Synopsis
On 2 July 2020, an Agusta A109E helicopter, registration G-ETPJ, landed at Boscombe Down Airfield after a routine flight during which a circuit breaker had tripped. While an engineer investigated the cause, the next flight crew noticed a burning smell in the rear baggage bay during their walk-around checks. The operator's subsequent investigation found evidence of a fire in the aft equipment bay, along with a chafed electrical cable. The cable was part of a design change made while the helicopter was on the UK military register as a Military Registered Civil Owned Aircraft (MRCOA). A protective strip that should have been installed as part of that change was missing.
History of the Flight
The helicopter was on a routine flight when the main Flight Test Instrumentation (FTI) circuit breaker tripped. The crew did not reset it, and the flight continued normally for approximately 20 minutes before landing. After landing, a ground engineer with the batteries on reset the circuit breaker, which tripped again almost immediately. While the engineer was investigating, the next flight crew began a walk-around check. Upon opening the baggage bay, they smelled burning and, with the engineer, removed a panel to access the rear equipment bay. They found a smouldering fire, which self-extinguished before the airfield fire service arrived.
Investigation
The damage was most severe where electrical cables entered the equipment bay through an aperture in the adjacent structure. A single cable connecting the battery and FTI busbars was chafed, and the exposed conductor had been arcing with the equipment bay liner, a composite material. This circuit was protected by a 10 Amp circuit breaker, the same one that had tripped in flight. The FTI installation was designed and installed several years earlier when the helicopter was on the UK military register. Upon moving to the civil register, the design was reviewed and a Supplemental Type Certificate (STC) was issued by EASA. The installation drawings specified a protective strip around the aperture to prevent chafing, but the strip was missing when examined after the incident. The operator's other Agusta 109, G-ETPI, had a similar installation with the protective strip correctly fitted.
Conclusion
The investigation established that the electrical power supply cable between the battery busbar and the FTI busbar chaffed against the equipment bay liner, causing a short-circuit, arcing, and the circuit breaker to trip. The equipment bay liner was melted and burned. The operator concluded that it was probable that fire damage occurred in flight, assisted by airflow through the cable aperture. Had the required protective strip been installed, it should have prevented the cable damage.