Incident
On 10 August 2015 at 13:48 local time, a Piper PA-28-161 Cadet, registered as HB-PQI, departed from runway 26 at Birrfeld Airfield (LSZF) on a training flight. The student pilot handled the takeoff. After the aircraft became airborne and began a climb, the pilot retracted the flaps. Shortly after turning crosswind, the engine lost power and began to misfire. The flight instructor immediately took control, adjusted the aircraft to a gliding attitude, and the engine subsequently stopped completely. Manual switching to the backup FADEC B system did not restore power. The instructor decided to perform a forced landing and successfully touched down in a field approximately 1.5 km west of Birrfeld. The aircraft came to a stop without damage. Both occupants were uninjured and able to exit the aircraft on their own.
Investigation
The Swiss Transportation Safety Investigation Board (STSB) conducted a summary investigation. The primary focus was the fuel boost pump, a mechanically driven gear pump that supplies Jet A-1 fuel to the high-pressure pump. The pump was removed and subjected to metallurgical examination.
Examination revealed multiple defects in the pump components:
- The drive gear, made of sintered steel, exhibited several cracks and a complete fracture. The fracture surface showed signs of fatigue (cyclic loading). The cracks originated at the load transfer points of the polygon shaft and were attributed to sintering defects; the sintering quality was insufficient with respect to homogeneity.
- The drive nut, also made of sintered steel, showed secondary cracks predominantly aligned with sintering defects, along with significant wear from friction at the load transfer surfaces.
- Wear marks on the shaft guide within the pump housing indicated rotation of the polygon shaft.
Similar incidents involving comparable pump failures had been documented by the French Bureau d'Enquêtes et d'Analyses (BEA) between 2009 and 2012, with a report published in June 2015.
Findings
The STSB concluded that the pump components were metallurgically limited in their suitability for continuous operation. Over time, such pumps could become mechanically unstable and fail. In this incident, fatigue cracks formed at the power transmission points of the polygon shaft on the drive gear. These cracks propagated during operation until a total fracture occurred, jamming the gear. The resulting defect in the fuel boost pump led to the engine power loss.
The STSB determined that no further investigation would yield additional findings relevant to preventing similar occurrences, and closed the investigation with this summary report.