What happened
On August 19, 2003, during the approach to runway 25L at Frankfurt/Main, passengers noticed white smoke emerging from behind the lower left cabin sidewall near row 24. The smoke was reported to the cabin crew, who discovered an active fire source near passenger seat 24A. The crew immediately removed the lower sidewall paneling and used two fire extinguishers to suppress the flames.
Because all cockpit instruments remained within normal operating parameters and no odor was detected in the flight deck, the flight crew declared an emergency but proceeded with the landing. Following a successful landing, no further smoke development was observed, and the aircraft taxied to its parking position for a standard passenger disembarkation.
The investigation
The BFU examined the area of the fire and found significant damage to a connector assembly, specifically the end resistance plug (P/N RD-AA2902) and its corresponding socket (Eqm-No. D19003J). The investigation revealed that pins 1 and 2 of the plug had melted and were missing, while the socket showed evidence of electrical arcing and oxidation. The surrounding thermal insulation mats also showed signs of fire damage.
The investigation focused on the in-seat video entertainment system, which had been retrofitted into the aircraft. The BFU analyzed the electrical path from the Area Distribution Box (ADB) to the seat electronics. The technical examination of the connector components and the insulation mats was conducted in a laboratory setting. The investigation also looked into the maintenance history of the seat configuration, noting that the connector had been relocated during a cabin reconfiguration in July 2003.
Findings
Moisture ingress into the connector was the primary cause of the incident. The connector assembly was located in an area of the cabin sidewall where condensation is expected to accumulate. Because both the plug and the socket were left unsealed on their rear sides, moisture and accumulated dirt were able to penetrate the connection.
This ingress led to a short circuit between pins 1 and 2, which carried a permanent 115V AC supply. The resulting electrical arc was powerful enough to burn a hole through the metal housing of the plug, igniting the nearby insulation film. While the insulation mats met the fire safety standards in place at the time of the aircraft's certification, the intense heat of the arc likely exceeded those testing parameters.
Safety action
To prevent recurrence, the German aviation authority (LBA) issued Airworthiness Directive (LTA) No. 2003-270. This directive mandated inspections and modifications for all Boeing 747-400 aircraft in the operator's fleet. The required engineering orders involved sealing the rear of the plug and socket with silicone and inserting a silicone washer to ensure a moisture-tight seal. The operator also implemented temporary measures to de-energize the wiring harness in the affected area until the permanent modifications could be completed.