What happened
On 18 October 2023, a Boeing 737-4K5 cargo aircraft, registration G-JMCZ, was operating a return flight to Belfast International Airport. After landing and taxiing to a holding position, the crew initiated after-landing procedures, which included starting the Auxiliary Power Unit (APU) to prepare for engine shutdown.
While the aircraft was stationary, the crew began transferring electrical power from the engine-driven generators to the APU generator. Upon transferring the right bus to the APU source, a loud noise, a flash of light, and fumes were observed. The crew noted an orange glow and what appeared to be flames emerging from behind the circuit breaker panel located behind the co-pilot's seat, accompanied by a strong smell of burning.
In response, the commander immediately transferred electrical power back to the engines. The co-pilot retrieved a handheld fire extinguisher and discharged it into the panel area. The crew notified Air Traffic Control and requested the airport fire service. After the extinguisher was used, the crew observed no further signs of fire or smoke and proceeded to their parking stand, where the fire service subsequently inspected the aircraft.
The investigation
Investigators examined the circuit breaker panel and identified that the R61 contactor, which provides power to the forward galley, had melted. The investigation found that the contactor's housing had been breached and its input terminals were destroyed. There was evidence of heat damage, including blistering paint on the underside of the compartment floor.
Technical analysis focused on the electrical transition that occurred during the incident. The investigation established that the aircraft's protection systems functioned as intended, as the associated circuit breaker tripped, successfully isolating the faulty component. No other issues were found within the aircraft's electrical systems during subsequent troubleshooting.