The National Transportation Safety Board today determined that the probable cause of a commuter aircraft accident near Monroe, Michigan was the FAA's failure to establish adequate aircraft certification standards for flight in icing conditions. Also cited was the FAA's failure to ensure that approved procedures for deicing system operation were implemented by U.S. air carriers, and to require the establishment of adequate minimum airspeeds for icing conditions, which led to a loss of control when the airplane accumulated a thin, rough accretion of ice on the wings.
Contributing to the accident, the Board found, were the flightcrew's decision to operate in icing conditions near the lower margin of the operating airspeed envelope (with flaps retracted), and Comair's failure to establish and adequately disseminate unambiguous minimum airspeed values for flap configurations and flight in icing conditions.
The accident occurred on January 9, 1997, when Comair flight 3272, an Embraer 120RT aircraft, crashed while on approach to Detroit Metropolitan Wayne County Airport. All 26 passengers and three crewmembers on board were killed and the aircraft was destroyed.
The Board's investigation revealed that, despite the accumulated lessons of several major accidents, the FAA failed to adopt a systematic, proactive approach to the certification and operational issues of turboprop aircraft icing. The icing certification process was found to be inadequate because it does not require manufacturers to demonstrate the airplane's flight handling characteristics under a sufficiently realistic range of adverse ice accretion/flight handling conditions. Additionally, the Board was critical of FAA policies that allow an air carrier to elect not to adopt the manufacturer's changes to the airplane flight manual. This, the Board said, can result in carriers using procedures that may not reflect the safest operating practices.
The Board noted that, consistent with Comair's procedures, the pilots did not activate the leading edge deicing boots during their descent and approach to the airport because they likely did not perceive that the airplane was accreting significant structural ice. Had they been aware of the circumstances of six previous EMB-120 icing accidents, and a relevant revision to the airplane flight manual by the manufacturer, it is possible that they would have operated the airplane more conservatively with regard to airspeed and flap configuration or activated the deicing boots when they knew they were in icing conditions.
Because the pilots were operating with the autopilot engaged during a series of descents, turns, and power and airspeed adjustments, they may not have perceived the airplane's gradually deteriorating performance. The Board stressed that disengagement of the autopilot while flying in icing conditions will enable pilots to sense the aerodynamic effects of ice accretion and enhance their ability to retain control of the aircraft.
The Board found that current operating procedures recommending that pilots wait until ice accumulates to an observable thickness before activating deicing boots results in unnecessary exposure to a significant risk for turboprop aircraft. Based primarily on concerns about ice-bridging, pilots continue to use procedures that increase the likelihood of potentially hazardous degraded airplane performance resulting from small amounts of rough ice on the leading edges.
As a result of this investigation, the Board made 19 recommendations to the FAA to remedy the problems that have been uncovered. Included was a recommendation that, jointly with NASA and other interested aviation organizations, the FAA organize an industry-wide training effort to educate manufacturers, operators and pilots of turboprop aircraft regarding the hazards of thin, possibly imperceptible, rough ice accumulations, the importance of activating deicing boots on entering icing conditions, and the necessity of maintaining minimum airspeeds in icing conditions.
In addition, the Board reiterated a 1996 recommendation calling on the FAA to revise icing certification testing regulations 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 safe operations cannot be demonstrated by the manufacturer, 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.
The NTSB's complete report on this accident, PB98-910404, may be purchased from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161, (703) 487-4650. The report also will be placed on the Board's web page (www.ntsb.gov) in the near future.