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Safety Recommendation Details

Safety Recommendation A-07-017
Details
Synopsis: On February 16, 2005, about 0913 mountain standard time,1 a Cessna Citation 560, N500AT, operated by Martinair, Inc., for Circuit City Stores, Inc.,2 crashed about 4 nautical miles east of Pueblo Memorial Airport (PUB), Pueblo, Colorado, while on an instrument landing system (ILS) approach to runway 26R. The two pilots and six passengers on board were killed, and the airplane was destroyed by impact forces and postcrash fire. The flight was operating under the provisions of 14 Code of Federal Regulations (CFR) Part 91 on an instrument flight rules flight plan. Instrument meteorological conditions (IMC) prevailed at the time of the accident.
Recommendation: TO THE FEDERAL AVIATION ADMINISTRATION: 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.
Original recommendation transmittal letter: PDF
Overall Status: Closed - Acceptable Action
Mode: Aviation
Location: Pueblo, CO, United States
Is Reiterated: No
Is Hazmat: No
Is NPRM: No
Accident #: DCA05MA037
Accident Reports: Crash During Approach to Landing, Circuit City Stores, Inc., Cessna Citation 560, N500AT
Report #: AAR-07-02
Accident Date: 2/16/2005
Issue Date: 2/27/2007
Date Closed: 6/14/2012
Addressee(s) and Addressee Status: FAA (Closed - Acceptable Action)
Keyword(s): General Aviation, Icing, Upset Recovery, Weather

Safety Recommendation History
From: NTSB
To: FAA
Date: 6/14/2012
Response: On January 23, 2012, technical staff from the FAA, Cessna, and the NTSB met to discuss the analyses that the FAA and Cessna had performed in response to this recommendation. NTSB staff had previously provided information to the FAA regarding the environmental conditions that we believed needed to be considered in these analyses. The analyses we reviewed were complete and very detailed, and the results were consistent with those previously presented to the NTSB. Based on these analyses, the FAA believes that the Cessna 560 has an adequate stall warning margin and that no modifications are needed. During these discussions, the FAA indicated that the Cessna 560 has a pronounced uncommanded roll characteristic but that this undesirable characteristic occurs before a stall has begun. The uncommanded roll remains within the controllability limits specified in the certification standards, although it is likely to surprise an unprepared pilot. The NTSB believes that such a characteristic is evidence of an airplane that has entered a stall, and the intent of this recommendation was to give pilots adequate warning to avoid such a situation. However, we acknowledge that, based on how the FAA defines a stall, an adequate margin exists. We also believe that the revisions related to handling in icing conditions, made to certification standards in 2007, would not permit the type of undesirable pre-stall roll found in the Cessna 560. Accordingly, Safety Recommendation A-07-17 is classified CLOSED—ACCEPTABLE ACTION.

From: FAA
To: NTSB
Date: 3/8/2012
Response: -From Michael P. Huerta, Acting Administrator: In our letter of March 27, 20 II, we stated (hat we were evaluating additional Supercooled Large Droplet (SLD) environmental conditions that could have been present at the time of the accident. The environmental conditions considered were not only from the input provided by the Board's technical staff, but also from an evaluation conducted by an independent engineering consulting firm that was retained by Cessna Aircraft Company. On January 23, 2012, we participated in an internet meeting with the Board's technical staff and personnel from Cessna Aircraft Company to present the analysis results and FAA's go forward position. During this meeting we reviewed the data that have been supplied to the Board. As discussed in the meeting, we believe the environmental conditions analyzed address the Board's concern regarding the use of larger droplets in the evaluation of this accident scenario. The analytical investigations utilized LEWICE to assess the ice thickness relative to the wing profile for two wing stations. A number of freezing drizzle droplet distributions were analyzed with droplet ranges from 9 to 417 microns and liquid water contents ranging from 0.1 to 0.4 grams per cubic meter. No freezing rain distributions were analyzed since there was no evidence of freezing rain associated with the accident. The aircraft configuration and descent profile are the same as the previous analysis. The results of the recent analytical evaluation were consistent with those previously presented. The results did not indicate icc accretions that would yield aircraft level effects beyond those already evaluated in flight tests and utilized to establish the existing stall warning system performance. Based on the results of previous and recent analytical evaluations, 1 believe that the FAA has effectively addressed this safety recommendation, and I consider our actions complete.

From: NTSB
To: FAA
Date: 6/2/2011
Response: The NTSB has previously acknowledged the findings about the angle of attack miscalibration, as well as Cessna’s and the FAA’s actions in response to this problem. On December 4, 2009, NTSB staff sent an e-mail to FAA technical staff that provided details of the testing in supercooled large droplet conditions that we believe need to be evaluated before concluding that the airplane has an adequate stall warning margin in icing conditions. We appreciate learning of the FAA’s activities since that time, including working with Cessna to evaluate the additional conditions. As part of this work, Cessna retained an independent engineering consulting firm to analyze the environmental conditions and potential ice shapes experienced by the accident aircraft that produced results similar to the NTSB’s findings from the accident. The FAA is currently reviewing an analytical investigation that Cessna conducted utilizing the environmental conditions from their consultant’s analysis. The NTSB is interested in the FAA’s offer to review this work with NTSB technical staff and to discuss appropriate FAA actions as a result of the analysis. Pending this meeting and the FAA’s revision of the stall warning margin in icing conditions based on the results of this work, Safety Recommendation A-07-17 remains classified OPEN – ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 3/27/2011
Response: CC# 201100127: - From J. Randolph Babbitt, Administrator: In our letter of September 22, 2009 we stated the actions taken to date adequately address the concern raised by Safety Recommendation A-07-17. In summary, those actions included a thorough investigation of the Cessna 560's angle of attack (AOA)/stall warning system and associated maintenance practices, flight testing and analytical investigations into the effects of suspected ice accretions, a review of the AFM procedures, and a review of deice boot maintenance practices. As a result of these actions we issued Airworthiness Directive (AD) 2007-23-13, later superseded by AD 2008-26-04, to address and identified an unsafe condition with the AOA/stall warning system. These actions address the concern raised in safety recommendation A-07-17. However, we are sensitive to the Board's concern with the ice accretions evaluated during this investigation, as highlighted in the Board's September 10, 2008 and April 27, 20 I0 letters. As stated in our last response, we are willing to consider the evaluation of additional environmental conditions that may have been present during the time of the accident. We arranged a telecon to discuss the issue of ice shapes with the Board's technical staff. On December 4, 2009 we received input from the Board's technical staff, via email, concerning additional environmental conditions for consideration in the evaluation of this accident. These conditions were based on a meteorological study performed by Mr. Ben Bernstein for the Board. Additionally, that email provided clarification on the Board's concem with the uncertainty of the environmental conditions at the time of the accident and the effect this uncertainty has on the ice shapes evaluated during this investigation. The Board suggests the Cessna 560 be analyzed using larger droplet distributions such as the proposed Supercooled Large Droplet freezing drizzle Median Volumetric Diameter (MVD) > 40 or the freezing rain MVD > 40. Since receiving this input from the Board on December 4, 2009 we have been working with Cessna to evaluate the additional conditions. Additionally: Cessna has provided the Wichita Aircraft Certification Office with an independent engineering consulting firm analysis of the environmental conditions and potential ice shapes experienced by the accident aircraft. This study reaffirms the Board's findings that freezing drizzle, but not freezing rain, was most likely present in the lower layer and presents a proposed distribution of water content and drop size for evaluation. Additionally, Cessna recently completed an analytical investigation utilizing the proposed distribution from their study and we are currently in the process of reviewing the results of this analysis to determine our future course of action. Once we complete our review we would like to meet and discuss the results with the Board's technical staff and discuss appropriate next steps. I will keep the Board informed of the FANs progress on these safety recommendations and provide an updated response by January 2012.

From: NTSB
To: FAA
Date: 4/27/2010
Response: The NTSB has acknowledged the findings about the angle of attack miscalibration, as well as Cessna’s and the FAA’s actions in response to this problem. The NTSB has also acknowledged the testing in icing conditions performed by Cessna and the FAA that included testing in SLD conditions; however, the NTSB believes that additional SLD conditions need to be evaluated before concluding that the airplane has an adequate stall warning margin in icing conditions. In a December 4, 2009, e-mail to FAA staff, NTSB staff described the technical details of the additional icing conditions that the NTSB believes need to be evaluated before concluding that the C-560 has an adequate icing stall warning margin. Pending the FAA’s evaluation of the icing stall warning margin for the C-560, using the additional conditions described in the December 4, 2009, e-mail and, if necessary, the FAA’s revision of the stall warning margin in icing conditions based on the results of this additional testing, Safety Recommendation A-07-17 remains classified OPEN -- ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 9/22/2009
Response: Letter Mail Controlled 10/5/2009 3:57:02 PM MC# 2090620: - From J. Randolph Babbitt, Administrator: In the FAA's May 17, 2007, letter we stated that we were working aggressively with Cessna to gain a better understanding of the factors that may have contributed to the subject accident. The FAA identified the following factors and presented an action plan to address each item: Stall warning system operation with ice accretions, including freezing drizzle; Maintenance actions associated with the stall warning system; Deice boot replacement and associated stall strip maintenance actions; Adequacy of current airplane flight manual (AFM); and Stall warning system design The FAA and Cessna completed an evaluation of each factor. In August 2007, representatives from the Board, Cessna, and the FAA met to discuss the evaluation findings. As per the action plan, the aircraft was flight tested with simulated thin rough ice accretions. Flight test and analytical investigations into the effects of SLD were also evaluated. Evaluation findings indicate that a properly configured and functioning angle-of-attack (AOA)/Stall warning system provides the required level of stall warning margin. The regulation requires the aircraft to have a minimum stall warning margin of seven percent under all specified environmental conditions and operational configurations. The most significant finding was that the setting of the AOA/Stall warning system on. Fielded aircraft often did not fall within type design limits. This finding was based on a survey of aircraft in the field. An incorrectly set AOA/Stall warning system could potentially render the system ineffective by providing an inadequate stall warning margin. Additionally, an improperly set AOA/stall warning system could allow the Cessna 560's pre-stall rolling tendency to occur prior to stall warning. During this evaluation the FAA also reassessed the approach and landing speeds presented in the AFM and we found the speeds acceptable. At the conclusion of the meeting, the Board representatives were given copies of all presentation materials and test data. Since the August 2007 meeting, the FAA has completed all items on its action plan. The following are the resulting actions: Issued AD 2007-23-13 which became effective on November 30, 2007. The AD required compliance with the following actions for all Cessna Model 560 Citation V and Ultra aircraft within 30 days after the effective date of the AD: Revise AFM Limitations to indicate the proper approach and landing speeds when operating with the ice protection systems "ON" independent of icing conditions. These speeds are currently in the normal procedures section of the AFM; Install a placard in clear view of the pilot that indicates the appropriate speeds for operating in icing and non-icing conditions; Expand current approach and landing performance charts making them applicable when operating with the ice protection system "ON independent of icing conditions; Develop a new AFM emergency procedure for recovery from inadvertent stall; Insert new wording in the AFM normal procedures section describing the Cessna 560's pre-stall rolling characteristics; o Remove ice bridging language from the AFM; and Revise preflight checklist procedures to verify security of the wing stall strips to address issues with deice boot replacement. Issued AD 2008-26-04 (enclosure 2) which became effective January 29, 2008. This AD superseded AD 2007-23-13 and required the following actions for all Cessna Model 560 Citation V and Ultra aircraft: Retain all actions from AD 2007-23-13; Require owners and operators to check and reset the AOA/Stall Warning System via Cessna Alert Service Letters ASL560-34-34 and ASL560-34-35, as applicable. This action provides a tighter tolerance for the AOA/Stall Warning system; Require owner/operators to provide Cessna the results of the AOA/Stall Warning recalibration performed via the ASLs; and Remove the placard and an AFM warning installed by the retained actions from AD 2007-23-13 once the AOA/Stall Warning system has been recalibrated. In addition to the actions required by AD 2008-26-04, Cessna has updated the aircraft maintenance manual (AMM). The AMM periodic check of the AOA/Stall Warning system has been enhanced and is similar to those contained in the ASLs. The AMM requires completion of the checks at every phase 5 inspection. In addition, a new procedure has been added to the AMM to periodically check the stall warning system icing bias function. The AMM requires this recurring inspection every 150 hours. These new maintenance procedures were released in May 2008, as part of revision 24 of the AMM. We believe the actions taken to date adequately address the concern raised by Safety Recommendation A-07-17. However, we understand the Board does not agree with this statement. In its letter of September 10, 2008, the Board noted it was pleased that the FAA had taken the actions stated above. However, the letter went on to state that the Board did not believe the FAA can conclude that the Cessna Model 560 airplane has an adequate stall warning margin in icing conditions. Specifically, their concern involves operation in SLD. The Board believes the FAA and Cessna need to consider additional icing conditions and additional locations of ice accumulations in SLD conditions. The Board recommends handling these discussions concerning this topic with face-to-face meetings between the FAA and the Board's technical staff. As noted in the Board's letter of September 10, 2008, the FAA did try to assess the impact of the suspect SLD conditions encountered during the Colorado accident. The testing and analysis conducted by the FAA and Cessna indicate that the encountered SLD ice accumulations during the accident should not have been worse than the SLD ice accumulations already flight tested. However, we recognize a degree of uncertainty in the environmental conditions and aircraft ice accumulations for this exposure. In an effort to address the Board's concerns, a telephone conversation was held on March 13, 2009, between personnel from the Board, Transport Airplane Directorate, Small Airplane Directorate, and the Wichita Aircraft Certification Office. Also, in attendance was the FAA Chief Scientific and Technical Advisor for Atmospheric Icing. During the conversation, Board personnel expressed their concern with the extent of chordwise coverage of the simulated ice accretions tested during the C560 evaluations. They presented ideas on additional ice accretions for consideration and took the action to provide the FAA the specifics of those ice accretions. The FAA agreed to examine the proposed ice accretions and have a follow-on discussion(s) with the Board in order to bring this issue to closure. I will keep the Board informed of the FAA's progress on this safety recommendation.

From: NTSB
To: FAA
Date: 10/1/2008
Response: 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.

From: NTSB
To: FAA
Date: 9/10/2008
Response: On August 9, 2007, staff from the Safety Board met with staff from the FAA and Cessna at the FAA’s Aircraft Certification Office in Wichita, Kansas, to discuss activities in response to this recommendation. The FAA and Cessna conducted an extensive evaluation that found that with the Appendix C icing conditions, the Cessna C-560 had an adequate stall warning margin. The evaluation then used test evaluation conditions for supercooled large droplet (SLD) icing developed by the aviation rulemaking advisory committee’s ice protection harmonization working group to perform additional tests with ice shape conditions not currently specified in Part 23 or Part 25. Much of this work consisted of putting 40-grit sandpaper on the leading edge of the airfoil and then performing flight tests to evaluate the stall warning margin. Again, an adequate stall warning margin was found even in these flight tests simulating SLD conditions. Given that the FAA/Cessna investigation could not find icing conditions with a lack of stall warning margin similar to the conditions found in the February 16, 2005, accident that prompted this recommendation, other possible causes of the lack of stall warning margin were investigated. If the airplane’s angle of attack (AOA) sensor is off calibration, the stall warning system will not be reliable. Cessna reviewed a number of in-service Cessna C-560 aircraft and found that all had AOA sensors out of calibration, and in every case, the out-of-calibration AOA had resulted in a stall warning’s being issued at a lower speed than it should have, thus reducing the stall warning margin. In some cases the AOA sensor problem had resulted in the stall warning speed’s being off as much as 7 knots, a significant error. Therefore, the FAA believes that the problem is with the AOA sensor, not with the stall warning speed margin in icing conditions. On November 15, 2007, the FAA issued AD 2007?23-13, which requires installing new minimum airspeed placards on C-560 aircraft to notify the flightcrew of the proper airspeeds for operating in both normal and icing conditions. The AD also requires revising the AFM to provide limitations and procedures for operating in icing conditions; for operating with anti-ice systems turned on, regardless of icing conditions; and for recognizing and recovering from an inadvertent stall. The FAA issued this AD 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. Cessna is also examining the problem with the AOA sensor and is developing a service bulletin to alert C-560 operators of the need to check the AOA sensor’s calibration and reset it if needed. The service bulletin may also include a continuing airworthiness requirement to periodically check the AOA sensor. The FAA plans to issue an AD to mandate this service bulletin when it is issued. The Safety Board considered the testing of SLD conditions performed with the C?560. The Board believes this was comprehensive work, and this testing may form the basis for certification tests to evaluate airplane handling in SLD conditions for future aircraft designs. However, the Board does not believe that this testing considered all of the SLD conditions of concern. Before concluding that the airplane has an adequate stall warning margin in SLD conditions, the Board believes that the FAA and Cessna need to consider additional icing conditions and additional locations of ice accumulation in SLD conditions. The details of the additional testing needed are technically complex and are best discussed in face-to-face meetings of the FAA and the Board’s technical staff. Although the Safety Board does not believe the FAA is yet able to conclude that the C-560 airplane has an adequate stall warning margin in icing conditions, the Board is pleased to learn that the FAA and Cessna have identified the problem with the AOA sensor and are taking actions to resolve this issue. Pending a meeting of FAA and Safety Board technical staff to discuss additional SLD testing conditions needing evaluation and, if necessary, revisions to the stall warning margin in icing conditions based on the results of this additional testing, Safety Recommendation A-07-17 is classified OPEN -- ACCEPTABLE RESPONSE.

From: FAA
To: NTSB
Date: 5/17/2007
Response: Letter Mail Controlled 5/31/2007 8:48:32 AM MC# 2070240: - From Marion C. Blakey, Administrator: FAA Comment. Since December 2006, Cessna and the Wichita ACO have been aggressively working to gain a better understanding of the factors that may have contributed to the subject accident. The following factors have been identified: Stall warning system operation with ice accretions, including freezing drizzle; Maintenance actions associated with the stall warning system; Deice boot replacement and associated stall strip maintenance actions; Adequacy of current Flight Manual; and Stall warning system design review. Below you will find an action plan with associated status and schedule to address each of the identified factors. Stall warning system operation with ice accretions: On December 12, 2006, Cessna was notified of the need to conduct additional evaluations of the Model 560 with simulated ice shapes. This included the possibility of testing ice shapes for conditions outside of the current certification standards for supercooled large droplets. Cessna and the FAA are currently evaluating the following: a. The adequacy of stall warning with artificial ice shapes representative of critical Appendix C icing conditions. This consists of shapes not tested during the ‘96/’98 investigation such as thin rough ice (sandpaper) and runback at near freezing temperatures; b. Stall characteristics with artificial ice shapes of item (a); and c. Additionally, we are re-examining the definition of the original ice shapes utilized during ‘96/’98 evaluations to verify their adequacy. The FAA asked Cessna to: a. Evaluate the adequacy of stall warning with artificial ice shapes representative of freezing drizzle icing conditions; b. Evaluate stall characteristics with artificial ice shapes of item (a); and c. Evaluate potential runback ice forming behind the protected wing leading edge in freezing drizzle. The ongoing flight tests with the Appendix C simulated ice accretions are nearly complete. Results from the latest flight tests appear consistent with those from the ‘96/’98 evaluations. The data reviewed thus far indicate adequate stall warning margin for a properly biased system. The data also show that if the stall warning system fails to bias, results are consistent with the accident investigation findings. A cursory examination was conducted on the 1996 ice shapes, but a more detailed analysis will be required. Once these critical Appendix C flight evaluations are completed and the data have been reviewed, Cessna and the FAA will discuss the need to examine icing conditions associated with freezing drizzle. Freezing drizzle is outside the required certification icing envelopes, but it may have been a factor in the subject accident. It is estimated Appendix C flight evaluations will be complete by the middle of November 2007. Field and maintenance manual evaluations of stall warning system: In addition to the flight evaluations for stall warning and handling characteristics above, Cessna and the FAA are evaluating a sample of representative in-service aircraft. The target sample size is 10 -15 aircraft. This evaluation will examine the adequacy of maintenance/inspection procedures associated with inspection and/or adjustment of the angle of attacWstal1 warning system. It will also provide data, via flight tests, on in-service stall warning system performance. Two aircraft have been evaluated thus far and the FAA and Cessna are working to obtain additional aircraft to complete the evaluation. The maintenance procedure review is estimated to be complete by the end of July 2007 and it is estimated the aircraft survey task will be complete by December 2007.