Accident to the Piper PA28 registered HB-PNP on 23/07/2020 at Basle-Mulhouse

At the end of the work to install a new GARMIN avionics suite and an autopilot, the ground tests took place normally. The GARMIN STC did not require a check flight; the Approval for Return to Service (ARS) was signed by a  technician from the maintenance workshop.

The owner of the aeroplane wanted to check that the newly installed systems were operating correctly and to familiarize himself with their use. He therefore asked an instructor pilot who was acquainted with the avionics suite installed, to carry out a first flight with him. They were accompanied by an electronics technician who had taken part in the work.

During the flight, the ammeter’s abnormally high indication of around 70 A (corresponding to the maximum mark of the ammeter) and more than 30 A higher than the normal values was not interpreted by the pilot and passengers as a sign of an electrical failure (overload). They thought that it was the consequence of the battery charging, believing that the latter was
partially discharged. 

The electrical overload procedure was therefore not considered and the flight was continued. The persons on board did not know that the battery had been totally recharged the day before. This lack of information may have contributed to their erroneous interpretation of the electrical overload situation indicated by the ammeter.

All of the damage observed and the information collected during the various examinations and tests indicate that incorrect insulation at the attaching point of the diode device cooler very probably resulted in an electrical continuity between this cooler and the aeroplane floor. As a result of this short-circuit, the aeroplane's alternator delivered its maximum power, explaining the excessive current observed by the pilot and his passengers during the event.

This excessive current very probably led to intense overheating of the diode device in the area of the short-circuit followed by the ignition of the ABS plastic protective cover. The flames then spread inside the compartment and the luggage hold. The tests showed that the ABS cover was an aggravating factor in the spread of the fire.

As soon as the persons on board smelt the “electrical” smell, the pilot complied with the in-flight fire emergency procedure and decided to land as quickly as possible. The flames moved into the cabin a short time later. The pilot and passengers evacuated the aeroplane during the landing run carried out at a high speed. The fire extinguisher located under the front right seat was not used. It is, however, very probable that its use would not have reached the source of the fire. There is a possibility that it might have slowed down the speed at which it spread.

Based on the elements collected during the investigation, it is not possible to say at what exact point in time the diode device cover ignited. As a consequence, the investigation cannot affirm that compliance with the overload emergency procedure after the pilot observed the ammeter indication would have prevented the fire. The latter might have started before the overload was observed around fifteen minutes after taking off.

The electrical consumption tests carried out at the end of the work and the ground tests carried out before the accident flight had proceeded without any malfunction. The latter, carried out with the GPU connected, would not have shown such a short-circuit as the alternator was not used. 

The personnel who carried out the work to install the new avionics suite stated that the diode device was not disassembled nor was it checked. The review of the aeroplane’s maintenance documentation indicated that no maintenance action (checks or disassembly) had been carried out on this diode device. Piper does not require any maintenance action on this equipment during the aeroplane’s service life. The examinations carried out found that the four insulating washers and four screws attaching the device to the floor of the aeroplane were present when the overheating occurred. Observations on another aeroplane of the same type equipped with a similar diode device found cracks and deformations of the insulating washer. 

The investigation was not able to determine the exact cause of the incorrect insulation between the diode device cooler and the aeroplane floor which led to the short-circuit. It might be explained by a combination of the following:

• wear of the insulating washers and,
• indirect consequence of the work carried out during the installation of the new avionics (device being accidentally knocked, introduction of ferrous residue during drilling operations, etc.).