Deicing guidance blamed in Circuit City accident
During the January 23 public meeting on the icing-related crash of a Cessna Citation 560 near Pueblo, Colo., NTSB members criticized the FAA and Cessna for not updating critical icing information used by pilots and certification engineers. The Cessna information to which the NTSB referred is instructions in the aircraft flight manual (AFM) that tell pilots in most circumstances to wait for ice to build to a depth of between one-quarter and half an inch before inflating deice boots. The FAA information is the FAR Part 25 Appendix C icing envelope, which has yet to recognize supercooled large droplet icing. Both subjects are longtime targets of NTSB recommendations.
Two Cessna Citation 560s owned by Circuit City Stores and operated by charter/management firm Martinair were flying to Pueblo, Colo., to refuel during a trip to the West Coast on Feb. 16, 2005. The first to attempt the approach crashed four miles east of Pueblo Memorial Airport after picking up ice. The pilots did not inflate the deice boots during the approach, according to the NTSB. They also failed to increase speed above Vref as specified in the AFM and the Citation, still on autopilot, got too slow and stalled, killing the six passengers and both pilots.
At the meeting, the Board focused on the following safety issues: “inadequate training on operations in icing conditions, inadequate deice boot system operational guidance, the need for automatic deice boot systems, inadequate certification requirements for flight into icing conditions and inadequate stall warning margins in icing conditions.”
The release of the full accident report was delayed for a few weeks due to questions about the wording of the probable cause. But the NTSB did provide a probable cause and recommendations stemming from the accident, although that information is still subject to editing.
Pending edits in the final report, the NTSB “determines that the probable causes of this accident were 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 FAA’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.”
During the descent and approach to Pueblo Memorial Airport, the accident Citation flew through two areas of icing, according to the NTSB. While the flight crew did inflate the boots during the first icing encounter, during the approach the Citation flew through an area of supercooled liquid droplet icing, the NTSB wrote in its summary of the accident, “which [is] most conducive to the formation of thin, rough ice on or aft of the protected surfaces, during about the last four-and-a-half minutes of the flight. 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.” The NTSB based its conclusion that the pilots allowed the airspeed to drop to 90 knots, four knots above the stall speed for the icing conditions and configuration, on its analysis of radar data. The stall, the NTSB believes, occurred at 90 knots and 1,500 feet agl.
At the meeting, NTSB member Robert Sumwalt brought up the issue of ice bridging, which the Board defines as when “ice in the shape of an inflated deice boot forms as the boot is cycled, which cannot be removed by subsequent cycles.” He expressed concern that an aircraft manufacturer would be reluctant to advise pilots to inflate deicing boots immediately after ice begins accreting instead of waiting for ice to build first. The AFM for the Cessna 560 provides different advice about when to inflate boots, depending on whether the jet is on approach with flaps and landing gear extended or in other flight regimes.
In the normal procedures section of the 560 AFM, the following procedure is listed for use of the anti-ice and deice systems: “The surface deice system should be used when ice buildup is estimated to be between one-quarter- and one-half-inch thickness. Early activation of the boots may result in ice bridging on the wing. If ice is allowed to accumulate in excess of one inch, boot cycling may not clear it.”
The same section lists in a note this procedure for the approach configuration: “When reconfiguring for approach and landing (i.e. flaps extended and gear down), and any ice accretion is visible on the wing leading edge, regardless of thickness, activate the surface deice system. Continue to monitor the wing leading edge for any reaccumulation.”
Frank Hilldrup, the NTSB investigator-in-charge on the Pueblo accident, explained that some manufacturers still require pilots to wait for ice to build before inflating the boots because “there is a concern among the manufacturers if you activate the deice boots at the first sign of accretion, it’s not going to get as clean a shed of the ice as you would if you let it build up.” Hilldrup explained that the NTSB has examined that claim, working with NASA researchers performing wind tunnel tests, and that “it’s true that you’re not going to get as clean a shed, you’re going to have more residual ice. However, we also found that the residual ice will get cleared on subsequent boot activations on a regular cycling.”
The NTSB also explained in the meeting and in the summary of the accident, there is “no evidence that modern turbine-powered airplanes are at risk for bridging. Bridging is no longer a valid reason to allow a buildup between boot cycles.”
According to the NTSB, the fact that ice bridging is no longer a concern “warrants activation of deice boots upon entering icing conditions [and] development of a system for automatically cycling deice boots.”
The FAA had proposed an airworthiness directive in the 1990s that would have forced Cessna and other manufacturers to tell pilots not to wait for ice to build up before inflating boots. “Can you explain to me why the FAA withdrew their NPRM to ask operators and, in this case, Cessna about incorporating guidance in their manuals to eliminate the buildup of ice and to do it on the first sign of ice?” NTSB member Deborah Hersman asked.
In 1981 the NTSB began recommending that the FAA update the FAR 25 Appendix C icing envelope. Appendix C came under increased scrutiny following the Oct. 31, 1994, American Eagle ATR 72 crash in Roselawn, Ind., and the NTSB again recommended updating Appendix C following that accident.
An aviation rulemaking committee has submitted recommendations on the issue of when boots should be inflated, according to an NTSB staffer, but the FAA has not acted on those recommendations because it has other rulemaking projects that take priority.