2.4
Certification Requirements for Flight Into Icy
Conditions
The Safety Board has previously identified concerns about inadequate flight test certification requirements. For example, the American Eagle flight 4184 accident
investigation revealed that SLD conditions can cause ice accretions that are more aerodynamically detrimental than those accretions that fall within the Part 25, Appendix C envelope. As a result, the Safety Board issued Safety Recommendation A-96-54, which asked the FAA to 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.

61 AC 23.1419-2C stated that the effect on pilot workload of continuously cycling the deice boots should be evaluated.

[C]onduct additional research to identify realistic ice accumulations and … determine the effects and criticality of such ice accumulations … the information
developed … should be incorporated into aircraft certification requirements.

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 an 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. As noted, Safety Recommendation A-98-100 only addressed turbopropeller-driven airplanes.

The circumstances of this accident clearly demonstrate that deice boot-equipped turbojet airplanes also require additional testing in an expanded Appendix C icing
certification envelope, which would include thin, rough ice accumulations and intercycle and residual ice. Therefore, the Safety Board believes that the FAA should, when the revised icing certification standards and criteria are complete, review the icing certification of all pneumatic deice boot-equipped airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. The new recommendation (A-07-16) will supersede Safety Recommendation A-98-100 and will be classified "Open—Unacceptable Response." 62

2.5
Inadequate Stall Warning Margins in Icing
Conditions
Stall warning systems are intended to provide flight crews with adequate warning of an impending stall to give them enough time to take necessary action to prevent a stall.

62 Safety Recommendation A-98-100 is on the Safety Board’s List of Most Wanted Transportation Safety Improvements. Accordingly, Safety Recommendation A-07-16 will automatically be placed on the Most Wanted List.

As a result of the Comair investigation, the Safety Board issued Safety Recommendation A-98-96, which recommended that the FAA require manufacturers and
operators of all airplanes certificated to operate in icing conditions to install stall warning systems that provide a cockpit warning before the onset of a stall when the airplane is operating in icing conditions. The FAA stated in an October 26, 2005, response letter that it was pursuing rule changes to require only that newly certificated airplanes have stall warning systems installed that provide a cockpit warning before the onset of a stall when operating in icing conditions and that it would take appropriate action on in-service airplanes only if an unsafe condition were identified.

The November 2005 NPRM proposed changes to 14 CFR 25.207 to require that only newly type-certificated airplanes be equipped with stall warning systems that provide a stall warning before the onset of a stall when the airplane is operating in icing conditions. In its comments on the proposed NPRM and in its May 2006 response letter to the FAA, the Safety Board stated that it was not acceptable for the FAA to wait until an accident or serious incident occurred to reveal that an unsafe condition existed on an in-service airplane and that, because the proposed changes did not address in-service airplanes, Safety Recommendation A-98-96 was classified "Open—Unacceptable Response." The Board continues to believe that not requiring in-service airplanes to be equipped with improved stall warning systems until an unsafe condition is identified is unacceptable and encourages the FAA to expedite issuance of a final rule that contains such a requirement.

Regarding the Cessna 560’s stall warning system, in 1996, the FAA conducted ice testing using 1/2-inch ice shapes installed on the protected surfaces. As a result of these tests, in early 1999, Cessna began incorporating a modified stall warning system on Cessna 560 airplanes (including the accident airplane) to provide a 5-knot increase in the stall warning margin for operations in icing conditions. However, as this accident has shown, these modifications were not adequate because they did not take into account the effects of thin, rough ice on the protected surfaces; therefore, additional modifications to the airplane’s stall warning system are necessary.

The Safety Board concludes that the Cessna 560 airplane’s stall warning system did not provide a stall warning before the upset. The Safety Board further concludes that the Cessna 560 airplane’s stall warning system does not provide a warning in all icing conditions, including those conditions in which thin, rough ice can accumulate on the protected surfaces. Therefore, the Safety Board believes that the FAA should require 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.

3. Conclusions
3.1 Findings
1. The captain and first officer were properly certificated and qualified under Federal regulations. No evidence indicated any preexisting medical or physical condition that might have adversely affected the flight crew’s performance during the accident flight. A review of the pilots’ 72-hour histories revealed that they slept well in the days leading up to the accident flight and went to bed early in preparation for an early departure. No evidence was found that fatigue degraded the performance of either pilot on the day of the accident.
2. The weight and balance of the airplane were within landing limits.
3. The recovered components showed no evidence of preexisting powerplant, structural, or system failures.
4. The Pueblo Memorial Airport local controller did not provide the accident flight crew or the Denver Flight Service Station with the pilot report reporting light to moderate icing; however, this was not a factor in the accident because cockpit voice recorder information indicated that the flight crew was aware of the icing conditions.
5. During the approach, the flight crew of the sister ship, which was following the accident flight, cycled the deice boots numerous times and maintained a high airspeed and, subsequently, landed safely, indicating the importance of taking these actions to counteract the hazardous effects of icing.
6. The flight crew did not increase the landing reference airspeed while operating in icing conditions, contrary to company procedures and manufacturer guidance.
7. The airplane encountered supercooled large droplet (SLD) conditions, which are most conducive to the formation of thin, rough ice on or aft of the protected surfaces, during about the last 4 1/2 minutes of the flight. Further, the airplane had residual ice on the wings after the deice boots were activated earlier in the flight, and this ice would have affected the overall thickness, roughness, and distribution of the SLD ice accumulation.
8. The flight crew did not activate the deice boots when configuring for the approach and landing, which was contrary to company procedures and manufacturer guidance.
9. The flight crew failed to maintain adequate airspeed during the final approach in icing conditions, which led to an aerodynamic stall from which they did not recover.
10. Pilots could benefit from the reinforcement during training of the Cessna Model 560 Citation V Airplane Flight Manual requirements to increase the airspeed and operate the deice boots during approaches when ice is present on the wings.
11. The briefing conducted late in the approach was a distraction that impeded the flight crew’s ability to monitor and maintain airspeed and manage the deice system.
12. All operators would benefit from an increased focus on providing monitoring skills in their training programs, including those operating under 14 Code of Federal Regulations Parts 121 and 135, as would pilots completing Federal Aviation Administration-approved training programs for Part 91 operations.
13. Ice bridging does not occur on modern airplanes; therefore, it is not a reason for pilots to delay activation of the deice boots.
14. Activating the deice boots as soon as an airplane enters icing conditions provides the greatest safety measure.
15. Manual operation of the deice boot system increases pilot workload, which can result in distraction during critical phases of flight, such as approach and landing.
16. 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.
17. The Cessna 560 airplane’s stall warning system did not provide a stall warning before the upset.
18. The Cessna 560 airplane’s stall warning system does not provide a warning in all icing conditions, including those conditions in which thin, rough ice can accumulate on the protected surfaces.
3.2 Probable Cause
The National Transportation Safety Board determines that the probable cause of this accident was the flight crew’s failure to effectively monitor and maintain airspeed and comply with procedures for deice boot activation on the approach, which caused an aerodynamic stall from which they did not recover. Contributing to the accident was the Federal Aviation Administration’s failure to establish adequate certification requirements for flight into icing conditions, which led to the inadequate stall warning margin provided by the airplane’s stall warning system.

4.1 New Safety Recommendations
As a result of its investigation of the February 16, 2005, accident involving a Cessna Citation 560, the National Transportation Safety Board makes the following recommendations to the Federal Aviation Administration:

Require that operational training in the Cessna 560 airplane emphasize the airplane flight manual requirements that pilots increase the airspeed and operate the deice boots during approaches when ice is present on the wings. (A-07-12)

Require that all pilot training programs be modified to contain modules that teach and emphasize monitoring skills and workload management and include opportunities to practice and demonstrate proficiency in these areas. (A-07-13)

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) (This safety recommendation supersedes Safety Recommendation A-98-9163 and is classified "Open—Unacceptable Response.")

Require that all pneumatic deice boot-equipped airplanes certified to fly in known icing conditions have a mode incorporated in the deice boot system that will automatically continue to cycle the deice boots once the system has been activated. (A-07-15)

When the revised icing certification standards (recommended in Safety Recommendations A-96-54 and A-98-92) and criteria are complete, review
the icing certification of pneumatic deice boot-equipped airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards. (A-07-16) (This safety recommendation supersedes Safety Recommendation A-98-10064 and is classified "Open—Unacceptable
Response.")

63 For more information about this recommendation, see sections 1.18.2.1 and 2.3.

64 For more information about this recommendation, see sections 1.18.2.2 and 2.4.

Require 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. (A-07-17)

4.2
Previously Issued Safety Recommendations
Reiterated in This Report
The Safety Board reiterates the following safety recommendations to the Federal Aviation Administration:

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. (A-96-54)65

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. (A-98-92)