What happened
On 5 September 2021, a Piper PA25-235 Pawnee, registration F-HPMO, was performing a glider towing flight near Bedous, France. While climbing to approximately 8,000 feet, the pilot experienced a reduction in engine power. Following the loss of power, the pilot instructed the glider pilot to release the tow cable and began an approach toward the departure aerodrome, Oloron-Herrère.
As the engine power continued to decline and began making abnormal noises, the pilot moved the throttle to idle and prepared for an off-field landing. After selecting a nearby field, the pilot briefly increased power to 1,500 rpm to test the engine's capability for the final approach. However, during the approach, the engine shut down completely. In an attempt to land in an alternative field, the aircraft struck a line of trees separating two fields, resulting in substantial damage to the aircraft wings.
The investigation
The investigation focused on a recent maintenance action performed during a 50-hour inspection one day prior to the accident. A mechanic had identified a leak in the exhaust system at the connection between the manifold and the muffler. To address this, the mechanic applied high-temperature-resistant tape and an aluminium sheet secured with metal clamps. This repair was performed without consulting the primary maintenance workshop or utilizing official technical documentation, and the repair was not recorded in the aircraft logbooks.
Post-accident examination of the wreckage revealed that the aluminium sheet used in the repair had melted. This melting caused significant damage to the wiring located beneath the exhaust, specifically affecting the magneto grounding cables. Additionally, engine disassembly showed that the exhaust valve of the left front cylinder was unlubricated and discolored, and the exhaust muffler bellows showed signs of high-temperature oxidation.
Findings
- The primary cause of the engine power loss was the failure of the magneto grounding cables due to heat damage from the melting aluminium sheet.
- The repair was fundamentally inadequate because the aluminium sheet used had a much lower temperature resistance than the operating temperatures of the exhaust (500 to 700 °C).
- The pilot's decision to proceed with the towing flight despite being aware of the recent exhaust repair and a previous flight involving a smell of heat contributed to the accident.