History of the Flight
On 5 September 2021, a Piper PA-25-235 Pawnee, registered F-HPMO, departed from Oloron-Herrère aerodrome for a glider towing flight. During the climb to approximately 8,000 ft, the pilot noticed a reduction in engine power. He instructed the glider pilot to release the tow cable and turned toward the departure aerodrome. As power decreased further and abnormal engine noises were heard, the pilot set the throttle to idle and decided to perform an off-field landing. The glider pilot provided the position of a nearby listed field. Upon sighting the field, the pilot released the tow cable, briefly increased power to 1,500 rpm to test the engine, then returned to idle. On final approach, he found the aircraft too low, applied power, and the engine shut down. He attempted to land in a field before the intended one. During the landing run, the aircraft hit a line of trees separating the two fields and came to a stop. The aircraft sustained substantial damage.
Aircraft Information
The aircraft was operated by Centre école vol à voile du Haut-Béarn (CEVVHB). Its mechanic, holding a Part-66 license, performed a 50-hour inspection on 4 September 2021. At the request of the airworthiness manager, he repaired a leak in the exhaust system at the connection between the exhaust pipe manifold and muffler. The repair involved high-temperature-resistant tape, an aluminium sheet, and two metal clamps. The mechanic did not consult the usual workshop or use supporting documents. He was unaware that the aluminium sheet had a temperature resistance much lower than the exhaust temperature (500-700 °C). The repair was not recorded in any logbook.
After the inspection, the first flight was uneventful. During the second flight, the pilot (who knew of the repair) detected a heat smell and aborted the flight, landing without incident. He informed another pilot. Unable to contact the mechanic, they briefly inspected the engine, particularly the repair area, finding no burn marks or damage. The second pilot then decided to perform subsequent flights.
Examination of Site and Wreckage
The wreckage was found at the edge of the listed field. Wings were substantially damaged from tree contact. Engine examination revealed that the aluminium sheet had melted, damaging wiring under the exhaust, including all magneto wiring in that area. No damage was found to the fuel supply line nearby.
Pilot Information
The 64-year-old pilot held an Airline Transport Pilot Licence (ATPL(A)) and a glider pilot licence. He had over 12,000 flight hours, with 5 hours on type in the previous three months. He was chief-pilot of the Oloron flying club and a tug pilot for CEVVHB.
Meteorological Information
Météo-France estimated weather at the accident site: wind from 360° at 5 kt, gusts to 10 kt, visibility >10 km, few clouds, temperature 29 °C.
Additional Observations
During engine disassembly, a CEVVHB representative noted that the exhaust valve of the left front cylinder was white and not lubricated, unlike others. The exhaust muffler bellows were substantially damaged with high-temperature oxidation, and the outer skin showed similar white coloration. Further examination by the responsible workshop had not been performed.
Conclusions
The investigation concluded that during the 50-hour inspection, the mechanic performed a temporary exhaust repair using an aluminium sheet with insufficient temperature resistance. This repair deteriorated in flight, causing the sheet to melt and damage the magneto grounding cables. During the accident flight, damage to these cables led to engine power reduction and eventual shutdown during final approach. Contributing to the accident was the pilot's decision to conduct a towing flight despite knowledge of the repair and a previous heat smell, without awaiting mechanic advice.