What happened
On 7 September 2011, a Boeing 757-21B, registration G-LSAI, was operating a commercial passenger flight from Leeds-Bradford to Larnaca, Cyprus. While cruising at FL3 90 in Bulgarian airspace, the aircraft experienced a loss of power to the left AC electrical bus, accompanied by multiple flight instrument failures. During this initial event, the crew observed a thin haze of smoke and a strong smell of electrical burning entering the flight deck, prompting them to don oxygen masks and goggles. The commander declared a MAYDAY and diverted the aircraft to Kavala Airport in Greece.
Approximately two hours and 20 minutes into the flight, while at FL3 80, the left AC bus lost power for a second time. Although no smoke or fumes were reported during this second occurrence, the crew was unable to restore power via the standard Quick Reference Handbook (QRH) procedures. The aircraft landed safely at Kavala without further incident, and the fumes had dissipated by the time the passengers disembarked.
The investigation
The investigation focused on the electrical generation system following the discovery of an open circuit during maintenance troubleshooting. Inspections of the wiring loom and the D1114J connector at the left pylon bulkhead revealed that a crimp terminal at pin 12 had become corroded and detached from its wiring. Chemical analysis of residue found within the connector showed high levels of copper, carbon, silicate, and oxygen, which is consistent with corrosion.
Further examination of the D1114J connector by the AAIB showed that the backshell was loose due to stripped threads. This defect allowed moisture to enter the connector and caused the wiring loom to vibrate, leading to mechanical damage at the attachment points. Additionally, the loss of continuity at pin 12 caused the generator's protection system to erroneously sense a fault, triggering the disconnection of the left integrated drive generator (IDG) from the bus.
Findings
- The intermittent disconnection of the left IDG from the left AC bus was caused by a corroded crimp terminal at the D1114J bulkhead connector.
- A loose connector backshell, caused by previous over-tightening, allowed moisture ingress and facilitated corrosion of internal components.
- The loose backshell also permitted vibration of the wiring loom, contributing to mechanical damage of the wires.
- The loss of electrical continuity at pin 12 caused the generator's differential protection logic to incorrectly trigger a disconnection.