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