1.18 Additional Information
1.18.1 Cessna 550 and 560 Airplanes Icing-Related Flight Testing and Subsequent Actions
In early 1996, the FAA conducted preliminary evaluations of the Cessna 560 stall speeds and characteristics when operating in icing conditions. The evaluations were conducted partially as the result of the following icing-related Cessna 55037 and 560 accidents:

38 On December 30, 1995, a Cessna 560 crashed while circling to land in icing conditions in Eagle River, Wisconsin.39 The investigation revealed that about
1/8 inch of rime ice had accumulated on the left wing and horizontal stabilizer leading edges.
On January 2, 1996, a Cessna 560 crashed while on final approach in icing conditions in Augsburg, Germany.40 The pilots reported that the airplane started to buffet, entered a stall, and rolled right. No stall warnings were activated during the flight. The investigation by the German Federal Bureau of Accidents Investigation (BFU) revealed that about 2 mm (0.078 inch) of ice had accumulated along the wing leading edges.

37 The airfoil used on model 560 series airplanes is a modified version of the airfoil used on model 550 series airplanes. The main spar height is the same; however, the leading edge and the upper surface curvature have been changed to improve high airspeed performance.

38 The FAA also noted that several test pilots from its training academy had reported concerns about the Cessna 560 s stall handling characteristics.

39 The description of this accident, CHI96FA067, can be found on the Safety Board s Web site at <http://www.ntsb.gov>.

40 For more information, see German Federal Bureau of Accidents Investigation, Report on the Accident to the Aircraft Cessna 560 on January 2, 1996, at Augsburg, File No. CX 001-0/96 (Braunschweig City, Germany: BFU, 1996).

On February 19, 1996, a Cessna 550 crashed while on final approach in Salzburg, Austria.41 On the basis of radar recordings and witness testimony, the BFU determined that the airplane entered a stall, banked left, and became uncontrollable. The BFU stated that heavy icing at low altitudes prevailed during the approach and that the accident 'happened because the minimum control airspeed was undershot during the final approach.' As a result of flight tests conducted in early 1996 with the Cessna 560, the FAA issued Priority Letter Airworthiness Directive (AD) 96-24-06, which was applicable to the Cessna 560, in November 1996. AD 96-24-06 required revising the Cessna Model 560 Citation V AFM to provide flight crews with airspeed limitations, deice system operational procedures, and landing performance information to be used during approach and landing when residual ice was present or expected to prevent the uncommanded roll of the airplane in such conditions.

In late 1996, with Cessna participation, the FAA conducted additional flight tests with Cessna 550 and 560 series airplanes to assess the airplanes stall speeds, warnings, and characteristics. The FAA conducted the flight tests using airplanes with and without 1/2-inch-thick ice shapes installed on the protected (that is, those surfaces with deice boots) and unprotected surfaces of the wing.42 The flight tests of the Cessna 550 revealed that the airplane had an acceptable stall warning margin with and without the ice shapes installed. The flight tests of the Cessna 560 revealed that the stall warning margin was insufficient with and without ice shapes installed.43

Specifically, the flight tests of the Cessna 560 revealed that, even without ice shapes installed, the stall warning system activated only about 1 to 2 knots before a
significant lateral roll tendency and subsequent stall occurred. The tests also showed that, with ice shapes installed, the stall warning system activated shortly after or concurrent with a stall and a subsequent significant lateral roll. The tests indicated that the stall speed increased from 3 to 5 knots with the ice shapes installed and that the stall warning system did not compensate for the increased stall speed.

In early 1999, Cessna began incorporating modified stall warning systems on the Cessna 560 airplane (including the accident airplane) to provide a sufficient stall warning margin for operations in icing conditions. The modifications resulted in a stall warning margin increase of about 5 knots and were outlined in Cessna Service Bulletins (SB) SB560-34-69 and SB560-34-70. On April 3, 2000, the FAA issued AD 2000-03-09,

41 An English translation of the full accident report was not available; however, an English summary of the findings and the probable cause was provided to investigators by the BFU.

42 At the time of the Cessna 560 certification flight tests, artificial ice shapes were not required to be installed on the protected surfaces of the airplane. According to the original Cessna 560 flight testing certification, the flight tests were conducted in natural icing conditions and with artificial ice shapes installed on the unprotected surfaces of the airplane. According to the FAA, 1/2-inch ice shapes were used for the 1996 tests because these shapes represented the type of ice accumulated during the most likely encountered icing conditions and because the Cessna Model 560 Citation V AFM instructed pilots to activate the deice boots when ice accumulation was estimated to be from 1/4-to 1/2-inch thick.

43 Title 14 CFR 25.207 requires that the stall warning begin at a speed exceeding the stall speed by a margin of not less than 5 knots.

1.18.2 Previous Icing-Related Safety Recommendations
The Safety Board has previously issued numerous icing-related safety recommendations, several of which are on the Safety Board s List of Most Wanted
Transportation Safety Improvements. Five previously issued icing-related safety recommendations are relevant to the Pueblo accident and are detailed in this section. One of these recommendations was issued as a result of the investigation of the October 31, 1994, accident involving American Eagle flight 4184, which crashed during a rapid descent after an uncommanded roll excursion during icing conditions.44 The other four relevant recommendations were issued as a result of the investigation of the January 9, 1997, accident involving Comair Airlines, Inc., flight 3272, which experienced a loss of control while maneuvering with ice accumulation on the wings.45

1.18.2.1 Deice Boot System Activation
During the Comair flight 3272 accident investigation, the Safety Board learned that the AFMs for many aircraft, including the Cessna 560, instructed pilots to delay initial deice boot activation until they observed 1/4- to 1/2-inch-thick ice accumulation on the wing surface. Further, the Cessna Model 560 Citation V AFM states, 'early activation of the boots may result in ice bridging on the wing.' Additionally, Advisory Circular (AC) 25.1419-1A, 'Certification of Transport Category Airplanes for Flight in Icing Conditions,' dated May 7, 2004, states, 'many AFMs specify ice accumulation thickness prior to activation of the deicer boot system. The practice originates from a belief that a bridge of ice could form if boots are operated prematurely.' However, the AC further states the following:

Although ice may not shed completely by one cycle of the boots, this residual ice will usually be removed during subsequent boot cycles and does not act as a foundation for a bridge to form. The AFM procedure for boot operations should be to operate the boots at the first sign of ice and not wait for a specific amount of ice to accumulate.

44 For additional information, see National Transportation Safety Board, In-flight Icing Encounter and Loss of Control, Simmons Airlines, d.b.a. American Eagle Flight 4184, Avions de Transport Regional (ATR) Model 72-212, N401AM, Roselawn, Indiana, October 31, 1994; Volume I Safety Board Report, Aircraft
Accident Report NTSB/AAR-96/02 (Washington, DC: NTSB, 1996). 45 For additional information, see National Transportation Safety Board, In-Flight Icing Encounter and Uncontrolled Collision With Terrain, Comair Airlines, Inc., Flight 3272, Embraer EMB-120RT, N265CA, Monroe, Michigan, January 9, 1997, Aircraft Accident Report NTSB/AAR-98/04 (Washington, DC: NTSB, 1998).

The icing tunnel tests, wind tunnel data, and existing icing research data revealed that thin (1/4 inch or less), rough ice accumulations on the wing leading edge deice boot surfaces could be, depending on distribution, as aerodynamically detrimental to an airplane s performance as larger ice accumulations and that such ice could be difficult for pilots to perceive. As a result of these findings, the Safety Board issued Safety Recommendation A-98-91, which asked the FAA to do the following:

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 emphasize that leading edge deicing boots should be activated as soon as the airplane enters icing conditions.

On July 16, 1999, the FAA issued Notice for Proposed Rulemaking (NPRM) 99-NM-136-AD, which was applicable to Cessna model 500, 501, 550, 551, and
560 series airplanes and proposed revising the applicable AFMs to include a requirement to activate the deice boots at the first sign of ice accumulation and to cycle the boots automatically, if the automatic mode was available, or to cycle the boots manually to minimize ice accumulation on the airframe. From July 1999 to March 2000, the FAA issued 18 similar NPRMs applicable to 14 CFR Part 23 airplanes and 21 NPRMs applicable to Part 23 and 25 airplanes equipped with pneumatic deice boots.46

In an August 12, 1999, letter to the FAA, Cessna requested that NPRM 99-NM-136-AD be withdrawn, contending that the affected airplanes service history in
icing conditions, the modifications made to the airplanes stall warning systems, and the minimum airspeed in icing conditions guidance contained in its AFM validated that the airplanes could operate safely in icing conditions and that, therefore, the NPRM was not warranted. In a September 25, 2000, response letter to Safety Recommendation A-98-91, the FAA stated that it withdrew NPRM 99-NM-136-AD in November 1999 because manufacturer s data indicated that the affected aircraft could operate safely with ice accretion on the protected surfaces. The FAA also withdrew six of the other NPRMs it issued from July 1999 to March 2000 for similar reasons.

On May 19, 2003, the FAA informed the Safety Board about recommendations an Aviation Rulemaking Advisory Committee (ARAC) Ice Protection Harmonization Working Group (IPHWG) had made to revise Parts 25 and 121. The proposed revisions

46 In a March 12, 2000, response letter, the Safety Board expressed concern that the proposed ADs would require that the deice boot system be activated at the first sign of ice accumulation, not as soon as the airplane enters icing conditions.

47 However, in the intervening 3 years, the FAA has taken no further action; therefore, on May 10, 2006, the Board classified Safety Recommendation A-98-91 'Open Unacceptable Response,' pending issuance of a final rule adopting the regulatory changes proposed by the ARAC IPHWG.

1.18.2.2 Certification Requirements for Flight into Icing Conditions
The Safety Board determined during the American Eagle flight 4184 accident investigation that SLD conditions can cause ice accretions that are more aerodynamically detrimental than those that were considered during the initial certification of many existing airplanes for flight in icing conditions (that is, those conditions that fell within the Part 25, Appendix C envelope).48 As a result, the Board issued Safety Recommendation A-96-54 (superseding Safety Recommendation A-81-116), 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.

Safety Recommendation A-96-54 was reiterated in the Comair flight 3272 accident report and is currently on the Safety Board s List of Most Wanted Transportation Safety Improvements. In a March 6, 2006, letter, the FAA stated 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.

The Safety Board noted in its May 10, 2006, letter that, although the work of the IPHWG is responsive to this recommendation, the actions are proceeding at an
unacceptably slow pace and that the FAA has not yet received the recommendations from the IPHWG, prepared regulatory analyses, issued an NPRM, analyzed comments, or completed the many other tasks involved in issuing new regulations. Pending development and issuance of regulatory requirements for both Part 23 and 25 airplanes to demonstrate that they can safely operate in SLD conditions for an unrestricted time or can detect the SLD and safely exit icing conditions, the Board classified Safety Recommendation A-96-54 'Open Unacceptable Response.'

The Safety Board also determined during the Comair flight 3272 accident investigation that the ice accretions and conditions considered during certification for

47 The ARAC IPHWG also stated that an operator s guidance could only state that the deice system should be activated at the first sign of ice accumulation if the operator had demonstrated through additional flight tests that the airplane could operate safely with some ice accumulation.

48 Part 25, Appendix C specifies the kind of icing conditions in which an airplane s ice protection system must be able to operate.

49 or residual ice accretions were not addressed in the icing certification rules. Therefore, the Board issued Safety Recommendations A-98-92 and -100, both of which are currently on the Board s List of Most Wanted Transportation Safety Improvements. Safety Recommendation A-98-92 asked the FAA to do the following:

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.

In a September 21, 2001, response letter, the FAA indicated that sufficient information and methods were not available at that time to provide additional guidance concerning the determination of critical ice shapes in aircraft certification and that, therefore, it would sponsor additional needed research. In an October 26, 2005, response letter, the FAA indicated that it had completed and would shortly issue a draft revision to AC 20-73, 'Aircraft Ice Protection,' 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, Safety Recommendation A-98-92 remains classified 'Open Unacceptable Response,' pending the receipt of information regarding any new research conducted in response to this recommendation.

The Safety Board also issued a safety recommendation concerning the determination of critical ice shapes, A-98-100, to the FAA, which stated the following:

When the revised icing certification standards [recommended in Safety Recommendation A-98-92] and criteria are complete, review the icing certification of all turbopropeller-driven airplanes that are currently certificated for operation in icing conditions and perform additional testing and take action as required to ensure that these airplanes fulfill the requirements of the revised icing certification standards.

On November 4, 2005, in response to Safety Recommendation A-98-100, the FAA issued an NPRM titled, 'Airplane Handling Characteristics in Icing Conditions,' which proposed to revise 14 CFR 25.143, 'Proof of Compliance,' by adding a new paragraph, (i)(1), that specifies the certification requirements for airplane performance or handling qualities for flight in icing conditions and the type of ice accretions (including the size, shape, and location) that must be used to demonstrate compliance in each phase of flight. Specifically, the NPRM stated that thin, rough ice accretions must be considered to show

49 Intercycle ice is ice that has accreted on the deice surface between boot activation cycles.

On November 4, 2005, the FAA also proposed AC 25-1X, 'Performance and Handling Characteristics in the Icing Conditions Specified in Part 25, Appendix C,' which was intended to provide guidance for implementing the regulations proposed in the NPRM. In its January 2006 comments on the proposed AC, the Safety Board noted that the results of the research conducted as a part of the Comair flight 3272 accident investigation are currently included in Appendix R of draft AC 20-73, 'Aircraft Ice Protection.' Appendix R, which is also referenced in Appendixes 1 and 2 of proposed AC 25.21-1X, includes guidance on determining critical ice shapes and their associated roughness, descriptions of intercycle and residual ice accretions, and the aerodynamic penalties associated with these ice shapes.50

Although the Safety Board agreed with the FAA s proposed regulatory changes, in its May 10, 2006, response letter, the Board noted that the FAA has not applied the new information to all in-service turbopropeller-driven airplanes. Although the FAA indicated that no airplanes have an unsafe condition, the Board stated that it was concerned that the FAA reached this conclusion based on its belief that no accidents or serious incidents had occurred related to this issue. However, the Board stated that, during the 1990s, a number of accidents had occurred involving airplanes that had passed the certification standards and for which the FAA believed there was no unsafe condition requiring action and that these accidents generated new information that the FAA could now use. 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. Pending receipt of this information, Safety Recommendation A-98-100 was classified 'Open Unacceptable Response.' 51

1.18.2.3 Stall Warning Margins
The Safety Board also determined during the Comair flight 3272 investigation that the stall warning did not activate until after the stall, as in this accident, because ice
accumulation on the airplane had increased the stall speed and that the stall warning system was not designed to account for the increase caused by the type of ice that had 50 For more information about the testing conducted to determine the effects of residual and intercycle ice accretions, see section 1.18.3.

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

Require 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.

In a September 21, 2001, response letter, the FAA stated that it was pursuing regulatory development projects that would require both new and in-service airplanes to have stall warning systems installed that provide a cockpit warning before the onset of a stall when operating in icing conditions. However, in an October 26, 2005, response letter, the FAA stated that, after further review, it had determined that such a requirement for all in-service airplanes would impose a cost burden not commensurate with the potential safety benefits and that, therefore, it would take appropriate action on in-service airplane designs only if an unsafe condition were identified.

The November 2005 NPRM proposed changes to 14 CFR 25.207 to require that 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 NPRM, the Safety Board stated that the proposed changes appear to address the intent of Safety Recommendation A-98-96 for newly type-certificated airplanes; however, the proposed changes do not address in-service airplanes. Further, in its May 2006 response letter, the 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. As a result, the Board classified Safety Recommendation A-98-96 'Open Unacceptable Response,' pending issuance of a final rule associated with the November 2005 NPRM that includes a requirement that both newly type-certificated and in-service airplanes be equipped with stall warning systems that provide a cockpit warning before the onset of a stall in icing conditions.

1.18.3 Residual and Intercycle Ice and Automatic Deice Boot
System Information
In May 2002, as a result of additional testing conducted during the Comair flight 3272 investigation, the FAA published a report titled, Effect of Residual and Intercycle Ice Accretions on Airfoil Performance,52 which presented the results of testing designed to characterize and evaluate the aerodynamic performance effects of residual and intercycle ice accretions resulting from the cyclic operation of a typical aircraft deice boot system.53 The tests were conducted using a scaled version of the EMB-120 airfoil outboard wing section, which is similar to the Cessna Citation airfoil, and liquid water

52 Federal Aviation Administration, Office of Aviation Research, Effect of Residual and Intercycle Ice Accretions on Airfoil Performance, DOT/FAA/AR-02/68 (Washington, DC: 2002).

53 Some pneumatic deice boot systems can be turned on and activated automatically at either slow/low (3-minute) or fast/high (1-minute) cycle intervals. Cessna Citation series airplanes are not equipped with automatic deice boot recycling systems.

Results from the icing tests showed that the intercycle ice accretions were much larger in size and surface extent than residual ice accretions and that the intercycle ice
shapes caused significant performance degradation. The testing showed that the deice boots generally removed the ice from the wing leading edge and left little residual ice. A single continuous maximum case was run with 1-minute boot cycles, which was found to be very effective in minimizing the size of the intercycle ice accretion. The tests also revealed that the deice boot system was equally effective at shedding ice when activated at the first sign of ice as when activated once a 1/4 inch of ice had accumulated. The FAA s test report recommended the 'early and often' approach to deicing (that is, activating the deice boots as soon as ice is detected and cycling the boots at 1-minute intervals) to limit the size of residual and intercycle ice accretions.

On May 7, 2004, the FAA issued AC 25.1419-1A, which stated the following.

The recommended AFM procedure for boot operation should be to operate the boots at the first sign of ice. The boots should be operated until icing conditions are exited and ice no longer adheres to the airframe.

On July 21, 2004, the FAA issued revised AC 23.1419-2C, 'Certification of Part 23 Airplanes for Flight in Icing Conditions,' which reiterated the FAA s technical
report findings and added the following:

For deicing systems that do not have a timer to cycle the system automatically once activated, the additional task of manually cycling deicing systems on pilot workload should be evaluated. A recent Part 23 applicant found that definition of airframe deicing boot intercycle and residual ice steered them toward one-minute boot cycles and the workload evaluation dictated an automatic timer for the boots.

Analysis
2.1 General
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 the pilots 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.

The weight and balance of the airplane were within landing limits.

The recovered components showed no evidence of any preexisting powerplant, structural, or system failures.

The PUB local controller did not provide the accident flight crew or the Denver FSS with the PIREP reporting light to moderate icing; however, this was not a factor in the accident because CVR information indicated that the flight crew was aware of the icing conditions.

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.

This analysis discusses the accident sequence, including the flight crew s performance. This analysis also discusses inadequate training on operations in icing
conditions, inadequate deice boot system operational guidance, the need for automatic deice boot systems, inadequate icing flight test certification requirements, and inadequate stall warning margins in icing conditions.

2.2 Accident Sequence
2.2.1 Descent Into Icing Conditions
Surface observations and radar data indicated that freezing drizzle conditions existed in the PUB area around the time of the accident and that temperatures were below freezing from the surface to 30,000 feet. Several PIREPs and NWS products transmitted around the time of the accident, including winter weather advisories that warned of freezing drizzle, confirmed the presence of icing conditions in the PUB area. CVR

An analysis of the CVR and meteorological information indicated that mixed icing conditions existed from about 21,000 to 14,000 feet. Radar data and CVR information indicated that the airplane was in this icing layer for about 5 1/2 minutes. At 0858:20, as the airplane was descending through about 18,000 feet, the first officer suggested to the captain that he might want to cycle the deice boots.54 After cycling the deice boots, the captain indicated that the deice boots might have shed a little of the ice but that some ice remained on the wing, indicating the presence of residual ice.

2.2.2 Approach to Landing
In accordance with the SimuFlite Cessna Citation V Technical Manual and the Cessna Model 560 Citation V AFM, pilots were trained that, when any residual ice is present or can be expected during approach and landing, Vref must be increased by 8 knots. The manuals also contained both a caution and a warning indicating that stall speeds increased during operations in icing conditions, and that, therefore, Vref must be increased.

At 0859:29, the CVR recorded the first officer state that the Vref was 96 knots. In the case of this flight, the Vref should have been increased from 96 to 104 knots because of the icing conditions. The CVR did not record either pilot mention increasing the airspeed at any point during the approach. Therefore, the Safety Board concludes that the flight crew did not increase the Vref while operating in icing conditions, contrary to company procedures and manufacturer guidance.

2.2.3 Final Approach
At 0908:25, while at an altitude of about 9,400 feet, the first officer reported that the flight was in IMC, and, about 1 minute later, while at an altitude of about 7,400 feet, he reported that clear ice had accumulated on the airplane s wing. CVR and meteorological information indicated that the airplane likely encountered SLD conditions from 9,400 to 6,100 feet (the calculated altitude at the time of the upset) and that the airplane was likely in these conditions for about 4 1/2 minutes. During this time, about 1 to 4 mm (0.039 to 0.156 inch) of additional ice could have accumulated on the wing leading edges. The Safety Board concludes that the airplane encountered 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. The Safety Board further concludes that the airplane had residual ice on the wings after the deice boots were activated earlier in the 54 As noted, the pilots had been trained to wait until 1/4- to 1/2-inch-thick ice accumulation was visible on the wing leading edges before activating the deice boots.

According to the airplane performance study, about 0910, the airplane started its final descent from 7,000 feet at an airspeed of about 155 knots. By about 0911:35, the airspeed had started to decrease. CVR evidence indicated that the landing gear was extended at 0911:10, followed by extension of the speedbrakes and selection of full flaps. At 0912:04, the first officer stated, 'and you are plus twenty five,' to which the captain replied, 'slowing.' On the basis of a Vref of 96 knots, the airspeed would have been about 121 knots at the time of the first officer s statement. At 0912:37, when the airplane was at an altitude of about 6,100 feet, the first officer told the captain that he might want to run the deice boots and that they had the Vref.