19970109000259A

On approach when crashed, weather may have been a factor. (.4)the flight was being vectored for the approach to runway 3R at Detroit metropolitan Wayne county airport (dtw) when the aircraft descended and impacted the ground. The aircraft struck the ground in a steep nose-down attitude in a level field in a rural area about 19 NM southwest of dtw. The flight carried 26 passengers and 3 crew members. There were no survivors and the airplane was destroyed by impact forces and a post crash fire. Instrument meteorological conditions prevailed at the time of the accident. The investigation revealed that it was likely that the airplane gradually accumulated a thin, rough glaze/mixed ice coverage on the leading edge deicing boot surfaces, possibly with ice ridge formation on the leading edge upper surface, as the airplane descended from 7,000 feet mean sea level (MSL) to 4,000 feet MSL in icing conditions, which may have been imperceptible to the pilots. The pilots had been instructed by air traffic control to slow to 150 knots and according to flight data recorder information, the airplane began to show signs of departure from controlled flight as it decelerated f rom 155 to 156 knots whle in a flaps-up configuration, and operational issues of turbopropeller-driven transport airplane icing. The icing certification process had been inadequate because it has not required manufacturers to demonstrate the airplane's flight handling and stall characterists under a sufficiently realistic range of adverse ice accretion/flight handling conditions. The aicraft manufacturer had issued a revision in April, 1996 to the approved flight manual which included activation of the leading edge deicing boots at the first sign of ice formation. The airplane operator did not incorporate the procedure, because it was contrary to the company's trained procedures and practices and of the belief that enacting the changes would result in potentially unsafe operation. Investigators; discussion with management personnel at each of the seven U.S.-based operators of the aircraft indicated that at the time of the accident only two of these operators had changed their procedures to reflect the information in the revision. The FAA, at the time of the accident, did not require manufacturers of all turbine-engine driven airplanes to publish minimum airspeed information fo r various flap configurations and phases and conditions of flight. During safety board investigation post accident interviews with company pilots. There were inconsistent answers on the complex and varied minmum airspeed requirements estiblished by the company for both icing and nonicing conditions. It was also noted that the pilots uncertanity of the appropriate airspee ds might have been associated withthe language used, the different airspeeds and criteria contained in the guidance, the company's methods of distribution, and the company's failure to incorporate the guidance as a formal, permanent reveision to the flight standards manual.