What happened
On 2 August 2013, a Boeing 737-377, registration G-CELF, departed Leeds Bradford Airport for Amsterdam Schiphol. Shortly after liftoff, the aircraft experienced a series of electrical malfunctions. The crew noted the disengagement of the auto-throttle, followed by the failure of the captain's primary flight instruments, including the electronic attitude director indicator, altimeter, and vertical speed indicator. The flight management computer also became inoperable.
During the initial climb, a distinct smell of electrical burning was reported in the passenger cabin. The crew identified that several circuit breakers had tripped, including those for the battery charger and hydraulic pump. Following a NITS briefing to the cabin crew, the commander requested an immediate return to Leeds Bradford. During the approach, the No 1 generator eventually tripped offline, which temporarily restored power to the essential flight instruments via the APU generator and the No 2 generator bus.
The investigation
An AAIB field investigation focused on the electrical system and the physical condition of the generator harness. Investigators discovered that the red phase ‘A’ ground cable had separated from the engine terminal stud due to a failure of its terminal lug. Further inspection of the harness revealed that other lugs were also cracked and nearing failure. The investigation also examined the battery charger, which had an overheated coil caused by excessive current draw in the remaining phases.
Metallurgical analysis of the failed lug revealed that fatigue cracks had initiated at multiple sites within grooves on the lug's surface. The study also involved examining the manufacturing process of the lugs and the impact of bending the components during maintenance.
Findings
- The primary cause of the electrical failure was the fatigue failure of the terminal lug on the No 1 generator phase ‘A’ ground cable.
- The failure was likely driven by a combination of high-frequency engine vibrations, a rough surface finish on the lugs caused by the manufacturing tool, and the effects of bending the lugs during installation.
- The loss of phase ‘A’ caused excessive current to flow through the remaining phases, leading to the overheating of the battery charger and the tripping of several circuit breakers.
- The aircraft's electrical system design lacked a mechanism to detect a single loss of phase, meaning the crew had no specific warning or checklist for this specific failure condition.