What happened
On December 7, 1995, a Socata TBM 700 was conducting a private instrument flight (IFR) from Linz to Braunschweig, Germany. The flight was intended for pilot training and a business engagement. On board were the pilot in command, a trainee, and four passengers.
During the NDB/DME approach to runway 08 at Braunschweig, the aircraft entered severe oscillations around the roll and yaw axes immediately after the descent was initiated. The pilot attempted to disconnect the autopilot and regain manual control, but was unable to stabilize the aircraft. As the aircraft descended uncontrollably, its airspeed dropped from approximately 13 and 90 knots. Just before impact, the pilot applied maximum power, but the engine's response delay likely prevented thrust from being realized before ground contact.
The aircraft struck a high-voltage power line approximately 3 NM from the runway threshold and impacted a wooded field at a shallow angle. The aircraft slid through a five-meter-wide stream, coming to rest with its rear fuselage partially submerged. The accident resulted in two serious and four minor injuries.
The investigation
The BFU examined the wreckage and conducted component testing of the autopilot system at the manufacturer's facilities in the United States. Investigators found that the engine was still producing power at the time of impact, evidenced by distorted propeller blades and a twisted hot section housing. The fuel system showed that clean Jet A1 fuel was under pressure at the time of the crash.
Regarding the flight controls, all surfaces and flaps were found to be in functional condition. However, the rudder trim was found in a position corresponding to high-speed cruise rather than the required approach configuration. The investigation also noted that the landing gear, which extends asymmetrically in this model, had been deployed just before the descent began, a process known to cause significant yawing.
Findings
- The aircraft entered a "Dutch-roll" state, characterized by coupled oscillations around the roll and yaw axes.
- The rudder trim was incorrectly set for the approach phase, placing the aircraft in a sideslip condition.
- The asymmetric deployment of the landing gear likely provided the additional yawing moment necessary to trigger the oscillations.
- The pilot's decision to disconnect the autopilot, while standard procedure for such aircraft, left the crew unable to manage the complex oscillations under IFR conditions.
- The presence of a trainee attempting to assist with flight controls may have further complicated the pilot's ability to recover.