What happened
On August 20, 2007, a Boeing 767-300 operating a scheduled flight from Chicago to Frankfurt experienced a serious malfunction. During the flight, the aircraft's Electronic Centralized Aircraft Monitor (EICAS) displayed an aft cargo overheat warning, which triggered the tripping of several circuit breakers, including those for the APU bleed power and cargo heating systems. Although the crew followed the relevant checklists, the error message persisted.
Upon landing at Frankfurt at 11:21 local time, the pilot started the Auxiliary Power Unit (APU) while taxiing to the gate. Shortly thereafter, the crew reported smoke and odors entering the passenger cabin. The smoke density increased significantly as the aircraft approached its parking position. The pilot notified ground personnel and requested emergency services and an accelerated evacuation. All 158 passengers and the crew successfully evacuated the aircraft via the main doors and an additional rear stairwell.
The investigation
The BFU investigation focused on identifying the source of the smoke and the cause of the electrical malfunctions. Firefighters used thermal imaging cameras to inspect the cabin, specifically focusing on the floor area near seat row 26. To access the area beneath the floor, the aft cargo compartment had to be manually unloaded because the electrical cargo handling system was inoperable due to power failures.
Investigators discovered an extinguished smoldering fire within the thermal insulation at the forward bulkhead of the aft cargo compartment. Upon inspecting the wiring harness in that area, investigators found evidence of a short circuit. Specifically, an insulated cable clamp had been worn through at two points, leading to charring and the welding of two adjacent cables.
Findings
The primary cause of the event was a short circuit caused by the relative movement of cables against an insulated cable clamp, which wore away the insulation.
Contributing factors included:
- The electrical circuit protection was not optimal, as the short circuit caused significant damage and fire before the breakers tripped.
- The intensification of the smoke occurred on the ground because, upon landing, additional electrical systems automatically switched on, increasing the current load and causing further sparking.
- The aircraft's maintenance program did not include periodic inspections or life-limit restrictions for these specific insulated cable clamps.
- The investigation noted that a similar fire had occurred on another Boeing 767-300 following maintenance work.