What happened
On July 19, 2020, a Piper PA-28-161 (registration HB-PMF) departed from Birrfeld Airport for a scenic flight of the Alps. The aircraft was carrying one pilot and one passenger. While cruising at 5,500 feet above sea level with 75% power, the engine failed near Zofingen. The pilot observed flashing FADEC indicators and attempted restart procedures, but the engine would not restart.
As the aircraft glided toward Triengen Airport, the pilot determined that no suitable landing sites were reachable at the destination. Consequently, the pilot executed an emergency landing in a cornfield near Reitable. During the touchdown, the aircraft struck a furrow in the field, causing the nose gear to collapse. The aircraft sustained light damage, and while there was minor damage to the surrounding field, the pilot and passenger remained uninjured.
The investigation
The investigation focused on the engine, which had been converted from a Lycoming O-320 to a Technify TAE 125-02-99 diesel engine. While the external components and fuel lines appeared in good condition, a detailed examination of the internal components revealed a mechanical failure within the high-pressure fuel pump.
Data from the engine's FADEC system showed that the pressure in the common-rail system plummeted from a target of 1063 bar to just 9 bar within two seconds. Flight path data from the collision warning device indicated that the pilot initiated a turning maneuver at approximately 100 feet above the ground, roughly 2.5 km before the runway, with a steadily decreasing turning radius.
Findings
- The engine failure was caused by a mechanical failure of the fuel high-pressure pump.
- A broken spring caused an inlet valve within the pump to become stuck in the open position.
- Debris from the broken spring further obstructed the valves on the high-pressure side of the pump.
- The pump had a time since new (TSN) of 592 hours, which was just below the 600-hour recommended replacement interval.
- The investigation noted that the pilot's decision to initiate a low-altitude turn carried the risk of an aerodynamic stall and limited the ability to adjust the landing point to a safer area, such as the adjacent meadow.