What happened
On 10 March 2007, an Eurocopter EC1s5 B1, registration G-ISSV, was undergoing a rotors-running refuelling procedure at Norwich Airport. The aircraft, which is primarily used for transporting oil industry personnel to offshore platforms, had recently completed a routine flight to North Sea platforms.
During the refuelling process, the crew noticed a strong, antiseptic-like odour. Shortly thereafter, an engineer observed smoke and six-inch flames emerging from the area of the electric hoist's fixed electrical connector on the upper right side of the fuselage. The flames were being blown toward the engine intake. The commander immediately shut down the engines, which caused the fire to extinguish. No injuries were reported to the crew, and the fire was extinguished before emergency services arrived.
The investigation
Investigators examined the aircraft in a maintenance hangar and found fire damage to the hoist electrical connector and the oil cooler support fairing. The investigation focused on the electrical supply to the hoist, which remains live as long as the aircraft generators are operational, regardless of the mission selector switch position.
Technical analysis of the connector revealed evidence of arcing between the 28 volt DC power contact and the body of the connector. The investigation also identified that the environmental seal on the connector had been damaged by deep cuts, likely caused by being compressed against a sharp edge during assembly. Furthermore, the presence of verdigris (copper oxidation) on the pins suggested that moisture had entered the connector.
Findings
- The fire was caused by the 28 volt DC electric hoist power supply shorting to the body of the hoist’s fixed electrical connector and earthing through the carbon fibre composite fairing.
- The short circuit was likely triggered by moisture ingress into the connector due to a damaged environmental seal.
- A significant contributing factor was that the electrical supply to the connector remains live whenever the aircraft generators are on line, leaving the crew with no way to isolate the power supply in flight.
- The orientation of the connector, with power and earth contacts positioned lowermost, may have increased the risk of a short circuit if moisture was present.