What happened
On November 14, 1997, at approximately 17:48 local time, a Piper PA 60-601P Aerostar, registered in the United States and based in Altenrhein, Switzerland, was conducting a private business flight to Erfurt, Germany. The flight was conducted under Instrument Flight Rules (IFR).
During the approach to runway 28 at Erfurt, weather conditions deteriorated significantly due to incoming fog from the west, reducing horizontal visibility to approximately 500 meters. The pilot was cleared for an ILS approach. Radar tracking confirmed the aircraft was on the correct approach path and glide slope until it descended below 2,000 feet NN, at which point it was no longer visible to radar.
Shortly after passing the outer marker, the pilot initiated a go-around procedure. During this maneuver, the aircraft entered an excessive flight attitude followed by a flat spin. The aircraft struck a field approximately 2,250 meters west of the runway threshold of runway 11 and caught fire. All four fatalities (the pilot and three passengers) occurred at the scene.
The investigation
The BFU investigation focused on the mechanical state of the engines and the fuel system. Investigators examined the wreckage and found that the fuel mixture control settings were incorrect. Specifically, the mixture levers were found in a significantly retarded position.
Technical analysis revealed that the engines had become extremely hot and lost power because the fuel-air mixture was set too lean. This led to the sequential failure of the right engine, followed by a severe power loss in the left engine. The investigation also identified that the fuel flow meters were defective and that the fuel pressure gauges for both engines were non-functional due to blocked pressure lines at the distribution valve. Furthermore, the investigation raised concerns regarding the aircraft's maintenance history, noting discrepancies in recorded flight hours and uncertainty regarding the timing of the last annual inspection.
Findings
- The primary cause of the accident was an excessive flight attitude and subsequent flat spin resulting from the sequential failure of both engines due to the incorrect operation of the fuel mixture control system.
- Contributing factors included heavy fog and darkness, which severely limited visibility.
- The pilot had limited experience in instrument flight (IFR).
- Defective fuel flow measurement and faulty settings on the fuel mixture control system.
- Inoperative fuel pressure gauges prevented the pilot from monitoring the engine's condition.
- Maintenance discrepancies, including unverified annual inspections and the use of non-authorized maintenance facilities.