What happened
On May 29, 2013, a privately operated Piper PA 28RT-201T, registration N68XM, departed Gap Tallard airport for Mulhouse Habsheim. Shortly after takeoff, the pilot requested an ascent to FL80 to clear cloud cover. Approximately seven minutes into the flight, the pilot contacted Marseille Information to report a suspected onboard fire and declared an emergency, stating an intention to return to Gap.
During the emergency descent, the pilot's communications became increasingly difficult. After reporting an altitude of 4,000 ft, the pilot indicated an intention to perform an emergency landing. Subsequent attempts by air traffic control to establish contact were unsuccessful. The aircraft eventually struck the tops of trees near a golf course at Col Bayard before crashing into the ground. The pilot was killed in the accident, and the aircraft was destroyed.
The investigation
The investigation focused on the condition of the engine and recent maintenance performed on the aircraft. Investigators found evidence of fire damage on the rear engine firewall and associated components. A metal fitting connecting the flexible fuel line from the electric pump to the rigid line leading to the fuel regulator was found loose, with only three threads engaged.
Testing confirmed that this loose fitting caused a significant fuel leak under pressure, spraying fuel onto hot engine components. The investigation also reviewed recent maintenance work conducted a month prior, which involved replacing several cylinders. The technician involved, an FAA-licensed mechanic, had limited experience with this specific aircraft type and Continental engines. It was noted that the mechanic worked in a restricted space with suboptimal lighting and was occasionally interrupted during tasks.
Findings
- The primary cause of the accident was a fuel leak caused by an insufficiently tightened fitting during recent maintenance.
- Vibration during flight likely exacerbated the looseness of the fitting, leading to the leak.
- The leaking fuel ignited upon contact with hot engine parts, creating a localized fire and producing smoke that entered the cabin.
- The fuel leak also caused a drop in fuel pressure at the injectors, resulting in a loss of engine power.
- While the pilot likely followed electrical fire procedures, the presence of smoke in the cockpit may have hindered the management of the emergency landing.
- The failure to follow the engine manufacturer's recommended flight check procedures after cylinder replacement may have prevented the detection of the developing anomaly.