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 Based on its findings, the NTSB issued Safety Recommendation A-96-54, which superseded recommendation A-81-116 and requested the following of the FAA: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.