NTSB · Safety Recommendation

Safety Recommendation A-96-054

Recommendation

TO THE FEDERAL AVIATION ADMINISTRATION: Revise the icing criteria published in 14 Code of Federal Regulations (CFR), Part 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 development in both the design and use of aircraft. Also, expand the appendix C icing certification envelop to include freezing drizzle/freezing rain and mixed water/ice crystal conditions, as necessary. A-96-54 supersedes recommendations A-81-116 and 118.

Safety Issue and Context

On October 31, 1994, about 1600 central standard time a Simmons Airlines Avions de Transport Regional ATR-72-210, operating as American Eagle Flight 4184, crashed into a soybean field 3 miles south of Roselawn, Indiana. The flight was on an instrument flight rules flight plan from Indianapolis, Indiana, to O'Hare International Airport, Chicago, Illinois, and had been placed in a holding pattern over Roselawn because of weather delays being experienced at O'Hare. The airplane's primary and secondary radar returns disappeared from the air traffic control radar shortly after the flight was cleared to continue the holding pattern and to descend from 10,000 to 8,000 feet.

Witnesses observed the airplane descend out of a low overcast and strike the ground in a steep nose-down attitude. All 64 passengers and 4 crewmembers were killed in the accident. The Safety Board investigated one such event that occurred on December 22, 1988, at Mosinee, Wisconsin.

Response History

June 21, 2017 · NTSB Correspondence · FAA

On February 11, 2015, we said that your November 4, 2014, publication of the final rule adding Appendix O to Part 25 satisfied this recommendation for Part 25 airplanes, but not for Part 23 airplanes. On March 14, 2016, you published a notice of proposed rulemaking (NPRM) describing an extensive rewrite of Part 23, including a requirement for a Part 23 airplanes certified for flight in icing conditions with freezing drizzle, freezing rain, mixed phase, or ice crystal conditions to show either compliance with Appendix O of Part 25 or with an equivalent means of compliance.

In our May 12, 2016, comments about this NPRM, we said that adopting this proposed requirement would satisfy Safety Recommendation A-96-54. We note that, on December 30, 2016, you published the associated final rule, and that it contained the revisions related to icing certification proposed in the NPRM. As a result, Safety Recommendation A-96-54 is classified CLOSED—ACCEPTABLE ACTION.

April 26, 2017 · Recipient Response · FAA

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May 12, 2016 · NTSB Correspondence · FAA

CC# 201600220, dated May 12, 2016: The National Transportation Safety Board (NTSB) has reviewed the Federal Aviation Administration’s (FAA) notice of proposed rulemaking (NPRM) titled “Revision of Airworthiness Standards for Normal, Utility, Acrobatic, and Commuter Category Airplanes,” which was published at 81 Federal Register (FR) 13451 on March 14, 2016. The NPRM proposes to amend airworthiness standards for normal, utility, acrobatic, and commuter-category airplanes by removing prescriptive design requirements and replacing them with performance based airworthiness standards.

The proposed standards would also replace the current weight and propulsion divisions in small airplane regulations with performance- and risk based divisions for airplanes with a maximum seating capacity of 19 passengers or less and a maximum takeoff weight of 19,000 lbs. or less. In addition, the NPRM proposes to adopt additional airworthiness standards to address certification for flight in icing conditions, enhanced stall characteristics, and minimum control speed to prevent departure from controlled flight for multiengine airplanes.

On April 8, 2016, the FAA also published on its website draft Advisory Circular (AC) 23.10 titled “FAA Accepted Means of Compliance Process for 14 [Code of Federal Regulations] CFR Part 23.” The draft AC provides guidance on how to submit a proposed means of compliance (MOC) with Part 23 for acceptance by the Administrator in accordance with proposed section 23.10, “Accepted Means of Compliance,” in the NPRM. This letter provides comments on the NPRM. We will submit separate comments regarding draft AC 23.10.

Icing Certification The NPRM proposes sections 23.230 and 23.1405 to improve standards for Part 23 airplanes certified for flight in icing conditions. Section 23.1405 would require that aircraft certified for flight in icing conditions demonstrate that the ice protection system would provide for safe operation and would also require these airplanes to be protected from stalling when the autopilot is operating in a vertical mode. Section 23.1405 would require that this demonstration be conducted in atmospheric icing conditions specified in Part 1 of Appendix C to Part 25.

As in the current rule, ice protection systems would have to be shown to be adequate in the icing conditions of Appendix C to Part 25. Freezing drizzle and freezing rain icing conditions would be optional icing conditions in which the airplane may be certificated to operate. The additional icing conditions for freezing drizzle and freezing rain were added in Appendix O to Part 25 in Amendment 25–140 and are not defined in proposed section 23.230. Instead, the NPRM states that the definition of these optional icing conditions in Appendix O to Part 25 will be a MOC.

The NPRM notes that it addressed Safety Recommendation A-96-54 but that it would still be possible to be certified for flight in icing conditions by only complying with Appendix C, not Appendix O. We issued Safety Recommendation A-96-54 on August 15, 1996: A-96-54 Revise the icing criteria published in 14 [CFR], Part 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. [Classified “Open—Acceptable Response”] We have previously indicated that Appendix O satisfied this recommendation for Part 25. The FAA references Part 1 of Appendix C as the basis for certifying for flight in icing conditions and indicates that Appendix O (which has the conditions in Safety Recommendation A-96-54) can be used as the MOC for airplanes certified for flight in freezing drizzle and freezing rain.

Adoption in the final rule of proposed sections 23.230 and 23.1405 will likely result in Safety Recommendation A-96-54 being classified CLOSED—ACCEPTABLE ACTION.

February 11, 2013 · NTSB Correspondence · FAA

After many years of delays and NTSB concern for the FAA’s slow pace in responding to this recommendation, on June 29, 2010, the FAA published a notice of proposed rulemaking (NPRM), titled “Airplane and Engine Certification Requirements in Supercooled Large Drop, Mixed Phase, and Ice Crystal Icing Conditions,” which addressed icing conditions for transport category airplanes and turbine engines. In our comments on this NPRM, we supported the Part 25 regulatory revisions proposed and stated our belief that the new rule would constitute an essential step in improving the safety of flight for airplanes operating in icing conditions.

We disagreed, however, with the NPRM’s exclusions of airplanes with a maximum takeoff weight (MTOW) of more than 60,000 pounds and of airplanes without reversible flight controls. We also emphasized that the proposed requirements should apply to both Part 25 and Part 23 operations. The FAA had anticipated issuing the final rule by February 2012 but, in its current letter, has revised this to the second quarter of 2013. The FAA received many comments on this NPRM and, as a result, is making additional changes to the rule.

To address Part 23 regulations, on February 19, 2010, the FAA chartered an Aviation Rulemaking Committee (ARC) to address revisions to icing regulations for Part 23 airplanes, specifying that the ARC should review the Part 25 revisions proposed in the NPRM and recommend necessary modifications for Part 23. The ARC transmitted a report with its recommendations to the FAA on February 19, 2011, and provided supplemental recommendations on April 27, 2011, and February 10, 2012. The FAA is currently reviewing the ARC’s recommendations and developing a plan to revise the regulations, but is unable at this time to provide a schedule for when these revisions will be proposed or completed.

We remain encouraged by the FAA’s activities in the last 3 years in response to this recommendation. We acknowledge the volume of comments and complexity of revisions that these comments may require for the final rule, but we urge the FAA to complete this work as soon as possible. We also acknowledge the work performed by the Part 23 ARC and the FAA’s current activities developing needed regulatory reforms, but we are concerned that, 16 years after this recommendation was issued, the FAA still is not able to provide even a schedule of when the proposed revisions to Part 23 will be published. Despite our concerns, we are encouraged by the recent progress.

Accordingly, pending the timely issuance of a final rule for Part 25 airplanes, and of an NPRM and final rule for Part 23 airplanes based on the Part 25 NPRM, Safety Recommendation A-96-54 remains classified OPEN—ACCEPTABLE RESPONSE.

August 27, 2010 · NTSB Correspondence · FAA

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 F

October 1, 2008 · NTSB Correspondence · FAA

Notation 8047: The National Transportation Safety Board has reviewed the Federal Aviation Administration (FAA) Notice of Proposed Rulemaking (NPRM), “Airworthiness Directives, Cessna Model 560 Airplanes,” which was published at 73 Federal Register 49359 on August 21, 2008. The notice proposes to supersede an existing airworthiness directive (AD), AD-2007-23-13, which applies to certain Cessna Model 560 airplanes to prevent an inadvertent stall due to the inadequate stall warning margin provided by an improperly adjusted stall warning system, which could result in loss of controllability of the airplane.

Existing AD-2007-23-13 requires installing new minimum airspeed placards to heighten flight crew awareness as to the proper airspeeds for operating in both normal and icing conditions and provides an optional terminating action for the placard installation. The optional terminating action consists of performing a functional test of the angle-of-attack (AOA) system, adjusting the calibration of the AOA system as needed, and submitting the AOA system test data report for the functional test to Cessna Aircraft Company (Cessna).

This action is intended to prevent an inadvertent stall due to the inadequate stall margin provided by an improperly adjusted stall warning system, which could result in loss of controllability of the airplane. In the proposed AD, the terminating action allowing for the removal of the placard would no longer be optional but required. The existing AD was issued after the Safety Board’s investigation of the February 16, 2005, accident involving a Cessna 560, which crashed on approach to Pueblo, Colorado, after encountering supercooled large droplet (SLD) icing conditions during the approach.

SLD conditions, and the ice accretions they can produce, were also identified as issues of concern to the Board in two previous investigations. These investigations showed that SLD conditions outside or near the edge of the Federal Aviation Regulation Part 25, Appendix C, icing certification envelope could cause thin, rough ice accretions that are more aerodynamically detrimental than accretions that occur within the icing certification envelope.

Additionally, these investigations showed that the effects of these ice accretions on airplane performance, stall, and handling characteristics could vary widely depending upon the size, distribution, and type of ice accumulated on the airplane’s surfaces. The circumstances of the Pueblo Cessna 560 accident and the previous accidents show that thin, rough ice that accretes prior to de-ice boot activation in SLD icing conditions, or in icing conditions near the edge of the Appendix C icing envelope, can reduce or eliminate stall warning margins, can alter an airplane’s flight handling characteristics, and can lead to an aerodynamic stall from which the flight crew may not recover.

The Safety Board’s report on the Pueblo accident concluded that the Cessna 560’s stall warning system did not provide a warning in all icing conditions, including those conditions in which thin, rough ice accumulates on the protected surfaces, and that the Cessna 560 stall warning system did not provide a stall warning prior to the upset. As a result of this investigation, the Board issued Safety Recommendation A-07-17 (currently classified “Open—Await Response”), which states the following:

Require the modification of the Cessna 560 airplane’s stall warning system to provide a stall warning margin that takes into account the size, type, and distribution of ice, including thin, rough ice on or aft of the protected surfaces. Additionally, the Safety Board reiterated Safety Recommendation A-96-54 (currently classified “Open—Unacceptable Response”), which was originally issued to the FAA as a result of the Roselawn, Indiana, ATR-72 accident:

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. Safety Recommendation A-96-54 was reiterated in the Monroe, Michigan, Comair 3272 accident report, and is currently on the Safety Board’s Most Wanted List of Transportation Safety Improvements.

The Board noted in its most recent correspondence with the FAA that the FAA 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. Utilizing the expanded Appendix C envelope being developed in response to A 96-54 to evaluate the stall warning margin of the Cessna 560 with thin, rough ice accretions on or aft of the protected surfaces as recommended in A 07-17 would provide the best level of safety in all icing conditions, and in icing conditions similar to those encountered by the airplane in the Pueblo accident.

` Based on its review of the proposed rule changes, the Safety Board is pleased that the FAA has performed a fleet survey and has identified a potential source of inaccuracy in the AOA system. Further, the Board is pleased that the FAA has proposed requirements for correcting the calibration of the AOA system and providing data regarding the calibration adjustment to the manufacturer. These actions would ensure that the stall warning system provides an accurate stall warning based on original and secondary icing certification flight testing. However, the Safety Board notes that the actions described in this NPRM and in AD-2007-23-13 do not address the actions recommended in A-07-17.

The Board is concerned that a reduction in stall warning margin produced by thin, rough ice accretions generated by SLD icing conditions or icing conditions near the edge of the Appendix C icing envelope can eliminate or seriously alter stall margin and the alert provided to the flight crew, even for a properly calibrated AOA system. The Safety Board appreciates the opportunity to comment on this NPRM addressing the safety of flight in icing conditions, and looks forward to working with the FAA soon to address the concerns presented in these comments.

July 23, 2007 · NTSB Correspondence · FAA

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

February 27, 2007 · NTSB Correspondence · FAA

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.

May 10, 2006 · NTSB Correspondence · FAA

The Safety Board appreciates the FAA's summary of the research results from its project to clarify the effects of mixed-phase icing conditions, as documented in the final report titled "Assessment of Effects of Mixed-Phase Icing Conditions on Thermal Ice Protection Systems" (DOT/FAA/AR-03/48), dated May 2003. The Safety Board notes that the FAA's Aviation Rulemaking Advisory Committee's (ARAC's) Ice Protection Harmonization Working Group (IPHWG) is continuing to develop a revision to Part 25 to require a demonstration that an airplane can safely operate in supercooled large drop (SLD) conditions for an unrestricted time or can detect SLD and safely exit icing conditions.

Although the work of the IPHWG is responsive to this recommendation, it is proceeding at an unacceptably slow pace. There does not appear to have been any progress since the FAA previously informed the Board of the status of this recommendation on September 15, 2003. The Board notes that this recommendation is 9 1/2 years old, and the FAA has not yet received the recommendations from the IPHWG, let alone prepared regulatory analyses, issued the NPRM, analyzed comments, or completed the many other tasks involved in issuing new regulations. The Safety Board has previously advised the FAA that the pace of progress on this recommendation is not acceptable.

The Board continues to investigate accidents where icing was a consideration, including current investigations of (1) a Cessna 560 which crashed while on approach to Montrose, Colorado, on February 16, 2005, killing eight people and (2) a Bombardier Challenger CL-600 which crashed during takeoff from Montrose, Colorado, on November 28, 2004, killing three people and seriously injuring three other people. In addition, the Board is participating in the investigation of a November 21, 2004, crash during takeoff of a Bombardier RJ200 in Baotou, China, resulting in 53 fatalities. Icing is being investigated as a significant factor that may have caused all three accidents.

The Board notes that although this recommendation specifically asks for action for both Part 23 and Part 25 airplanes, the FAA's activities to date have been only for Part 25 airplanes. Pending development and issuance of regulatory requirements for both Part 23 and Part 25 airplanes to demonstrate that they can safely operate in SLD conditions for an unrestricted time or can detect the SLD environment and safely exit icing conditions, Safety Recommendation A-96-54 remains classified OPEN -- UNACCEPTABLE RESPONSE.

February 2, 2006 · NTSB Correspondence · FAA

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:

Requi

February 2, 2006 · NTSB Correspondence · FAA

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.

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, 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, chord wise and span wise, 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, 11 the Safety Board issued Safety Recommendation A-96-54, which requested the following of the FAA:

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.

January 3, 2006 · NTSB Correspondence · FAA

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:

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, an

October 26, 2005 · Recipient Response · FAA

Letter Mail Controlled 10/27/2005 2:12:40 PM MC# 2050501 - From Marion C. Blakey, Administrator, FAA, 10/26/05 The Aviation Rulemaking Advisory Committee's (ARAC) Ice Protection Harmonization Working Group is continuing to develop a revision to 14 CFR Part 25 that includes regulatory requirements to demonstrate an airplane can safely operate in certain supercooled large drop (SLD) conditions for an unrestricted time or can detect SLD and safely exit icing conditions. In 2002, the FAA sponsored research to clarify the effects of mixed-phase icing conditions.

The results are documented in the final report entitled "Assessment of Effects of Mixed-Phase Icing Conditions on Thermal Ice Protection Systems" (DOT/FAA/AR-03/48), dated May 2003. The report is available at the FAA William J. Hughes Technical Center's full-text technical reports web page at actlibrary.tc.gov. The research examined unprotected airfoil surfaces, fully-evaporative systems, and running wet systems. The test results do not suggest that ice accretions on unprotected surfaces in mixed-phase clouds would be more hazardous that those in a pure liquid phase icing environment.

For fully-evaporative thermal systems the power required in mixed-phase and glaciated conditions was lower than for purely liquid clouds. This may be attributed to ice particles bouncing from the surface and loss of water due to splashing. Also, the additional heat energy required for melting the adhering ice crystal is minor when compared with the much larger heat energy required for a fully-evaporative thermal ice protection system. For running-wet systems the local power density required at the stagnation areas was higher for mixed phase conditions. The additional local power density is required to melt ice crystals that adhere along the stagnation line of protected surfaces.

However, the overall power required was virtually the same for all-liquid and mixed-phase conditions. Although the research raises some questions with regard to running wet systems, there is no history of airframe ice protection system problems in mixed-phase conditions. Therefore, the FAA does not find compelling evidence to include mixed-phase icing conditions in the certification requirements for airframe ice protections systems. An ARAC working group is, however, examining engine icing events believed to have occurred in mixed-phase conditions to determine if there is a need to develop mixed-phase icing requirements for engine inlets (14 CFR Part 25) and engines (14 CFR Part 33).

I will keep the Board informed of the FAA's progress on this safety recommendation

October 16, 2000 · Recipient Response · FAA

Letter Mail Controlled 10/19/2000 3:19:55 PM MC# 2001561 - From Jane F. Garvey, Administrator: The Ice Protection Harmonization Working Group (IPHWG) is continuing its efforts to define an icing environment that includes supercooled large droplets (SLD). The IPHWG began work on these tasks in February 1999. The IPHWG has received SLD data from the Federal Aviation Administration (FAA), National Aeronautics and Space Administration (NASA), Canada's Atmospheric Environment Service, and The Boeing Company. The FAA collected and consolidated SLD data from the atmosphere around the Great Lakes Region and existing SLD data and provided analyses of the SLD data to the IPHWG.

The IPHWG will use these data to define an icing environment that includes SLD. The icing environment definition will support the development of a proposed icing regulation by the IPHWG. Presentations by the FAA's William J. Hughes Technical Center, NASA, Canada's Atmospheric Environment Service, and The Boeing Company on SLD analytical methods have been given to the IPHWG. The IPHWG has identified potential shortcomings of these analytical methods when applied to ice accretions, which form in SLD conditions.

The IPHWG will consider potential shortcomings when determining an acceptable means to assess the ability of aircraft to operate safely in an icing environment either for the period of time to exit or to operate without restriction in SLD aloft and in SLD at or near the surface. The FAA provided a draft mixed?phase report to the IPHWG in September 1998. The final report was published in December 1998. Proceedings from the FAA?sponsored Mixed Phase and Glaciated Icing Conditions Workshop, which was held in December 1998, were published in July 1999 and subsequently provided to the IPHWG.

The IPHWG will consider this information in determining the need for a rulemaking initiative to address mixed?phase conditions. The IPHWG anticipates having an adequate data set available define an atmosphere that includes SLD by February 2001. I will keep the Board informed of the FAA's progress on this safety recommendation.

November 4, 1998 · NTSB Correspondence · FAA

Further the accident report of an in-flight icing encounter and uncontrolled collision with terrain of an Embraer EMB-120RT in Monroe, Michigan (report adopted 11/4/1998): As a result of its investigation of the 1994 Roselawn accident, the Safety Board issued Safety Recommendations A-96-54 and A-96-56 (currently classified “Open—Acceptable Response”), which, respectively, stated that the FAA should do the following: Revise the icing criteria published in 14 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. 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.

(The status of these recommendations is discussed in section 1.18.3.) Further, based on a perceived depletion of the FAA’s technical expertise, the 1993 U.S. General Accounting Office report entitled “Aircraft Certification: New FAA Approach Needed to Meet Challenges of Advanced Technology” recommended that the FAA should hire more technical subject matter specialists in various areas, including that of environmental icing. (See section 1.17.2.2.)

After the Roselawn accident, the FAA developed a three-phase, multi-pronged plan to address icing-related concerns, including operational issues, forecasting/defining icing conditions, certification issues, validating simulation methods, identifying the aerodynamic effects of accretion, and identifying visual cues to various hazardous icing conditions and (about 2 years after the Roselawn accident) hired its current Environmental Icing National Resource Specialist (NRS).

In January 1998, the FAA’s Environmental Icing NRS updated the Safety Board on the FAA’s progress with its plan, indicating that the first two phases have been completed and progress is being made in several aspects of Phase III (specifically in the areas of understanding the effects of various ice accretions, operational issues such as bridging, and development of ice detection/protection equipment). The Safety Board notes that the FAA’s three-phase plan could potentially satisfy the need for a comprehensive review of all aspects of structural icing in turbopropeller-driven transport airplanes.

However, the regulatory/certification changes addressed during Phase III have encountered delays. FAA personnel reported to the Safety Board that their attempts to produce an advisory circular (AC) that would appropriately revise methods of compliance with Parts 23/25 and Part 25 appendix C were not successful;193 therefore, they changed their approach to the problem and issued two of three proposed ACs addressing changes to methods of compliance and are going through the rulemaking process for the needed regulatory changes.

According to FAA personnel, ACs addressing methods of compliance with Parts 23 and 25 were issued on August 19, 1998, and March 31, 1998, respectively, and the newly created AC 25.1419 is currently in draft form, with no estimated issue date available. FAA personnel estimated that the rulemaking process will probably not be completed until January 2000.

In response to the Safety Board’s Safety Recommendations A-96-54 and A-96- 56, the FAA assigned ARAC working groups to accomplish, in part, the following: to establish criticality of ice accretions on airplane performance and handling qualities, to develop icing certification criteria for the safe operation of airplanes in icing conditions that are not covered by the current certification envelope, and to consider the development of a regulation requiring the installation of ice detectors or equivalent means to warn flightcrews of ice accumulations.

The Safety Board appreciates the efforts of the FAA Environmental Icing NRS and the ARAC working groups, and the Safety Board concludes that the work conducted by the FAA Environmental Icing NRS and the ARAC icing-related working groups is of crucial importance to the future safety of icing operations.

Consequently, the Safety Board believes that the FAA should expedite the research, development, and implementation of revisions to the icing certification testing regulations to ensure that airplanes are adequately tested for the conditions in which they are certificated to operate; the research should include identification (and incorporation into icing certification requirements) of realistic ice shapes and their effects and criticality. Further, the Board reiterates Safety Recommendation A-96-54 and A-96-56 to the FAA.