Notation 7677B: The National Transportation Safety Board has reviewed the Federal Aviation (FAA) Notice of Proposed Rulemaking (NPRM), Airworthiness Directives; The Cessna Aircraft Company Models 208 and 208B Airplanes, which was published at 72 Federal Register 4663 on February 1, 2007. The NPRM proposes to supersede Airworthiness Directive (AD) 2006-06-06, which applies to Cessna Aircraft Models 208 and 208B.
On January 17, 2006, as a result of the preliminary findings from two foreign investigations of fatal accidents involving Cessna 208 series airplanes in icing conditions, and a previous assessment of Cessna 208 icing incidents and accidents, the Safety Board issued the following safety recommendations to the FAA:
Require all operators of Cessna 208 series airplanes to maintain a minimum operating airspeed of 120 knots during flight in icing conditions, even if a descent is required to do so. (A-06-01) (Urgent)
Prohibit all operators of Cessna 208 series airplanes from conducting flight into any icing conditions determined to be more than light icing. (A-06-02) (Urgent)
Require all operators of Cessna 208 series airplanes to disengage the autopilot and fly the airplane manually when operating in icing conditions. (A-06-03) (Urgent)
On March 16, 2006, the FAA adopted Airworthiness Directive (AD) 2006-06-06. The AD affects all Cessna 208 and 208B aircraft, and requires incorporation of changes to the Airplane Flight Manual (AFM) for these aircraft as well as placement of placards in the cockpit providing information about operations under icing conditions. The changes to the AFM (1) require a minimum airspeed of 120 knots indicated air speed (KIAS) when the flaps are up for all phases of flight in icing conditions, (2) prohibit operation of the aircraft in icing conditions that are moderate or greater, and (3) require disengagement of the autopilot if icing conditions are encountered. The AD defines “moderate icing conditions” for the Cessna 208 series of aircraft specifically as any one of the following conditions:
• Airspeed in level flight at constant power decreases by 20 KIAS.
• Engine torque required to maintain airspeed increases by 400 ft. lbs.
• 120 KIAS cannot be maintained in level flight.
• Ice accretion of ¼ inch observed on the wing strut.
On November 15, 2006, the Safety Board noted that the issuance of AD-2006-06-06 fully met the intent of Safety Recommendations A-06-1, -2, and –3, and classified these Recommendations “Closed – Acceptable Action.”
In commenting on the NPRM that would supersede AD-2006-06-06, the Safety Board seeks to ensure that the safety improvements obtained through that AD are not diminished. The Safety Board is satisfied that the intent of Safety Recommendations A-06-01 and A-06-02 are retained in the new NPRM, but is concerned that the new “Low-Speed Awareness System” proposed in the NPRM, which will supersede the requirement to disconnect the autopilot in icing conditions, may not provide a level of safety equivalent to disconnecting the autopilot in icing conditions, as recommended in A-06-03 and as required in AD-2006-06-06.
The Intent of A-06-01 and A-06-02 as Addressed in the NPRM
Although the NPRM no longer uses the term “moderate icing conditions” to identify icing conditions in which continued flight is prohibited, it does retain specific and quantitative performance-based measures for determining when to exit icing conditions. In addition, the changes to the AFM proposed in the NPRM would oblige pilots to immediately act to exit icing conditions if any of a number of qualitative criteria describing the icing environment are met. Specifically, the proposed changes to the AFM would require pilots to take immediate actions to exit icing conditions under the following conditions:
1. Unusually extensive ice accretions on the airframe in areas that do not normally collect ice.
2. Accumulation of ice on the upper or lower surface of the wing aft of the protected area.
3. Heavy ice accumulations on the windshield, or ice accumulations aft of the curved sections on the windshield.
4. Ice accumulations aft of the protected surfaces of the wing struts.
5. Visible rain or drizzle at temperatures below +5º C outside air temperature.
6. Droplets that splash or splatter on impact at temperatures below +5º C outside air temperature.
The NPRM proposes the following additional quantitative performance-based measures for determining when to exit icing conditions:
1. Airspeed of 120 KIAS cannot be maintained in level flight.
2. Airspeed decrease of 10 KIAS that cannot be prevented by increase to max continuous power.
3. Minimum en route IFR altitude (MEA) or minimum obstruction clearance altitude (MOCA) (if applicable) on current leg falls into area “C” of the icing service ceiling chart contained in Section 5 of the Known Icing AFM Supplement.
The Safety Board believes that these changes to the AFM retain the intent and equivalent level of safety envisioned in Safety Recommendations A-06-01 and A-06-02 and mandated in AD-2006-06-06, and so has no objection to their inclusion in the final rule.
The Intent of A-06-03 as Addressed in the NPRM
In the letter transmitting Safety Recommendation A-06-03 to the FAA, the Safety Board stated the following:
It is difficult for pilots to effectively monitor control inputs made by an autopilot and detect changes in the magnitude and direction of these inputs as an airplane’s performance degrades due to icing conditions … The Safety Board concludes that manually flying the Cessna 208 in icing conditions is necessary to enable pilots to sense the aerodynamic effects of icing and enhance their ability to retain control of the airplane.
The Safety Board has made similar observations in the course of other accident investigations, and previously recommended that crews of other aircraft types be required to disconnect the autopilot in icing conditions. For example, recommendation A-94-184, issued as a result of the investigation of an accident involving an ATR-72-210 aircraft, recommended that the FAA prohibit the use of the autopilot on ATR-42 and ATR-72 aircraft in icing conditions. Recommendation A-98-97, issued as a result of the investigation involving an Embraer EMB-120RT aircraft, recommended that the FAA require all operators of turbopropeller-driven air carrier airplanes to require pilots to disengage the autopilot and fly their airplanes manually when they activate the anti-ice systems. Recommendation A-98-101, also issued as a result of the EMB-120RT accident investigation, recommended the following to the FAA:
Review turbopropeller-driven airplane manufacturers’ airplane flight manuals and air carrier flightcrew operating manuals (where applicable) to ensure that these manuals provide operational procedures for flight in icing conditions, including the activation of leading edge deicing boots, the use of increased airspeeds, and disengagement of autopilot systems before entering icing conditions (that is, when other anti-icing systems have traditionally been activated).
Recommendation A-06-51, issued as a result of the investigation of an accident involving a Saab-Scania AB SF340B+ aircraft, recommended that the FAA require all operators of turbopropeller-driven airplanes to instruct pilots, except during intermittent periods of high workload, to disengage the autopilot and fly their airplanes manually when operating in icing conditions.
As these recommendations indicate, the Safety Board has consistently determined in its investigations of icing-related accidents that safety is enhanced when pilots are part of the control-loop while operating in icing conditions. The changes in aircraft handling qualities that can accompany ice accretion are indicators that airplane performance is being degraded and that corrective action is required. However, pilots are more likely to be aware of these indicators if they are in direct control of the aircraft.
AD-2006-06-06 currently requires pilots of Cessna 208 series airplanes to disconnect the autopilot “at first indication of ice accretion.” This requirement fully met the intent of Safety Recommendation A-06-03, and so, in November 2006, the Safety Board classified A-06-03 “Closed – Acceptable Action.” However, AD-2006-06-06 also states the following:
Although we have not previously proposed limitations on autopilot use in icing, the limitation on autopilot use is an interim action until an acceptable low speed annunciation system is developed and installed on the airplane.
The Safety Board notes that although the NPRM retains the requirement to disconnect the autopilot at first indication of ice accretion for airplanes that do not have the “Low Air Speed Awareness System (SK208-171),” it also states that the autopilot need not be disconnected at first indication of ice accretion on airplanes that do have the system. Because the NPRM would require the eventual installation of the system on all Cessna 208 series aircraft, pilots would then not have to disconnect the autopilot. Since closing Safety Recommendation A 06-03, the Safety Board has re-evaluated its position regarding the Low Air Speed Awareness System and is concerned that it may not provide the same level of icing awareness as called for by A-06-03 and mandated in AD 2006-06-03 (that is, for pilots to disconnect the autopilot and fly their airplanes manually in icing conditions). Further, Recommendation A-06-51, issued as a result of the investigation of the Saab-Scania accident, reiterates the Safety Board’s concerns about not using the autopilot when in icing conditions.
Revision 9 to “Supplement S1 — Known Icing Equipment” to the AFM, contains the following guidance under Low Air Speed Awareness System:
An advisory annunciator is installed just above the annunciator panel … and is labeled BELOW ICING MIN SPD. This annunciator illuminates when the propeller anti-ice switch is in the ON position and the indicated airspeed is less than 108 ± 2 kts. This annunciator will illuminate with a white background color. After initially obtaining 108 ± 2 kts after takeoff, any subsequent airspeed decrease below this value will cause the annunciator to illuminate with an amber background color and flash. An aural horn will also sound to alert the pilot to the need to take appropriate action to increase airspeed. For approaches with flaps at 10º or 20º the horn may be cancelled by pushing the switch light. The horn aural will cease and the light will illuminate white color and be steady ON.
The Safety Board notes that the low air speed alert provided by this system will not activate until the airplane is well below the minimum flaps-up airspeed for flight in icing conditions (120 KIAS), and approximately 2 knots below the minimum speed to be used when climbing to exit icing conditions. Operated in this fashion, the low speed alert in effect acts as a stall-warning system for icing conditions. Consequently, the NPRM would allow the autopilot to remain engaged while the airspeed degrades due to ice accretion to below the minimum safe airspeed. During such performance degradation, the pilot would not have direct control of the airplane. Accordingly, he or she would not be able to sense the aerodynamic effects of the ice accretion and would therefore be at increased risk of losing control of the airplane. This situation is exactly that addressed by Safety Recommendation A-06-03, and that which AD-2006-06-06 addressed satisfactorily by requiring pilots to disconnect the autopilot at first indication of ice accretion.
Revision 9 to “Supplement S1 -- Known Icing Equipment” to the AFM also contains the following guidance:
The autopilot may be used in icing conditions if not prohibited (see limitations section). However, every 10-15 minutes the autopilot should be disconnected to detect any out of trim conditions caused by ice buildup. If significant out of trim conditions are detected, the autopilot should remain off for the remainder of the icing encounter so that the pilot may monitor for additional force buildup.
This guidance only partially addresses the intent of Safety Recommendation A-06-03. The guidance is not mandatory (it is contained in the “Normal Procedures” section of the AFM Supplement, and not in the “Limitations” section), and permits “significant out of trim conditions” to appear as a result of ice accretion before the pilot needs to fly manually to “monitor for additional force buildup.” This procedure introduces a delay in the pilot’s recognition of the airplane’s handling degradation as a result of ice buildup and shortens the amount of time available to exit icing conditions before the ice accretion becomes critical. By flying the airplane manually at the first sign of ice accretion, as required by AD-2006-06-06, the pilot continuously monitors the airplane’s handling qualities and can detect degradation when it first appears. Furthermore, flying the airplane manually increases the likelihood that pilots will detect any degradation in airplane performance (such as a decay in airspeed or an increase in the power required to maintain airspeed) at the earliest opportunity.
For these reasons, the Safety Board believes that the Low Air Speed Awareness System does not address the intent of A-06-03 and does not provide the same level of safety as the autopilot disconnect requirement in AD 2006-06-06. Furthermore, the system as designed does not provide an alert until the airspeed has decayed to at least 10 knots below the minimum safe airspeed in icing conditions, at which point the airplane performance may already be degraded due to ice accumulation, such that a flaps-up climb at 110 KIAS to escape the icing conditions (as permitted by the AFM) may not be possible. Therefore, the Safety Board believes that installation of the low speed awareness system should not provide relief from the requirement to disconnect the autopilot at first indication of ice accretion.
NPRM Requirements Not Related to Safety Recommendations A-06-01 through A 06-03
The NPRM also requires the following in regard to pilot training:
The pilot-in-command must successfully complete specific training for flight into icing conditions provided by Cessna Aircraft Company within the 12 calendar months preceding any flight into known or forecast icing conditions.
Safety is enhanced when pilots are aware of the serious risks associated with aircraft icing of all types – both ground icing, and in-flight ice accretion – and when they have the knowledge and skills to manage those risks effectively. The training requirement contained in the NPRM is promising in this regard, and together with the airplane limitations intended by Safety Recommendations A-06-01 through A-06-03 and required in AD-2006-06-06, should improve the safety of cold-weather operations with Cessna 208 series airplanes. The Safety Board therefore agrees with the pilot training requirement proposed in the NPRM.
The Safety Board appreciates the opportunity to comment on this NPRM.