What happened
On December 1, 1994, at approximately 18:00 local time, a Piper PA 34 Seneca, operated by a Dutch flight school, crashed near Radscheid, Germany. The aircraft was conducting an Instrument Flight Rules (IFR) training flight, traveling from Maastricht-Beek via Stuttgart and Geneva.
At 16:50, the crew received instructions from Brussels Air Traffic Control to descend from FL 100 to FL 60. Radar data indicates that the aircraft did not immediately initiate the descent. Approximately 45 seconds later, the aircraft's flight parameters changed violently. Radar tracking showed a sudden reversal in course, a rapid descent rate of 8,500 ft/min, and a significant drop in airspeed. During this period, unusual noises were heard on the radio frequency. The aircraft eventually broke apart in mid-air, leaving a debris trail spanning 2 km. All three occupants—a flight instructor, a student pilot, and a second student—were killed upon impact.
The investigation
The BFU investigation focused on the aircraft's structural integrity and the functionality of the autopilot system. Investigators examined the wreckage, which included separated sections of the wings, tail, and fuselage. Analysis of the airframe revealed no evidence of external impact, primary structural failure, or control system failure prior to the event.
However, the investigation identified a defect in the autopilot's microswitch within the control column. This defect prevented the autopilot from automatically disconnecting when the manual electric trim was engaged. This specific malfunction had been previously noted in the aircraft's maintenance logs but had not been repaired because the error was not reproducible on the ground. The investigation also reviewed radar data, radio communications, and the aircraft's technical logs.