Accident Details
The flight was being vectored for an approach to runway 3R at Detroit Metropolitan Wayne County Airport (DTW) when the aircraft descended and impacted the ground. The airplane struck a level field in a rural area approximately 19 nautical miles southwest of DTW in a steep nose-down attitude. The flight carried 26 passengers and 3 crew members; there were no survivors. The airplane was destroyed by impact forces and a post-crash fire. Instrument meteorological conditions prevailed at the time of the accident.
Icing Conditions and Flight History
The investigation determined that the airplane likely accumulated a thin, rough glaze/mixed ice coverage on the leading edge deicing boot surfaces, possibly with ice ridge formation on the upper surface, as it descended from 7,000 feet mean sea level (msl) to 4,000 feet msl in icing conditions. This accumulation may have been imperceptible to the pilots. Air traffic control had instructed the pilots to slow to 150 knots. According to flight data recorder information, the airplane began to show signs of departure from controlled flight as it decelerated from 155 to 156 knots while in a flaps-up configuration.
Certification and Regulatory Issues
The investigation disclosed that the FAA failed to adopt a systematic and proactive approach to the certification and operational issues of turbopropeller-driven transport airplane icing. The icing certification process was found inadequate because it did not require manufacturers to demonstrate the airplane's flight handling and stall characteristics under a sufficiently realistic range of adverse ice accretion and flight handling conditions. Additionally, at the time of the accident, the FAA did not require manufacturers of all turbine-engine driven airplanes to publish minimum airspeed information for various flap configurations and phases and conditions of flight.
Operator Procedures
The aircraft manufacturer had issued a revision in April 1996 to the approved flight manual that included activation of the leading edge deicing boots at the first sign of ice formation. However, the airplane operator did not incorporate the procedure because it was contrary to the company's trained procedures and practices and due to a belief that enacting the changes would result in potentially unsafe operation. Investigator interviews with management personnel at each of the seven U.S.-based operators of the aircraft indicated that only two had changed their procedures to reflect the revision at the time of the accident. Postaccident interviews with company pilots revealed inconsistent answers regarding the complex and varied minimum airspeed requirements established by the company for both icing and nonicing conditions. The pilots' uncertainty about appropriate airspeeds may have been associated with the 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 revision to the flight standards manual.