The 1994 in-flight icing encounter and subsequent loss of control and crash of a commuter airliner in Roselawn, Indiana, which took 68 lives, prompted the NTSB to examine the issue of airframe structural icing and conclude that the icing certification process has been inadequate because the process has not required manufacturers to demonstrate the airplane's flight handling and stall characteristics under a realistic range of adverse ice accretion/flight-handling conditions. The Federal Aviation Administration (FAA) has not adopted a systematic and proactive approach to address certification and operational issues of turbine-engine, transport-category airplane icing.
As a result of the NTSB's investigation of the in-flight encounter with icing and subsequent uncontrolled collision with terrain of Comair flight 3272, an Embraer 120RT, near Monroe, Michigan, on January 9, 1997, in which all 29 persons onboard the airplane were killed, the NTSB asked the FAA to review the icing certification of all turbopropeller-driven airplanes currently certificated for operation in icing conditions and to perform additional testing.
The consequences of operating an airplane in icing conditions without first having thoroughly demonstrated adequate handling/controllability characteristics in those conditions are sufficiently severe that they warrant a thorough certification test program, including the application of revised standards to airplanes currently certificated for flight in icing conditions.
A widespread and firmly held, but incorrect, belief in the aviation community is that the activation of deice boots should be delayed until the buildup of 1/4 to 1/2 inch of ice to prevent "ice bridging." As a result, in many cases, flight crews have not activated the deice boots until after the buildup of dangerous levels of icing on the aircraft.
Revise Icing Certification Criteria and Testing
As a result of the Roselawn, Indiana, accident, the NTSB called on the FAA to revise (1) the icing criteria and icing testing requirements necessary for an airplane design to be approved within the United States and (2) the operational requirements that specify under what icing conditions it is permissible to operate an aircraft. More than 10 years ago, this work was referred to an aviation rulemaking advisory committee (ARAC), which has recommended to the FAA changes to the design requirements for new airplanes to evaluate performance and handling characteristics in icing conditions. In March 2002, 6 years after it started this work, the ARAC approved a concept to revise the icing criteria in the design requirements for new airplanes. In March 2006, the ARAC completed its final report on recommended rulemaking and advisory material related to supercooled large drop (SLD) conditions and ice crystal/mixed-phase conditions. Based on the recommendations and report of the ARAC, the FAA is preparing a notice of proposed rulemaking (NPRM) that will propose (1) revisions to Part 25 certification requirements for newly type certificated transport aircraft that will define the SLD environment and (2) the performance requirements for aircraft that encounter such conditions. The FAA anticipates publishing the NPRM in early 2010. In March 2009, the FAA began to form an aviation rulemaking committee (ARC) to make recommendations for similar revisions to Part 23 requirements, but that ARC has not yet met.
Currently, there are four rulemaking activities concerning icing:
On August 3, 2009, the FAA published a final rule, applying to new transport aircraft designs, that became effective on September2, 2009. Although the new rule does not address existing airplane designs, the FAA is considering similar rulemaking that would apply to those designs. Under the revised certification standards, new transport aircraft designs must incorporate one of three methods to detect icing and to activate the airframe ice protection system:
The standards further require that, after initial activation, the ice protection system must operate continuously, automatically turn on and off, or alert the pilots when the system should be cycled.
The pace of the FAA's activities in response to all of these recommendations remains unacceptably slow, despite indications of several NPRMs recently issued or scheduled for issuance shortly.
Apply Revised Icing Requirements to Currently Certificated Aircraft
The icing certification regulations and advisory material developed by the FAA are sufficiently developed to determine whether additional action is required for any airplanes currently certificated and in service. The FAA has issued over 65 airworthiness directives (AD) concerning specific icing issues for specific aircraft, but has stated that no unsafe conditions exist that warrant actions beyond those that have already been completed or are in the process of being completed. The NTSB is concerned that the FAA has reached this conclusion based on a lack of accidents or serious incidents. During the 1990s, a number of accidents occurred involving airplanes that had passed the certification standards and for which the FAA believed there was no unsafe condition requiring action. Before another accident or serious incident occurs, the FAA should evaluate all existing pneumatic deice boot-equipped airplanes in service using the new information available, such as critical ice shapes and stall warning margins in icing conditions.
Activate Deice Boots as soon as the Airplane Enters Icing Conditions
In Advisory Circular 25.1419-1A, "Certification of Transport Category Airplanes for Flight in Icing Conditions," the FAA states that, although ice may not be completely shed by one cycle of the boots, the residual ice will usually be removed by subsequent cycles and does not act as a foundation for a bridge of ice to form. Further, information gathered at a 1997 Airplane Deice Boot Bridging Workshop, icing tunnel tests, and flight tests revealed that ice bridging did not occur on modern airplanes equipped with deice boots that quickly inflate and deflate. The icing tunnel tests also revealed that thin (1/4 inch or less), rough ice accumulations on the wing leading edge deice boot surfaces could be as aerodynamically detrimental to an airplane's performance as larger ice accumulations.
The NTSB has investigated many icing accidents in which the airplane stalled prematurely and the stall warning system did not activate before the stall because of ice accumulation on the wing leading edges. Accident investigations, NTSB accident data, and existing icing information clearly show that delaying the activation of the deice boots can create an unsafe condition. The NTSB concludes that ice bridging does not occur on modern airplanes; therefore, it is not a reason for pilots to delay activation of the deice boots. The FAA's final rule that became effective on September 2, 2009, fully addressed this issue for new type certificated airplanes. The FAA needs to take similar action for existing airplanes that are in service and use pneumatic boot deicing systems.
Complete efforts to revise icing certification criteria, testing requirements, and restrictions on operations in icing conditions. Evaluate all aircraft certified for flight in icing conditions using the new criteria and standards. For in-service aircraft, require manufacturers and operators of deice boot-equipped airplanes to revise the guidance to emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions.
A-96-54 (FAA)
Issued August 15, 1996
Added to the Most Wanted List: 1997
Status: Open-Unacceptable Response
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. (Source: In-flight Icing Encounter and Loss of Control Simmons Airlines, d.b.a. American Eagle Flight 4184 Avions de Transport Regional (ATR) Model 72-212, N401AM, Roselawn, Indiana, October 31, 1994 [NTSB-AAR-96-01]).
A-96-56 (FAA)
Issued August 15, 1996
Added to the Most Wanted List: 1997
Status: Open-Unacceptable Response
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 flight crews should be provided with the means to positively determine when they are in icing conditions that exceed the limits for aircraft certification. (Source: In-flight Icing Encounter and Loss of Control Simmons Airlines, d.b.a. American Eagle Flight 4184 Avions de Transport Regional (ATR) Model 72-212, N401AM, Roselawn, Indiana October 31, 1994.[NTSB-AAR-96-01])
A-07-14 (FAA)
Issued February 27, 2007
Added to the Most Wanted List: 2008
Status: Open-Acceptable Response
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. (Source: Crash During Approach to Landing, Circuit City Stores, Inc., Cessna Citation 560, N500AT, Pueblo, Colorado, February 16, 2005.[NTSB-AAR-07-02])
A-07-16 (FAA)
Issued February 27, 2007 (Superseded A-98-100)
Added to the Most Wanted List: 2008
Status: Open-Unacceptable Response
When the revised icing certification standards 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. (Source: Crash During Approach to Landing, Circuit City Stores, Inc., Cessna Citation 560, N500AT, Pueblo, Colorado, February 16, 2005. [NTSB-AAR-07-02])
