Incident
On August 20, 2007, a Boeing 767-300 was taxiing to the gate at Frankfurt after a scheduled flight from Chicago. During taxi, heavy smoke developed in the passenger cabin. All 158 passengers and the crew evacuated the aircraft at the gate without injury. The aircraft sustained light damage.
Flight and Smoke Development
Five and a half hours after departure, the Electronic Centralized Aircraft Monitor (EICAS) displayed the message "AFT CARGO OVHT" during flight. In conjunction, several circuit breakers tripped: BULK CARGO HEAT OVRD, CARGO HEAT AFT CONT, CARGO HEAT OVRD AFT, and APU BLEED PWR. Switching the AFT CARGO HEAT switch per the checklist did not clear the fault. The aircraft landed uneventfully at 11:21 local time. On the way to the gate, the pilot started the auxiliary power unit (APU). Shortly after, the cabin crew reported smoke and odor. The smoke intensified until arrival at the gate. The pilot informed ground personnel, requested fire department notification, and expedited passenger deplaning. Visibility in the cabin was significantly reduced when the pilot left the cockpit.
Fire Department Response and Findings
The fire department arrived as passengers deplaned quickly, some without hand luggage, using an additional stair at the rear door. Firefighters observed smoke emanating from the ventilation duct in the foot area of seat row 26 on the right side. A thermal imaging camera found no fire source near seat row 26. To access the area under row 26, the aft cargo compartment had to be unloaded. After removing side panels of the cargo compartment wall, a smoldering fire was discovered in the thermal insulation at the forward bulkhead of the cargo compartment. The fire had self-extinguished.
Damage to Aircraft
A damaged cable clamp showed evidence of a short circuit between the clamp and current-carrying wires. The insulation of the clamp was chafed in two places with soot traces. Two wires in the cable bundle adjacent to the clamp were fused. The affected systems included: AFT CARGO HEAT SYS, BULK CARGO HEAT SYS, VACUUM BLOWER SYS, Lavatory Flush Control SYS 2, APU Air Supply Shutoff Valve, Aft Cargo Handling SYS, Eppl Wires for seat row 28HJ, SATCOM System, and APU Battery Charger SYS. The smoldering fire in the insulation was below the cable damage site.
Causes
The immediate causes of the serious incident were: a short circuit in flight that tripped the aft cargo heat and APU power circuit breakers; chafed insulation of a cable clamp causing a short-circuit fire in a cable bundle behind the aft cargo compartment forward panel; after landing, automatic activation of additional systems caused a second short-circuit fire; this fire ignited a smoldering fire in insulation mats, producing heavy smoke. The systemic causes were: non-optimal electrical circuit protection allowed a short circuit leading to system damage and smoldering fire; the aircraft maintenance program did not include periodic inspection or life limitation for insulated cable clamps.
Recommendations
The investigation recommended that the FAA require Boeing to: (1) check and modify circuit protection for power supplies running through the area forward of the aft cargo compartment; (2) replace insulated cable clamps with plastic clamps in the area forward of the aft cargo compartment.