accretion on airplane performance could vary widely depending on the size, distribution, and type of ice accumulated on the airplane’s surfaces. However, the Board learned that manufacturers are not required to demonstrate an airplane’s flight handling characteristics or stall margins using thin, rough ice that can accrete on protected surfaces before the activation of the deice boot system or between activation cycles. As a result of its findings, the Board issued Safety Recommendation A-98-92, which asked the FAA (in cooperation with the National Aeronautics and Space Administration and other interested aviation organizations) to do the following: [C]onduct additional research to identify realistic ice accumulations, to include intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of the deicing boots, and to determine the effects and criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft certification requirements and pilot training programs at all levels. The Safety Board also issued Safety Recommendation A-98-100, which asked the FAA to review the icing certification of all turbopropeller-driven airplanes currently certificated for operation in icing conditions, perform additional testing, and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards asked for in Safety Recommendation A-98-92. The FAA indicated in a March 6, 2006, response to Safety Recommendation A-96-54 that the ARAC IPHWG is continuing to develop a revision to Part 25 to require a demonstration that an airplane can safely operate in SLD conditions for an unrestricted time or can detect SLD and safely exit icing conditions. However, the FAA has still not received the recommendations from the IPHWG, prepared regulatory analyses, issued the NPRM, analyzed comments, or completed the many other tasks involved in issuing new regulations. The FAA indicated in an October 26, 2005, response to Safety Recommendation A-98-92 that it had completed and would shortly issue a draft revision to AC 20-73, which included the certification guidance on determining critical ice shapes, descriptions of intercycle and residual ice accretions, and the aerodynamic penalties associated with these ice shapes. Although the FAA issued AC 20-73A on August 16, 2006, it has still not provided the Safety Board with information regarding any new research conducted in response to this recommendation. Regarding Safety Recommendation A-98-100, the FAA issued a notice of proposed rulemaking (NPRM) in November 2005, which proposed to expand 14 CFR Part 25 to include specific certification requirements for airplane performance or handling qualities for flight in icing conditions and to specify the ice accumulations that must be considered for each phase of flight. Further, the FAA proposed changes to AC 25-1X, which intended to provide guidance for implementing the regulations proposed in the NPRM. In May 2006, the Safety Board expressed concern that, although it agreed with the proposed regulatory changes, the FAA had not applied the new standards to all in-service turbopropeller-driven aircraft. The FAA further indicated that no airplanes have an unsafe condition in icing environments despite a number of accidents in the 1990s that involved airplanes that had passed the certification standards. The Board stated that, to meet the intent of Safety Recommendation A-98-100, the FAA would need to formally evaluate (perhaps by conducting flight tests) all in-service turbopropeller-driven aircraft to ensure that these aircraft comply with all current icing certification criteria for new aircraft. The Board asked the FAA to provide a list of the aircraft that it had formally evaluated and a summary of the findings and resultant actions. To date, this information has not been received. The circumstances of the Comair flight 3272, American Eagle 4184, and Pueblo accidents and the icing tunnel test data show that the ice shapes used during initial certification flight tests were not adequate because the tests did not account for thin, rough ice on the wing. The 1996 ice shapes tests on the Cessna 560 were also inadequate because, although tests were conducted with ice shapes on the protected surfaces, tests were not conducted using thin, rough ice. Therefore, additional ice sizes, distribution patterns, and types need to be considered during flight testing to more adequately gauge an airplane’s performance in icing conditions. The Safety Board concludes that existing flight test certification requirements for flight into icing conditions do not test the effects of thin, rough ice on or aft of an airplane’s protected surfaces, which can cause severe aerodynamic penalties. The circumstances of this accident clearly show that the actions requested in Safety Recommendations A-96-54 and A-98-92 are needed to improve the safety of all airplanes operating in icing conditions. Therefore, the Safety Board reiterates Safety Recommendations A-96-54 and A-98-92.

From: NTSB
To: FAA
Date: 5/10/2006
Response: On September 21, 2001, the FAA indicated to the Safety Board that a working group co chaired by the FAA and NASA produced a report in September 2000 titled, Report of the 12A Working Group on Determination of Critical Ice Shapes for the Certification of Aircraft. The report concluded that sufficient information and methods were not available at that time to provide additional guidance concerning the determination of critical ice shapes in aircraft certification. The FAA indicated in 2001 that it was sponsoring the additional research that was needed, as recommended in Safety Recommendation A-98-92. On July 11, 2002, the Safety Board classified Safety Recommendation A-98-92 "Open-Acceptable Response," pending completion of the needed research and incorporation of the results into aircraft certification requirements and pilot training programs. In its current letter, the FAA references tests performed during December 1999 and March 2000, but does not mention any additional research or testing conducted since the FAA's September 2001 letter. The FAA also indicates that it has completed, and will shortly issue for comment, a draft revision to AC 20-73, "Aircraft Ice Protection," which includes the certification guidance on determining the critical ice shapes during aircraft certification. While the material in the revised AC will be appropriate for use by certification engineers and airframe manufacturers, the FAA believes that safe operation of an airplane in icing conditions is not dependent upon the pilot's having the same information. In lieu of providing them the AC, the FAA, in collaboration with NASA, produced and distributed icing training materials to pilots. The Safety Board notes that in the February 1, 2005, annual report to Congress, Regulatory Status of the National Transportation Safety Board's "Most Wanted" Recommendations to the Department of Transportation (DOT), the DOT stated, much as the FAA's September 2001 letter to the Board did, that a working group co-chaired by the FAA and NASA produced the September 2000 report and concluded that sufficient information and methods were not available to provide the needed guidance concerning the determination of critical ice shapes in certification. The DOT indicated to Congress that the FAA-sponsored additional research had been completed. The Safety Board asks the FAA to clarify this situation. In the February 1, 2005, report to Congress, the DOT indicated that additional research had been conducted and completed, and that these research results would be included in new guidance. The FAA's October 26, 2005, letter provided details of the research conducted in December 1999 and March 2000, and indicated that new guidance material for aircraft certification would be issued shortly. However, the FAA's recent letter does not describe any results from research conducted since its September 2001 letter, nor does it indicate that these results will be included in the new guidance material. The Safety Board finds the FAA's delay unacceptable if in fact it completed its research 6 years ago, but has not yet issued the resulting guidance material. If the FAA has learned additional information from the research conducted since 2001 that addresses the issues its working group identified, the Safety Board would appreciate (1) learning of the results and (2) receiving a statement from the FAA that the new AC will include this guidance. Pending our receiving such information and the issuance of the revised AC 20-73 including the research results, Safety Recommendation A-98-92 is classified "Open-Unacceptable Response."

From: FAA
To: NTSB
Date: 3/1/2006
Response: In its 3/1/2006 annual report to Congress, Regulatory Status of the National Transportation Safety Board's "Most Wanted" Recommendations to the Department of Transportation, the DOT wrote: The FAA has completed a draft revision to advisory circular (AC) 20-73, which includes certification guidance relative to the effects and criticality of deicing boot inter cycle and residual ice accumulations, and ice accumulations on unprotected surfaces aft of protected surfaces. It is anticipated that the AC will be published by summer 2006.

From: NTSB
To: FAA
Date: 2/2/2006
Response: Notation 7744A: The National Transportation Safety Board has reviewed the Federal Aviation Administration (FAA) notice of proposed rulemaking (NPRM), Airplane Performance and Handling Characteristics in Icing Conditions, published in the Federal Register (Vol. 70, No. 213) on November 4, 2005. The Safety Board has evaluated the NPRM, which applies to newly certificated designs, in light of current, open recommendations related to aircraft icing and in regard to other concerns identified during the course of its investigations. The Safety Board is providing comments on the following sections: 25.21, Proof of compliance; 25.143, General (controllability and maneuvering); 25.207, Stall warning; and Part 25, Appendix C. The Safety Board will also comment on the Aviation Rulemaking Advisory Committee (ARAC) Flight Test Harmonization Working Group findings in its comments on the relevant sections. The Safety Board has evaluated the proposed sections that detail additional rule changes specific to the phases of flight, as listed in section 25.21, and agrees that they are appropriate and acceptable to fully implement the overall regulation change in section 25.21. The Safety Board therefore will not offer individual comments on those sections. Title 14 Code of Federal Regulations (CFR) Part 25, which the NPRM proposes to revise, contains airworthiness standards for type certification of transport-category airplanes. The NPRM proposes to expand 14 CFR Part 25 to include specific certification requirements for airplane performance or handling qualities for flight in icing conditions and to specify the ice accretions that must be considered for each phase of flight. The FAA states that the proposed revisions will ensure that minimum operating speeds determined during certification of all future transport-category airplanes will provide adequate maneuvering capability in icing conditions for all phases of flight and all airplane configurations. In short, the NPRM proposes to require that the same airplane handling characteristics that apply in non-icing conditions will continue to apply in icing conditions. Additionally, the proposed revisions will harmonize U.S. and European airworthiness standards for flight in icing conditions. Previous Icing Certification Recommendations The Safety Board has been concerned for many years about the inadequacy of the existing certification regulations for flight in icing conditions, which have not required manufacturers to demonstrate an airplane’s flight handling, stall characteristics, minimum airspeeds, and stall margins under a sufficiently realistic range of adverse ice accretions. Additionally, the regulations have not required airplanes to be tested with the thin, rough ice that can accrete on protected surfaces prior to the activation of ice protection systems or between activation cycles of the ice protection systems, especially in larger water droplet environments. These issues came to light during the Board’s investigations of two accidents involving loss of control in icing conditions: United Express flight 2415, a British Aerospace BA-3101, in Pasco, Washington (December 26, 1989), and COMAIR flight 3272, an Embraer EMB-120RT, in Monroe, Michigan (January 9, 1997). United Express flight 2415 involved an ice-contaminated tailplane stall and loss of control at low altitude. Contributing to the loss of control was the accumulation of ice on the airframe, which degraded the aerodynamic performance of the airplane. The United Express 2415 investigation resulted in Safety Recommendation A-91-87: Amend the icing certification rules to require flight tests wherein ice is accumulated in those cruise and approach flap configurations in which extensive exposure to icing conditions can be expected, and require subsequent changes in configuration, to include landing flaps. [Status: “Open—Acceptable Response”] Several years later, the Safety Board investigated the Comair 3272 accident, which involved a loss of control while the airplane was maneuvering with ice accretions on the wings. The Safety Board determined that the probable cause of the accident was the following: The FAA’s failure to establish adequate aircraft certification standards for flight in icing conditions…and the FAA’s failure to require the establishment of adequate minimum airspeeds for icing conditions, which led to the loss of control when the airplane accumulated a thin, rough accretion of ice on its lifting surfaces. As a result of its investigation, the Safety Board issued Safety Recommendation A-98-94 to the FAA: Require manufacturers of all turbine-engine driven airplanes (including the EMB 120) to provide minimum maneuvering airspeed information for all airplane configurations, phases, and conditions of flight (icing and nonicing conditions); minimum airspeeds also should take into consideration the effects of various types, amounts, and locations of ice accumulations, including thin amounts of very rough ice, ice accumulated in supercooled large droplet icing conditions, and tailplane icing. [Current status: “Open—Unacceptable Response”] During the Comair 3272 accident investigation, the Safety Board also noted that the ice accretions and icing conditions considered during certification for flight in icing conditions were not representative of the most critical ice accretions that would be encountered while operating in icing conditions. The current icing certification rules required the certification applicant to demonstrate safe flight using a limited number of icing conditions within the Appendix C icing envelope. The effects of delayed ice protection system activation, intercycle ice accretions, or residual ice accretions were not addressed in the icing certification rules. Icing conditions near the edge of the Appendix C envelope, which are less likely to be encountered, can produce thin, rough ice accretions that research has shown can be as aerodynamically detrimental as the much larger ice shapes that form during flight in the more likely icing conditions. Such thin, rough ice accretions produced at the edge of the envelope icing conditions are dangerous in that they may not be noticed by the flightcrew, or they may not be perceived as a threat based on the crew’s experience accreting ice while flying in more typical, center-of-the-envelope icing conditions. As a result of these findings, the Safety Board issued Safety Recommendation A-98-92 to the FAA: With the National Aeronautics and Space Administration and other interested aviation organizations, conduct additional research to identify realistic ice accumulations, to include intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of the deicing boots, and to determine the effects and criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft certification requirements and pilot training programs at all levels. [Status: “Open—Acceptable Response”] This recommendation is currently on the Safety Board’s Most Wanted list of safety improvements. In addition to these loss-of-control issues, the Safety Board has addressed the need for an adequate stall warning system for operations during icing conditions. The Board found during the Comair 3272 investigation that the stall warning system installed on the airplane did not provide an adequate warning to the pilots because the system was not designed to account for aerodynamic degradation due to ice accretions or to adjust its warning to compensate for the reduced stall margin caused by ice accretions on the lifting surfaces of the airplane. The lack of a system to warn pilots of impending stall in icing conditions in this accident, and in several other similar incidents, led the Safety Board to issue Safety Recommendation A-98-96: Require the manufacturers and operators of all airplanes that are certificated to operate in icing conditions to install stall warning/protection systems that provide a cockpit warning (aural warning and/or stick shaker) before the onset of stall when the airplane is operating in icing conditions. [Status: “Open—Acceptable Response”] Following are the Safety Board’s comments on the proposed regulatory changes, presented in light of past accident and incident investigations where loss of control due to ice accretion was a factor or causal or the lack of a stall warning system was an issue. Section 25.21, Proof of Compliance The Safety Board notes that current Part 25, Subpart F, regulations for airplanes with approved ice protection state that airplanes must be able “to safely operate” in icing conditions. However, no standard set of criteria currently defines what “to safely operate” means in terms of performance and handling characteristics. The Safety Board agrees with the NPRM that a loss of control is the greatest threat to safety in icing conditions and warrants a change to the regulations to define safe operations for flight in icing conditions. Additionally, the Safety Board agrees with ARAC’s Flight Test Harmonization Working Group (FTHWG) that to ensure safe flight in icing conditions, airplanes with ice accretions should comply with Part 25, Subpart B. The NPRM seeks to address this issue by adding paragraph (g)(1) to section 25.21, which will require flight in icing conditions to comply with the same handling requirements and most of the performance requirements that are currently required for flight in non-icing conditions. The NPRM also states that compliance must be demonstrated using the ice accretions defined in Appendix C of Part 25, and assuming normal operation of the airplane and its ice protection system as specified in the Airplane Flight Manual (AFM). By referencing the AFM, this paragraph requires the AFM to include the limitations and operating procedures that are specific to operations in icing conditions. The Safety Board will comment on the use of Appendix C ice accretions for evaluating handling and performance later in these comments. The Safety Board agrees with the FAA that airplane operations in icing conditions should not diminish the safety margins, handling qualities, and performance of the airplane, and that the airplane should be as safe to operate in icing as in non-icing conditions. The Safety Board believes further that the proposed change to section 25.21, by adding specific performance and handling requirements that must be satisfied to certify an airplane for operation in icing conditions (for example, requirements for takeoff performance and landing climb performance), will provide an additional margin of safety for these airplanes. The Safety Board therefore endorses the proposed changes to the section 25.21 regulations, and the following associated sections listed in the NPRM, which will also impose the same performance and handling requirements for icing conditions as for non-icing conditions: 25.103, Stall speed; 25.105, Takeoff; 25.107, Takeoff speeds; 25.211, Takeoff path; 25.119, Landing climb: All-engines operating; 25.121, Climb: One-engine inoperative; 25.123, En route flight paths; and 25.125, Landing. The Safety Board notes that the requirements of these sections will establish minimum maneuvering speeds while in icing conditions, and will therefore satisfy that part of Safety Recommendation A-98-94 that addresses the need to provide minimum maneuvering airspeeds for all airplane configurations and phases of flight in icing conditions. Section 25.143, Controllability and Maneuverability—General This section of the NPRM addresses three issues of interest to the Safety Board: minimum control speed, tailplane stall, and visual detection of ice accretions. Minimum Control Speed. The NPRM states that it proposes to revise section 25.143 by adding a new paragraph (c) that will require a certification applicant to show that an airplane with ice accretions appropriate to the phase of flight and with the critical engine inoperative is safely controllable and maneuverable during takeoff, approach and go-around, and approach and landing. The NPRM states that the requirements proposed in paragraph (c) are intended to ensure that using the minimum control speeds for non-icing conditions will not degrade controllability and maneuverability when the same speeds are used for icing conditions. An additional paragraph, (i)(1), is also proposed, which defines the ice accretions that must be used in demonstrating compliance with 25.143. Paragraph (i)(1) states that controllability must be demonstrated with the ice accretion that proposed Appendix C defines as most critical for that particular phase of flight. The NPRM notes that by using the most critical ice accretion to determine minimum control speeds for each flight phase, certification applicants can minimize the number of ice accretions that must be tested by using one accretion that is shown to be the most critical accretion for several flight phases. The Safety Board believes that inclusion of the requirement for certification applicants to demonstrate that the minimum control speeds provide adequate controllability and maneuverability with ice accretions on the airplane [new paragraph (c)] will provide an additional level of safety for operations in icing conditions, provided that the most critical ice accretion used [new paragraph (i)(1)] is validated as such, and is tested for the respective phase of flight, as indicated in Appendix C. Section 25.143 also provides information about the thin, rough layer of ice defined as “sandpaper” ice in the proposed revision to Appendix C. The NPRM states that such ice has been shown to have a more detrimental effect on handling qualities for airplanes with unpowered control systems than larger ice accretions. In some cases, the NPRM goes on to say, a small, rough ice accretion has resulted in control surface hinge moment reversals that required the flightcrew to apply extremely high forces to the controls to regain control of the airplane. The NPRM states that certification applicants must consider sandpaper ice as a critical ice shape in showing compliance with proposed paragraph 25.143(i). The Safety Board agrees that thin, rough ice accretions must be considered when evaluating an airplane’s controllability and performance in icing conditions. However, the Safety Board believes that the sandpaper ice used to demonstrate compliance with proposed paragraph 25.143(i) should accurately represent the thin, rough ice accreted in the most severe and edge of the-envelope icing conditions. The Safety Board believes that use of these extreme conditions should be required to evaluate critical ice shapes. Inclusion of the edge of the envelope conditions is particularly important when determining the chordwise extent of the thin, rough, sandpaper-type ice accretions. In its letter commenting on proposed Advisory Circular 25-1X, the Safety Board noted that the results of research sponsored by the FAA and NASA are currently included in Appendix R of draft AC 20-73, Aircraft Ice Protection. Appendix R includes guidance on determining critical ice shapes and their associated roughness, and descriptions of ice accreted before activation of an ice protection system and between ice protection cycles, and the aerodynamic penalties associated with these ice shapes. The Safety Board noted that by issuing the revision to AC 20-73, and by including information like that found in Appendix R, the FAA is providing useful information for better evaluating an airplane’s performance and handling capabilities in icing conditions. The Safety Board believes that, by providing examples of thin, rough ice accretions, intercycle ice accretions, and residual ice, by including paragraph 25.143(i)(1) in the regulations, and by ensuring that the most extreme conditions are examined, the FAA will satisfy the intent of Safety Recommendation A-98-92. Tailplane Stall. The NPRM proposes to add paragraph 25.143(i)(2), which would require applicants to evaluate an airplane’s susceptibility to ice-contaminated tailplane stall (ICTS) by conducting a pushover maneuver down to a zero g load factor (or the lowest load factor obtainable if limited by elevator power) with the critical ice accretion on the airplane. This issue was also described in the NPRM as an area of non-consensus in the FTHWG, as some members believed that the requirement to maintain a push force throughout the maneuver should be limited to higher values of load factor during the push down maneuver (0.5g and 0.25g were suggested alternatives). The FTHWG did agree, however, that the test should be conducted down to zero g. Safety Board investigations have shown that loss of control in icing conditions can be an extremely sudden and dynamic event. The Safety Board believes that a control force reversal, whether ice-induced or not, would be unacceptable within the flight envelope of any airplane (which includes load factors to zero g). The Safety Board supports the proposal as written in the NPRM to require a push force on the control column through the entire ICTS evaluation pushover maneuver. The Safety Board believes that incorporation of the zero-g pushover test into the regulations for flight in icing conditions will adequately address the intent of Safety Recommendation A-91-87. The Safety Board therefore endorses the inclusion of paragraph (i)(2) to the icing certification requirements of section 25.143. Visual Detection of Ice Accretion. The FAA also proposes to add paragraph 25.143(j), which would address airplane controllability from the time an airplane first enters icing conditions until the ice protection system is activated and performing as intended. This paragraph allows two different means of compliance, based on the method used to detect the icing conditions. If the means of detecting the icing conditions is dependent upon the pilot recognizing a specified amount of ice on a reference surface (for example, 1/4 inch of ice on the wing or 1/2 inch of ice on the windshield wiper), the requirements of section 25.143 must be met with the ice accretion defined in the proposed Appendix C, part II(e), and by proposed AC 25-1X, accounting for the time prior to activation of the ice protection system. If other methods are used to detect the icing conditions—initial ice accretion on an ice probe, for example—the requirements for demonstrating compliance would be less stringent for smaller ice accretions. This portion of the proposed regulation is based on the expectation that the aircraft would be in icing conditions for a brief time. The less stringent handling requirements involve the following maneuvers with the pre-activation ice shape that is expected before the ice protection system activates: (1) demonstrating controllability in a pull-up maneuver to 1.5g load factor and (2) demonstrating no longitudinal force reversal in a pushover maneuver to 0.5g load factor. The NPRM mentions that the FTHWG did not concur with the proposal to impose a different set of compliance requirements based on the means of detecting icing conditions. In the non-consensus discussion, the Air Line Pilots Association argued that any means of visual ice detection, whether trace amounts or a specified amount, relies on the flightcrew to monitor conditions outside the cockpit, and hence should be subject to the more stringent requirements. The Safety Board believes that several factors that the flightcrew may encounter during icing conditions, such as higher workload and reduced visibility, can considerably delay their recognition of icing conditions when a visual means is used, regardless of the amount of ice being observed. The flightcrew’s failure to recognize ice accretion, their perception that the ice accretion is not a threat, and the presence of clear ice have all been factors in accidents and incidents. Thin, rough ice, which can develop quickly and is difficult to detect visually, particularly that near the edge of the Appendix C icing envelope, can be enough to cause considerable aerodynamic penalty, particularly if the surface used for visual detection is a lifting surface like the wing. Hence, full controllability and maneuverability evaluation of the airplane with the ice accretion that would be expected with a considerably delayed visual detection is warranted. The Safety Board believes that for airplanes that rely on a visual means of ice detection, the more stringent rules should apply. Section 25.207, Stall Warning The NPRM proposes to revise section 25.207 to require that “the means for providing a warning of an impending stall must be the same for both icing and non-icing conditions.” Additionally, the NPRM proposes to add a paragraph to require that stall warnings in icing conditions be sufficient to allow the pilot to take corrective action to prevent stalling in both straight and turning flight, using the same recovery techniques as in icing conditions, and describes the ice accretions that must be considered in demonstrating compliance. The proposed rule also specifically addresses the time between the start of an icing encounter and activation of the ice protection system. The NPRM states that this time period is dependent upon the means of detecting the icing conditions, and if that means is dependent upon the pilot recognizing a specified amount of ice on a reference surface, the time period could be longer. The NPRM proposes that, if the pilot is visually recognizing a specific amount of ice (hence, imposing a potentially longer period before the ice protection activates), the same stall warning requirements would apply as when the ice protection system is fully active. However, if the detection of icing conditions does not depend upon the pilot recognizing a specified amount of ice on a reference surface (for example, because an ice detector is being used), the proposed rule also allows for a different method of stall warning (for example, airframe buffet) to be used while the airplane is accreting ice and before the ice protection system activates. In a case like this, a more stringent testing procedure (that is, a longer period of time prior to activation of the ice protection system) will be required to demonstrate compliance. Ensuring that all airplanes are equipped with stall warning devices that will provide adequate stall warning while those airplanes operate in icing conditions is the subject of previously described Safety Recommendation A-98-96. The Safety Board believes the proposed rules rectify a serious shortcoming in the current rules, which do not consider ice accretions on the airplane in evaluating the stall warning system. The Safety Board believes the proposed changes will satisfy the intent of this recommendation, and supports its inclusion in the certification rules. Proposed Appendix C, Part II The proposed changes to Appendix C state that the most critical ice accretions for each phase of flight must be determined considering the atmospheric conditions of part I of Appendix C and the flight conditions (such as angle of attack, configuration, and airspeed.). Ice accretions to be determined include those on the unprotected surfaces and on the protected surfaces as appropriate to normal ice protection system operation; these ice accretions must include takeoff ice, final takeoff ice, en route ice, holding ice, landing ice, and sandpaper ice. The proposed change also specifies that the ice must form before the ice protection system activates and is performing its intended function. The Safety Board believes that the proposed changes to Appendix C do not ensure that the airplane is tested in the most severe icing conditions as defined in the icing atmospheric envelope of Appendix C. As a result of its investigation of the Roselawn accident, the Safety Board issued Safety Recommendation A-96-56 to the FAA, which recommended the following: Revise the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate, or are otherwise shown to be capable of safe flight into such conditions. If the manufacturer cannot demonstrate safe operations, operational limitations should be imposed to prohibit flight in such conditions and flightcrews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification. [Status: “Open—Unacceptable Response”] This recommendation is also on the Safety Board’s Most Wanted list of safety improvements. Although the proposed rule specifies the icing conditions that must be considered in determining the ice accretions for each phase of flight, the intent of Safety Recommendation A-96-56 is to ensure that the testing occurs in the range of conditions in which the airplane is authorized to operate—in particular, in the most hazardous icing conditions—and that the testing ensures that airplane performance is evaluated at the limits of the Part 25 Appendix C envelope for droplet size and liquid water content. The Safety Board agrees that the most critical ice shape must be determined for each phase of flight and its corresponding flight conditions, and that the resulting critical ice shape may be different for each phase of flight. The Board believes further that the additional requirements to examine the accretions for each phase of flight would be positive enhancements to the evaluation procedure for certification for flight in icing conditions. However, past Safety Board investigations of icing accidents and incidents have shown that some aircraft may not have been exposed to the full range of Appendix C conditions for all phases of flight during icing certification, and hence, have not been subject to the most critical ice shapes. The Board therefore believes that the proposed rules should also require applicants to justify their selection of the most critical ice shape for each phase of flight to ensure that the most critical shape is being used for the respective phase of flight. For each phase of flight, the applicant should be required to demonstrate that the shape, chordwise and spanwise, and the roughness of the shapes considered in selection of a critical ice shape accurately reflect the full range of Appendix C conditions that have been examined in terms of mean effective drop diameter, liquid water content, and temperature during the respective phase of flight. Additionally, the FAA should review the justification and selection of the most critical ice shape for each phase of flight. Finally, the Safety Board wishes to stress that super-cooled large droplet (SLD) conditions can be more hazardous than conditions considered during current icing certification, and accident experience has shown that SLD conditions can cause ice accretions that are more aerodynamically detrimental than those developed while flying within the Part 25 Appendix C envelope. Additionally, the accident and incident history has shown that many of the smaller turboprop airplanes spend much more of their operational time in regions of the atmosphere that are likely to contain SLD conditions than the larger, turbojet type airplanes. As a result of the Roselawn investigation, the Safety Board issued Safety Recommendation A-96-54, which requested the following of the FAA: Revise the icing criteria published in 14 Code of Federal Regulations (CFR), Parts 23 and 25, in light of both recent research into aircraft ice accretion under varying conditions of liquid water content, drop size distribution, and temperature, and recent developments in both the design and use of aircraft. Also, expand the Appendix C icing certification envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary. [Status: “Open—Unacceptable Response”] This recommendation is also currently on the Safety Board’s Most Wanted list of safety improvements. The proposed rules do not address this recommendation, and the Safety Board continues to await the FAA’s proposed regulatory changes to 14 CFR Part 25 in response to A 96-54 to include SLD conditions in the icing certification envelope. The Safety Board believes that the proposed rulemaking is an essential step in improving the safety of flight in icing conditions for airplanes certified under Part 25. The Safety Board appreciates the opportunity to comment on this notice of proposed rulemaking.

From: NTSB
To: FAA
Date: 2/2/2006
Response: The National Transportation Safety Board has reviewed the Federal Aviation Administration (FAA) notice of proposed rulemaking (NPRM), Airplane Performance and Handling Characteristics in Icing Conditions, published in the Federal Register (Vol. 70, No. 213) on November 4, 2005. The Safety Board has evaluated the NPRM, which applies to newly certificated designs, in light of current, open recommendations related to aircraft icing and in regard to other concerns identified during the course of its investigations. Previous Icing Certification Recommendations The Safety Board has been concerned for many years about the inadequacy of the existing certification regulations for flight in icing conditions, which have not required manufacturers to demonstrate an airplane's flight handling, stall characteristics, minimum airspeeds, and stall margins under a sufficiently realistic range of adverse ice accretions. Additionally, the regulations have not required airplanes to be tested with the thin, rough ice that can accrete on protected surfaces prior to the activation of ice protection systems or between activation cycles of the ice protection systems, especially in larger water droplet environments. These issues came to light during the Board's investigations of two accidents involving loss of control in icing conditions: United Express flight 2415, a British Aerospace BA-3101, in Pasco, Washington (December 26, 1989), and COMAIR flight 3272, an Embraer EMB-120RT, in Monroe, Michigan (January 9, 1997). United Express flight 2415 involved an ice-contaminated tailplane stall and loss of control at low altitude. Contributing to the loss of control was the accumulation of ice on the airframe, which degraded the aerodynamic performance of the airplane. The United Express 2415 investigation resulted in Safety Recommendation A-91-87: Amend the icing certification rules to require flight tests wherein ice is accumulated in those cruise and approach flap configurations in which extensive exposure to icing conditions can be expected, and require subsequent changes in configuration, to include landing flaps. [Status: "Open-Acceptable Response"] Several years later, the Safety Board investigated the Comair 3272 accident, which involved a loss of control while the airplane was maneuvering with ice accretions on the wings. The Safety Board determined that the probable cause of the accident was the following: The FAA's failure to establish adequate aircraft certification standards for flight in icing conditions ... and the FAA's failure to require the establishment of adequate minimum airspeeds for icing conditions, which led to the loss of control when the airplane accumulated a thin, rough accretion of ice on its lifting surfaces. As a result of its investigation, the Safety Board issued Safety Recommendation A-98-94 to the FAA: Require manufacturers of all turbine-engine driven .airplanes (including the EMB-120) to provide minimum maneuvering .airspeed information for all airplane configurations, phases, and conditions of flight (icing and nonicing conditions); minimum airspeeds also should take into consideration the effects of various types, amounts, and locations of ice accumulations, including thin amounts of very rough ice, ice accumulated in supercooled large droplet icing conditions, and tailplane icing. [Current status: "Open-Unacceptable Response"] During the Comair 3272 accident investigation, the Safety Board also noted that the ice accretions and icing conditions considered during certification for flight in icing conditions were not representative of the most critical ice accretions that would be encountered while operating in icing conditions. The current icing certification rules required the certification applicant to demonstrate safe flight using a limited number of icing conditions within the Appendix C icing envelope. The effects of delayed ice protection system activation, intercycle ice accretions, or residual ice accretions were not addressed in the icing certification rules. Icing conditions near the edge of the Appendix C envelope, which are less likely to be encountered, can produce thin, rough ice accretions that research5 has shown can be as aerodynamically detrimental as the much larger ice shapes that form during flight in the more likely icing conditions. Such thin, rough ice accretions produced at the edge of the envelope icing conditions are dangerous in that they may not be noticed by the flightcrew, or they may not be perceived as a threat based on the crew's experience accreting ice while flying in more typical, center-of-the-envelope icing conditions. As a result of these findings, the Safety Board issued Safety Recommendation A-98-92 to the FAA: With the National Aeronautics and Space Administration and other interested aviation organizations, conduct additional research to identify realistic ice accumulations, to include intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of the deicing boots, and to determine the effects and criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft certification requirements and pilot training programs at all levels. [Status: "Open-Acceptable Response''] This recommendation is currently on the Safety Board's Most Wanted list of safety improvements.

From: NTSB
To: FAA
Date: 1/3/2006
Response: Notation 7744: The National Transportation Safety Board has reviewed the Federal Aviation Administration (FAA) proposed Advisory Circular (AC) 25.21-1X, Performance and Handling Characteristics in the Icing Conditions Specified in Part 25, Appendix C, published in the Federal Register (Vol. 70, No. 213) on November 4, 2005. The Safety Board has evaluated the proposed AC in light of current, open recommendations related to aircraft icing and in regard to other concerns identified during the course of its accident and incident investigations. The intent of the proposed AC is to provide acceptable methods of compliance with proposed certification requirements for performance and handling characteristics of transport-category airplanes affected by flight in the icing conditions defined in Appendix C of Title 14, Code of Federal Regulations (CFR) Part 25. The AC states that the means of compliance in this document are intended to provide guidance to supplement the engineering and operational judgment that must form the basis of any compliance findings relative to handling characteristics and performance in Appendix C icing conditions. The AC also states that it provides one means, but not the only means, of complying with the airworthiness standards revisions proposed in Notice No. 05-10, titled “Airplane Performance and Handling Qualities in Icing Conditions,” published in the same edition of the Federal Register. The Safety Board is closely evaluating this notice of proposed rulemaking (NPRM), which contains the changes to the CFR requirements that must be adhered to by applicants. The Safety Board will comment on Notice No. 05-10 separately. The materials contained within the AC, commented upon in this letter, are a strictly advisory means of compliance for the applicants. The notice of the proposed AC states that the AC is based on recommendations submitted to the FAA by the Aviation Rulemaking Advisory Committee (ARAC), which was tasked by the FAA to provide advice and recommendations on “harmonizing” certain sections of Part 25 (including section 25.21) with the counterpart standards contained in Joint Aviation Requirements (JAR). The goal of the “harmonization tasks” is to ensure the following: 1. Where possible, standards and guidance do not require domestic and foreign parties to manufacture or operate to different standards for each country involved; and 2. The standards and guidance adopted are mutually acceptable to the FAA and the foreign aviation authorities; 3. The guidance contained in the proposed AC has been harmonized with that of the Joint Aviation Authority (JAA), and provides a method of compliance that has been found acceptable to both the FAA and JAA. The Safety Board has issued several recommendations to the FAA regarding performance and handling in icing conditions that pertain to the proposed AC. These include Safety Recommendation A-91-87, which recommended the following to the FAA: Amend the icing certification rules to require flight tests wherein ice is accumulated in those cruise and approach flap configurations in which extensive exposure to icing conditions can be expected, and require subsequent changes in configuration, to include landing flaps. The flight testing described in the proposed AC describes flight testing with simulated ice accretions as the primary means for showing compliance with the regulations, and states that the objective of the natural atmospheric icing testing is to corroborate the handling characteristics and performance results obtained in flight tests with simulated ice accretions. The natural atmospheric icing condition testing described in paragraph 3u(2)(b) of the proposed AC would include accreting ½ inch ice in flight while at the trimmed holding speeds with the airplane configured with flaps and gear retracted. The airplane configuration would then be changed by subsequent flap deployment positions and further accretions of ice while at each new flap setting. After the ice accreted an additional ¼ inch, several maneuvers, including bank-to-bank rolls and decelerations to the speed reached 3 seconds after the activation of a stall warning, would be performed. This testing would conclude with ice accreting while the airplane was configured with the landing flap setting in the landing gear down position. The handling maneuvers would then start with the airplane flying at the VREF trim speed. The testing specified in the AC guidance is of the type that the Safety Board recommended as mandatory in Safety Recommendation A-91-87. The proposed testing guidelines for approach and landing with successive ice accretions in different airplane configurations address in part Safety Recommendation A-91-87. However, the Safety Board notes that the proposed AC does not address scenarios when ice is accreted with flaps deployed, and with flaps subsequently retracted. The Safety Board is concerned with this type of ice accretion scenario, which played a role in the 1994 ATR-72 accident in Roselawn, Indiana. Also in reference to Safety Recommendation A-91-87, the Safety Board notes that the zero-g pushover flight test maneuver, described in paragraph 3i(3)(a) of the proposed AC, is a reasonable means for evaluating an airplane’s susceptibility to ice-contaminated airplane stall. Additional flight testing with simulated ice shapes is described to evaluate the potential for ice-contaminated tailplane stall while operating in a sideslip. The Safety Board believes that the types of flight tests mentioned above would adequately address the intent of regulations recommended in A-91-87. The Safety Board also notes that paragraph 3i(2)(g) describes an additional acceptable flight test program using simulated ice accretion shapes that would include the evaluation of an approach and landing, wherein satisfactory controllability should be demonstrated during a landing conducted at an airspeed of VREF minus 5 knots, using a heavy airplane weight and forward center of gravity. The Safety Board believes this is a useful way to evaluate airplane performance during maneuvers that may be performed at less than the minimum reference speed. During the Comair 3272 accident, which occurred in Monroe, Michigan, the Safety Board issued recommendation A-98-92, which recommended the following to the FAA: With the National Aeronautics and Space Administration and other interested aviation organizations, conduct additional research to identify realistic ice accumulations, to include intercycle and residual ice accumulations and ice accumulations on unprotected surfaces aft of the deicing boots, and to determine the effects and criticality of such ice accumulations; further, the information developed through such research should be incorporated into aircraft certification requirements and pilot training programs at all levels. This recommendation is currently on the Safety Board’s Most Wanted list of safety improvements. The results of research sponsored by the FAA and NASA are currently included in Appendix R of draft AC 20-73. Appendix R is also referenced in Appendix 1, Airframe Ice Accretion, and Appendix 2, Simulated Ice Shapes, of proposed AC 25.21-1X. Appendix R includes guidance on determining critical ice shapes and their associated roughness, and descriptions of ice accreted in the time prior to activation of an ice protection system, intercycle ice accretions, and the aerodynamic penalties associated with these ice shapes. The Safety Board notes that by issuing the revision to AC 20-73, and including information like that found in Appendix R, the FAA is providing useful information for better evaluating an airplane’s performance and handling capabilities in icing conditions. The Safety Board is aware that research by other agencies, universities, and entities is ongoing in the area of in-flight ice accretions and their effects on aircraft, and encourages the FAA to continue to incorporate into the advisory materials additional information and results from ongoing research as they become available. The intent of A-98-92 would be partially met with the inclusion of Appendix R in AC 20-73, which has not yet been issued. The importance of Appendix R is further enhanced by its repeated reference in draft AC 25.21-1X. The Safety Board will be evaluating the associated NPRM to determine how this information will be integrated with the proposed certification requirements. In addition to the testing described above, the Safety Board is concerned that the proposed AC does not ensure that the airplane is tested in the most severe icing conditions that are defined in the icing design envelope (Appendix C). As a result of its investigation of the Roselawn accident, the Safety Board issued Safety Recommendation A-96-56 to the FAA, which recommended the following: Revise the icing certification testing regulation to ensure that airplanes are properly tested for all conditions in which they are authorized to operate, or are otherwise shown to be capable of safe flight into such conditions. If the manufacturer cannot demonstrate safe operations, operational limitations should be imposed to prohibit flight in such conditions and flightcrews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification. This recommendation is also on the Safety Board’s Most Wanted list of safety improvements. Whereas the proposed AC provides guidance on several flight-testing procedures that can be used to evaluate the performance and handling qualities of an airplane in icing conditions, the intent of Safety Recommendation A-96-56 is to ensure that the testing occurs in the range of conditions in which the airplane is authorized to operate—in particular, in the most hazardous icing conditions—and that the testing ensures evaluation at the limits of the Part 25 Appendix C envelope. The Safety Board notes that Appendix 1 of the proposed AC states that the most critical ice accretion in terms of handling characteristics and/or performance should be determined. The parameters to be considered should include the flight conditions (for example, airplane configuration, speed, angle of attack, altitude) and the icing conditions of Appendix C (for example, temperature, liquid water content, mean effective drop diameter). Similarly, Appendix 2 of the proposed AC provides guidance for using simulated ice accretions and states that the simulated ice accretions used for flight testing should be those that have the most adverse effects on handling characteristics. The simulated accretions should represent natural icing conditions in terms of location, general shape, thickness, and texture. The Safety Board agrees that the most critical ice shape must be determined for each phase of flight and its corresponding flight conditions, and that the resulting critical ice shape may be different for each phase of flight. Past Safety Board investigations of icing accidents and incidents have shown that some aircraft may not have been exposed to the full range of Appendix C conditions for all phases of flight, and hence, have not been subject to the most critical ice shapes during icing certification. Appendix 1 of the proposed AC specifies that icing conditions of Appendix C be considered in developing the critical ice accretions used in certification testing, and states the applicant should substantiate the conditions that result in the formation of the critical ice accretion. The Safety Board believes that the proposed AC should also provide means for applicants to justify their selection of the most critical ice shape for each phase of flight to ensure that the most critical shape is the one being used for the respective phase of flight. For each phase of flight, the applicant should ensure the shape, chordwise and spanwise, and the roughness of the shapes, considered in selection of a critical ice shape, accurately reflect the full range of Appendix C conditions that have been examined in terms of mean effective drop diameter, liquid water content, and temperature during the respective phase of flight. Additionally, the FAA should review the justification and selection of the most critical ice shape for each phase of flight. The proposed AC specifies that icing conditions of Appendix C be considered in developing the critical ice accretions used in certification testing. However, means are not provided to ensure that the full range of Appendix C conditions are examined by the applicant in determining the most critical ice shape for each phase of flight. The Safety Board therefore notes that the intent of A-96-56 will not be met, and is awaiting the proposed changes to 14 CFR Part 25 in response to Safety Recommendation A-96-56, as well as proposed changes to 14 CFR Part 25 in response to recommendation A-96-54 to include regulatory requirements that an airplane can safely operate in super-cooled large droplets (SLD) or detect and safely exit the SLD conditions. Finally, the Safety Board wishes to stress that SLD conditions can be more hazardous than those considered during current icing certification, and accident experience has shown that SLD conditions can cause ice accretions more aerodynamically detrimental than those developed while flying within the Part 25 Appendix C envelope. Additionally, the accident and incident history has shown that many of the smaller, turboprop airplanes spend much more of their operational time in regions of the atmosphere that are more likely to contain SLD conditions than the larger, turbojet type airplanes. As a result of the Roselawn investigation, the Safety Board issued recommendation A-96-54, which requested the following of the FAA: Revise the icing criteria published in 14 Code of Federal Regulations (CFR), Parts 23 and 25, in light of both recent research into aircraft ice accretion under varying conditions of liquid water content, drop size distribution, and temperature, and recent developments in both the design and use of aircraft. Also, expand the Appendix C icing certification envelope to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary. This recommendation is also currently on the Safety Board’s Most Wanted list of safety improvements. The proposed AC does not address this recommendation, and the Safety Board continues to await the FAA’s proposed regulatory changes to 14 CFR Part 25 in response to A 96-54. The Safety Board appreciates the opportunity to comment on this proposed AC.

From: FAA
To: NTSB
Date: 10/26/2005
Response: Letter Mail Controlled 10/27/2005 2:12:40 PM MC# 2050501 Marion C. Blakey, Administrator, FAA, 10/26/05 With the National Aeronautics and Space Administration (NASA), the Goodrich Deicing and Specialty Systems Division, The University of Illinois at Urbana-Champaign (UIUC), and Empresa Brasileira de Aeronautica - Embraer S.A., the FAA performed wind tunnel tests in the Goodrich Icing Wind Tunnel to identify realistic preactivation and deicing boot inter cycle and residual ice accumulations. These tests were performed during December 1999 and March 2000. Tests to determine the aerodynamic effects and criticality of deicing boot intercycle and residual ice accumulation were performed in the NASA Low Turbulence Pressure Tunnel (LTPT) by NASA, UIUC, and the FAA. These tests investigated the aerodynamic effects and criticality of deicing boot intercycle and residual ice accumulations determined during December 1999 Icing Wind Tunnel tests of a 36 inches chord NACA 23012 model. NASA and the FAA also performed tests in the NASA Low Turbulence Pressure Tunnel to investigate the aerodynamics and criticality of ice accumulations on unprotected surfaces of modem airfoils. The ice accumulations were obtained during tests in the NASA Icing Research Tunnel. In addition, UIUC and the FAA performed extensive wind tunnel and computerized fluid dynamics investigations relative to the location, size, and shape of ice accumulations that may accumulate on unprotected surfaces aft of deicing boots. The FAA has also collaborated with Mr. Frank T. Lynch and Mr. Abdollah Khodadoust in the article entitled "Effects of ice accretions on aircraft aerodynamics," Progress in Aerospace Sciences 37 (2001), pages 669-767. These individuals are members of the American Institute for Aeronautics and Astronautics aerodynamic technical committees and are well recognized throughout industry for their work at the Douglas and Boeing Companies. The article was a Progress in Aerospace Sciences invited paper and is considered the most comprehensive and scholarly review available on published information concerning the aerodynamic effects of ice buildups. A copy of the article is enclosed for your information. The FAA has completed a draft revision to AC 20-73, Aircraft Ice Protection that includes certification guidance relative to the effects and criticality of deicing boot intercycle and residual ice accumulations and ice accumulation on unprotected surfaces aft of protected surfaces. The research discussed above is included in the revision to AC 20-73, Appendix R, "Ice Shapes." A copy of the revised information is enclosed for the Board's information. It is anticipated that the revision to AC 20-73 will be published by December 2005. The FAA does not plan to revise the aircraft certification requirements since there has never been a question that the critical ice shapes should be considered during certification. The FAA believes that guidance on determining the critical ice shapes is needed. The 43 pages of material contained in draft AC 20-73 for determining certification ice shapes provide the needed information. The material in the draft AC is appropriate for use by certification engineers and airframe manufacturers. Safe operation of the airplane in icing conditions is not dependent upon the pilot having similar information. In lieu of providing the AC material to pilots, the FAA has worked with NASA to produce icing training materials for pilots. A CD-ROM self-guided training aid entitled "A Pilot's Guide to In-Flight Icing," and a pilot training video entitled "Super Cooled Large Droplet Icing," have been widely distributed to the aviation community.

From: FAA
To: NTSB
Date: 2/1/2005
Response: In its 2/1/2005 annual report to Congress, Regulatory Status of the National Transportation Safety Board's "Most Wanted" Recommendations to the Department of Transportation, the DOT wrote: A working group co-chaired by the FAA and the National Aeronautics and Space Administration's Glenn Research Center produced a report in September 2000, titled "Report of the 12A Working Group on Determination of Critical Ice Shapes for the Certification of Aircraft." The report concluded that sufficient information and methods were not available to provide additional guidance material concerning the determination of critical ice shapes in certification. As recommended in the report, the FAA sponsored additional research that has been completed. Pertinent ice shape information from the research will be incorporated in advisory material for use during certification. The draft revision to the advisory circular is expected to be published in March 2005.

From: NTSB
To: FAA
Date: 7/11/2002
Response: The FAA reports that a working group co-chaired by the FAA and the NASA's Glenn Research Center produced a report in September 2000, titled "Report of the 12A Working Group on Determination of Critical Ice Shapes for the Certification of Aircraft." The report concluded that sufficient information and methods were not available to provide additional guidance material concerning the determination of critical ice shapes in certification. As recommended in the report, the FAA is sponsoring additional research and has indicated that it plans to incorporate data from the additional research into advisory material. Pending completion of the research and incorporation of the results into aircraft certification requirements and pilot training programs, Safety Recommendation A-98-92 remains classified "Open--Acceptable Response."

From: FAA
To: NTSB
Date: 9/21/2001
Response: Letter Mail Controlled 10/22/2001 11:44:27 AM MC# 2010866 A working group cochaired by the FAA and the National Aeronautics and Space Administration's Glenn Research Center produced a report in September 2000, entitled "Report of the 12A Working Group on Determination of Critical Ice Shapes for the Certification of Aircraft" (Report No. DOT/FAA/AR-0037). The report is available at the FAA's William J. Hughes Technical Center's Full-Text Technical Reports web page (actlibrary.tc.faa.gov). The report contains the results of the survey of publicly available data on aerodynamic effects of ice accretions. Glaze ice was evaluated in detail in the report. Information concerning other types of ice accretions may be found in an extensive list of references compiled as part of the report. The report concluded that sufficient information and methods were not yet available to provide additional guidance material concerning the determination of critical ice shapes in certification. As recommended in the report, the FAA is sponsoring additional research. The FAA plans to incorporate data from the additional research into advisory material. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: NTSB
To: FAA
Date: 3/12/2001
Response: The Safety Board is pleased that the FAA is taking the action recommended and would appreciate receiving a copy of the report that was scheduled for release during the fall of 2000. Pending receipt of a copy of that report, the conduct of research that was identified in the report as necessary for the certification process, and the incorporation of this information into aircraft certification requirements and pilot training programs at all levels, Safety Recommendation A-98-92 remains classified Open--Acceptable Response.

From: FAA
To: NTSB
Date: 9/25/2000
Response: Letter Mail Controlled 10/02/2000 3:16:36 PM MC# 2001437 A working group cochaired by the FAA and NASA's Glenn Research Center produced a draft report that contained the results of the survey of publicly available data on aerodynamic effects of ice accretions. Glaze ice was evaluated in detail in the draft report. Information concerning other types of ice accretions may be found in an extensive list of references compiled as part of the draft report. The draft report identifies the need for additional research before the development of guidance material related to the identification and evaluation of critical ice accretions during the certification process. The final report will be released during the fall of 2000. The FAA will expedite the development of guidance material referenced in this recommendation action plan. It is anticipated that the draft guidance material will be available for public comment by March 2003. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: NTSB
To: FAA
Date: 3/9/2000
Response: THE SAFETY BOARD STRONGLY BELIEVES THAT THIS RECOMMENDATION SHOULD BE COMPLETED IN FEWER THAN THE FAA'S PROPOSED 5 YEARS AND URGES THE FAA TO EXPEDITE THIS EFFORT AS MUCH AS POSSIBLE. HOWEVER, CONSIDERING THE POTENTIAL FOR POSITIVE RESULTS, A-98-92 IS CLASSIFIED "OPEN--ACCEPTABLE RESPONSE." THE SAFETY BOARD WOULD APPRECIATE A COPY OF THE IN-FLIGHT ICING WORKING GROUP REPORT.

From: FAA
To: NTSB
Date: 2/26/1999
Response: ONE ELEMENT OF THE FAA IN-FLIGHT AIRCRAFT ICING PLAN PUBLISHED IN APRIL 1997 CALLS FOR THE DEVELOPMENT OF GUIDANCE MATERIAL RELATED TO THE IDENTIFICATION AND EVALUATION OF CRITICAL ICE ACCRETIONS DURING THE CERTIFICATION PROCESS. A WORKING GROUP TO ADDRESS THIS TASK WAS FORMED IN NOVEMBER 1997. THE GROUP IS CO-CHAIRED BY THE FAA AND NATIONAL AERONAUTICS AND SPACE ADMINISTRATION'S (NASA) LEWIS RESEARCH CENTER AND HAS REPRESENTATION FROM INDUSTRY AND ACADEMIA. THE GUIDANCE WILL PROVIDE A BASIS FOR DETERMINING THE MOST ADVERSE SHAPES FOR RELEVANT AERODYNAMIC CHARACTERISTICS AND FOR EVALUATING WHETHER OR NOT THE CERTIFICATION ICE SHAPES ARE APPROPRIATELY CHOSEN SO THAT THE AERODYNAMIC IMPACT WILL BE THE MOST SEVERE. A REPORT FROM THE GROUP IS EXPECTED DURING THE THIRD QUARTER OF 1999, WHICH WILL INCLUDE A SURVEY OF INFORMATION IN THE PUBLIC DOMAIN ON GAZE ICE SHAPES. IT IS ANTICIPATED THAT THE REPORT WILL IDENTIFY THE NEED FOR ADDITIONAL RESEARCH PRIOR TO THE DEVELOPMENT OF THE GUIDANCE MATERIAL. THE RESEARCH AND DEVELOPMENT OF THE GUIDANCE MATERIAL WILL TAKE APPROXIMATELY 5 YEARS TO COMPLETE. THE GUIDANCE IS INTENDED FOR AIRCRAFT MANUFACTURERS AND FAA CERTIFICATION ENGINEERS DURING THE CERTIFICATION PROCESS. THE FAA WILL ALSO EVALUATE THE GUIDANCE FOR ITS RELEVANCY TO PILOTS AND WILL DETERMINE THE BEST MEANS FOR RELAYING THE RELEVANT INFORMATION TO PILOTS.

From: NTSB
To: FAA
Date:
Response: At the 1997 Board meeting addressing the NTSB’s Most Wanted List of Transportation Safety Improvements (MWL), the Board voted to place Safety Recommendations A-96-54 through A-96-56, A-96-62, and A-96-69 on the MWL under the issue category “Airframe Structural Icing.” The Board voted to add Safety Recommendations A-98-92, A-98-94, A-98-95, A-98-99, A-98-100, A-07-14 at later dates. Safety Recommendation A-07-16 was added to the MWL upon its adaptation because it supersedes Safety Recommendation A-98-100.