Aircraft Apparently Entered Stall At 41,000 Feet

The National Transportation Safety Board Wednesday released the following update on its investigation of the October 14, 2004 crash of Pinnacle Airlines flight 3701 in a residential area in Jefferson City (MO). The two crewmembers, who were the only occupants on board, were killed, and impact forces and a postcrash fire destroyed the airplane. There were no injuries on the ground. The on-scene portion of the investigation finished on October 20, 2004. 

The two GE CF34-3B1 engines were shipped to a General Electric Aircraft Engine facility in Lynn, Massachusetts for detailed examination. The examination found that the cores of both engines were free to rotate and there was no indication of any pre-existing problems that would have led to the accident.

The flight data recorder (FDR) data indicate that while the airplane was at 41,000 feet, the stick shaker and stick pusher activated several times before the airplane entered an aerodynamic stall. Almost simultaneously, both engines shut down. The air-driven generator was automatically deployed and supplied the backup alternating current power to the airplane.

According to the emergency checklist for a dual engine failure, there are two ways to restart or relight the engines. One option is to use a windmill restart, which requires at least 300 knots indicated airspeed and the core of the engine to be either 12 percent rpm above 15,000 feet or 9 percent rpm below 15,000 feet. The FDR data show that the computed airspeed did not get above 300 knots and that there was no measured rotation of the engine core.

The second option is to use auxiliary power unit (APU) bleed air, which has to be accomplished at 13,000 feet or below. The target best glide speed depends on the weight of the aircraft and is either 190 knots indicated airspeed or 170 knots indicated airspeed. The FDR data indicate that the APU was on after the aerodynamic stall and that the airspeed was sufficient for an APU start. The FDR and CVR indicated that the flight crew tried to start the engines several times but were unsuccessful.

The operations group is still conducting interviews and developing the 72-hour history for the flight crew. The operations group has scheduled interviews with the Federal Aviation Administration principal operations inspector and several managers for the operator. The systems, powerplants, and aircraft performance will visit the airplane manufacturer.

FMI: www.ntsb.gov