What happened
On the night of February 16, 2000, a Swearingen SA 227 AC Metroliner, operated by a German cargo company, was conducting a flight from Luton, England, to Paderborn-Lippstadt, Germany. The crew, consisting of a captain, a co-pilot, and a loadmaster, was on final approach when the landing gear was extended.
During this phase of flight, the loadmaster, located in the cargo compartment, noticed smoke or mist within the cabin and alerted the flight crew. In response, the pilots utilized oxygen masks and continued the landing procedure without further difficulty. Once the aircraft had landed and the engines were shut down, the air in the cabin cleared. No injuries were reported, though the aircraft sustained minor damage.
The investigation
The BFU investigation was initiated after the aircraft underwent temporary repairs in Paderborn and was moved to the operator's maintenance facility. Investigators examined the aircraft's underfloor area and interviewed witnesses.
Laboratory analysis of the damaged components, conducted by the LKA Niedersachsen, revealed that an electrical cable (TLTLAV2-16) used for the toilet pump motor had been resting against a hydraulic line bracket for the nose gear. The investigation found that the cable insulation had been compromised, and the metal wall of the hydraulic pipe had a small crater, approximately 0.2 mm in diameter. Traces of melted copper particles were found on the hydraulic line, and hydraulic fluid had accumulated in the floor area, accompanied by singed cable sleeving.
Findings
- The investigation established that the primary cause was the improper routing and handling of electrical cables during a previous reconfiguration of the aircraft from a surveillance role to a passenger/cargo configuration.
- An electrical cable was in direct contact with a hydraulic line bracket, leading to insulation failure through either heat or vibration-induced chafing.
- The resulting electrical arcing between the bare copper conductor and the metal hydraulic pipe created a small crater in the pipe wall.
- This puncture allowed high-pressure hydraulic fluid (120 to 150 bar) to escape, creating a mist that entered the cabin.
- Although an aerosol of hydraulic fluid was present, a full-scale fire did not occur because the electrical energy from the short circuit was too low to ignite the fluid and was insufficient to trip the circuit breaker (CB542).