Incident Overview
On 2 July 2021, a Boeing 777-336ER (registration G-STBA) operated by a commercial air transport cargo carrier was preparing for departure at London Heathrow Airport when a fire cargo aft warning illuminated during pre-start procedures. The aircraft had 16 crew members on board and no passengers. The commander, aged 53, held an Airline Transport Pilot's Licence and had 17,000 total flying hours, with 10,300 hours on type.
Crew Response
Upon receiving the warning and associated fire bell, the crew initiated the fire cargo aft checklist from the Quick Reference Handbook (QRH). The checklist indicated that with cargo doors open, extinguishers should only be discharged if a fire was confirmed. The additional co-pilot was sent to investigate. He first observed the rear cargo door open with no visible fire or fumes, then went outside to liaise with loading crew. Due to the long airbridge jetty, significant time elapsed before reaching the ground.
Meanwhile, cabin crew reported acrid fumes in the passenger cabin. The commander paused the fire cargo aft checklist and began the smoke, fire, and fumes checklist. Shortly after, the warning indications cleared. A public address call of "Attention Crew! At Stations" was made, directing cabin crew to their allocated exit positions. Some crew moved to the aft cabin near the fumes. A PAN call was made to air traffic control requesting fire service assistance.
Evacuation and Injury
The additional co-pilot approached the rear cargo hold and saw two loading team members running clear, reporting acrid fumes in the hold. Rescue and Fire Fighting Service (RFFS) arrived, and the RFFS commander advised clearing the aircraft. The co-pilot passed this to the commander, who ordered a precautionary disembarkation via the airbridge at the left No 2 passenger door. The APU was shut down and the aircraft secure checklist completed. During disembarkation, one cabin crew member tripped on a cargo net securing cargo in the passenger cabin, sustaining minor injuries. Total time from warning to all crew off was approximately 8 minutes.
Cargo Investigation
RFFS examined the rear hold cargo and identified a refrigerated container as the source of fumes. The container battery pack was determined to be the origin. The container was taken to a warehouse and later returned to the manufacturer. The manufacturer's investigation reported that smoke resulted from improper handling causing a significant impact to the back of the container, leading to a short circuit between two batteries. The impact deformed the container and tore a bracket in the battery compartment, allowing batteries to shift sideways. This caused battery cables to rub against a secondary bracket, shearing and heating them, producing the short circuit. The manufacturer stated that the cables were "short circuit proof" and designed to produce smoke but no fire in such events, preventing further damage. Data showed the container was last charged without abnormalities between 0755 and 0839 hrs on the day of the incident.
Analysis and Safety Actions
The commander initially used the "Attention Crew! At Stations" call, but in retrospect considered a rapid disembarkation announcement more appropriate. The operator's standard rapid disembarkation PA was designed for passengers; the commander felt it might cause confusion with only crew on board. The container manufacturer decided to consider reinforcing battery attachment and potentially modifying the container. The operator decided to introduce comprehensive damage checks on all similar containers before loading.
Conclusion
The incident was caused by a short circuit in the battery pack of a refrigerated container in the aft cargo hold, leading to cable heating and smoke. The crew executed a precautionary disembarkation with one minor injury.