What happened
On 24 August 2010, an Airbus A321-231, registration G-MEDJ, was conducting a scheduled commercial passenger flight from Khartoum, Sudan, to Beirut, Lebanon. While cruising at Flight Level 360 over northern Sudan under night instrument meteorological conditions, the aircraft experienced a series of severe electrical malfunctions.
The flight crew reported that the captain's primary flight displays, navigation displays, and ECAM units began to flicker, grey out, or go blank. Simultaneously, a continuous chattering sound was heard from the rear circuit breaker panels. The aircraft also experienced an uncommanded rudder trim movement, which caused the plane to adopt a left-wing-low attitude and deviate approximately 20 nm from its intended track.
During the event, the crew observed an 'ELEC GEN 1 FAULT' message. Upon selecting the No 1 generator to the OFF position, the electrical symptoms and the chattering noise ceased, though the left-wing-low attitude persisted. The crew eventually reset the generator to ON, which caused the symptoms to return, prompting them to switch the generator OFF again. The aircraft was subsequently flown manually for the remainder of the flight and landed safely in Beirut with no injuries to the 7 crew members or 4 and 42 passengers on board.
The investigation
Because the incident was reported several weeks after the occurrence, the AAIB was unable to access data from the cockpit voice or flight data recorders, as the information had been overwritten. Investigators instead relied on the operator's Flight Data Monitoring (FDM) programme and a Post-Flight Report.
Technical examination focused on the No 1 generator's Integrated Drive Generator (IDG) and its associated jumper lead. Investigators discovered that the jumper lead had sustained significant damage, with the outer protective layer worn down. Forensic analysis using a scanning electron microscope revealed evidence of arcing on one of the internal wires.
Testing on a manufacturer's 'iron bird' test rig demonstrated that simulating electrical shorts in the jumper lead could produce symptoms similar to those reported by the crew, specifically flickering displays and incremental rudder trim offsets, provided the co-pilot's displays were powered by DMC 3.
Findings
- The primary cause of the electrical malfunctions was an electrical fault within the No 1 generator's jumper lead, where the wire braid had chafed against the internal insulated wires.
- This electrical instability caused intermittent power interruptions, leading to the flickering and blanking of the captain's electronic flight instrument displays.
- The electrical transients caused the Flight Augmentation Computers to reset, resulting in uncommanded rudder trim inputs that induced the aircraft's leftward roll and track deviation.
- The crew lacked specific procedures to diagnose the uncommanded rudder trim, as the existing checklists did not account for this specific symptom.