What happened
On 2 July 2020, an Agusta A109E helicopter, registration G-ETPJ, was conducting a routine flight when the circuit breaker for the Flight Test Instrumentation (FTI) tripped. The crew elected not to reset the breaker and continued the flight, landing at Boscombe Down Airfield approximately 20 minutes later.
Following the landing, a ground engineer attempted to reset the circuit breaker with the batteries active, but the component tripped again immediately. During simultaneous walk-around inspections by the next flight crew, a burning odor was detected within the rear baggage bay. Upon removing the access panel to the aft equipment bay, the crew and engineer discovered a smouldering fire, which extinguished itself prior to the arrival of the airfield fire service.
The investigation
The AAIB examined the damage within the equipment bay, noting that the most significant thermal damage was concentrated where electrical cables passed through an aperture in the structure. Investigators identified a single cable, which supplied power between the battery and FTI busbars, had become chafed. This chafing allowed the exposed conductor to arc against the composite liner of the equipment bay.
Regarding the origin of the installation, the FTI system had been implemented while the aircraft was previously on the UK military register. When the aircraft transitioned to the civil register, the design underwent a review and received an EASA Supplemental Type Certificate (STC). The installation drawings for this design change specified the use of a protective strip around the aperture to prevent cable contact with the structure. However, during the post-incident inspection, this protective strip was found to be absent.
An inspection of the operator's other Agusta A109E, registration G-ETPI, which featured a similar FTI setup, confirmed that the required protective strip was correctly installed on that aircraft.
Findings
- The electrical cable connecting the battery and FTI busbars chafed against the equipment bay liner.
- This chafing caused electrical arcing and a short circuit, which triggered the 10 Amp circuit breaker to trip.
- The absence of a required protective strip allowed the cable to contact the adjacent structure.
- It is probable that the fire began during the flight, potentially aided by airflow through the cable aperture.