What happened
On 2 July 2021, during pre-start procedures at London Heathrow Airport, a Boeing 777-336ER, registration G-STBA, triggered a fire cargo aft warning and an audible fire bell. The aircraft was operating a cargo flight with a crew of 16 and no passengers.
Following the warning, the crew initiated the relevant Quick Reference Handbook (QRH) checklists. An additional pilot investigated the rear of the aircraft, initially finding no visible fire or fumes. However, cabin crew subsequently reported acrid fumes within the passenger cabin. The commander transitioned to the smoke, fire, and fumes checklist. During the incident, a public address call, "Attention Crew! At Stations," was used to alert the crew, which resulted in some members moving toward the rear of the aircraft near the source of the fumes.
Upon the advice of the London Heathrow Fire and Rescue Service (RFFS), the commander ordered a precautionary disembarkation via the airbridge. During the evacuation, one cabin crew member sustained minor injuries after tripping on a cargo net. The entire crew successfully exited the aircraft within approximately eight minutes of the initial warning.
The investigation
Investigators examined the cargo hold and identified a refrigerated container as the source of the smoke. Upon inspection of the container by the RFFS and the manufacturer, it was discovered that the battery pack had been compromised.
Technical analysis revealed that the container had sustained a significant impact during handling. This collision caused a bracket within the battery compartment to tear from its mounting, allowing the batteries to shift. This movement caused the battery cables to rub against a secondary bracket, resulting in a short circuit. The heating of these cables produced the smoke that triggered the aircraft's detection systems.
Findings
- The fire cargo aft warning was triggered by smoke produced by heating cables within a refrigerated container's battery pack.
- A short circuit in the container's battery pack was caused by physical damage to a mounting bracket resulting from an impact during container handling.
- The aircraft's electrical safety systems functioned as designed, as the cables were engineered to produce smoke rather than an open flame during a short circuit.
- The use of the "Attention Crew! At Stations" announcement resulted in crew members moving toward the area containing fumes.