What happened
On July 23, 2020, a Piper PA-28RT-201T, registered HB-PNP, was conducting a local test flight at Bâle-Mulhouse airport following the installation of new avionics and an autopilot system. The flight was intended to verify the operation of the new equipment and familiarize the owner with the updated systems.
Approximately 15 minutes after takeoff, the pilot noticed an abnormal ammeter reading of nearly 70 A, significantly higher than the expected 10 to 30 A. Believing the high reading was simply due to a discharged battery being recharged, the crew continued the flight. During a climb to 7,000 ft, an electrical odor was detected in the cabin. Shortly thereafter, smoke and flames emerged from the luggage hold and spread into the cabin. The pilot initiated an emergency descent and managed to land on runway 33. During the high-speed landing roll, the occupants attempted to evacuate. The owner, seated in the front right, was fatally injured upon impact with the runway, while a passenger sustained severe burns.
The investigation
The BEA focused its investigation on the aircraft's electrical power system, specifically the area behind the luggage hold where the fire originated. Technical examinations of the wreckage revealed that the fire started in a compartment containing the battery, ventilation system, and the landing gear hydraulic pump.
Investigators examined a diode device (part number 79412-011) located between the battery and the alternator. The metal cooler of this device showed significant thermal damage, including melting of the aluminum alloy. Laboratory analysis using Energy Dispersable Spectrometry (EDS) confirmed that the heat was intense enough to melt the steel attaching screws and the fluorine-based insulating washers. Furthermore, testing on an identical aircraft revealed that worn or cracked insulating washers could allow the cooler to make electrical contact with the aircraft floor, creating a short-circuit.
Findings
- The primary cause of the fire was an electrical short-circuit caused by a loss of insulation between the diode device cooler and the aircraft floor.
- This short-circuit caused the alternator to deliver maximum power, leading to intense overheating that ignited the ABS plastic protective cover of the diode device.
- The crew's failure to recognize the ammeter spike as an electrical overload contributed to the progression of the fire, as they incorrectly assumed the high current was a normal battery charging process.
- The lack of information regarding the recent full recharge of the battery meant the crew did not realize the high ammeter reading was an anomaly.
- The ABS plastic cover acted as an aggravating factor by facilitating the spread of the fire.
Safety action
- The BEA has recommended that Piper implement a maintenance procedure to ensure the continued integrity of the diode device insulating washers.
- A recommendation was made for Piper to assess replacing the ABS cover with a more fire-resistant material.
- The BEA also recommended that EASA and the FAA work together to ensure manufacturers control the risk of fire resulting from short-circuits at diode device attachment points.