What happened
On 16 September 2019, a Boeing 737-400, registration ZS-JRM, departed O.R. Tambo International Aerodrome (FAOR) for a scheduled service to Cape Town. While the aircraft was in the rotation phase on Runway 03L, the crew observed the illumination of master caution lights and noted that both autopilot systems failed to engage.
Following the initial failures, the crew performed after-takeoff checklists and consulted the Quick Reference Handbook (QRH). They identified that the number 1 engine generator had failed and the transfer bus was inoperative, which subsequently triggered several other electrical system failures. To manage the situation, the crew requested to maintain a lower altitude before climbing to flight level 110 to avoid turbulence.
After assessing the aircraft's remaining operational systems, the crew decided to return to O.R. Tambo. During the landing roll at FAOR, the captain's flight instruments and primary communications failed. Upon shutting down the aircraft, a physical inspection revealed that the generator control unit (GCU) for the number 1 engine had shifted out of its mounting rack. There were no injuries among the 130 passengers or the 6 crew members on board.
The investigation
SACAA AIID investigators examined the mechanical state of the electrical components and the maintenance history of the aircraft. The investigation focused on the physical condition of the number 1 engine's Generator Control Unit (GCU).
Investigators found that the GCU had been installed on the aircraft approximately seven months prior, having accumulated 167 hours of operation since its installation. The aircraft had also undergone a C-check less than two months before the incident. The investigation looked into why the unit had moved from its designated position in the electronic equipment rack during the high-stress rotation phase of takeoff.
Findings
- The primary cause of the multiple electrical system failures was the broken lever latch on the number 1 engine's GCU.
- This latch had developed a fatigue crack on its hook, which eventually led to a structural failure (shear lip).
- The failure of the latch allowed the GCU to move forward out of its rack, which physically disconnected the electrical connectors.
- The loss of the number 1 generator and the transfer bus caused the loss of several electrical buses, including the 115V AC and 28V DC buses.
- The aircraft remained controllable and capable of returning to base because the electrical system 2 remained operational via the right engine generator.
Safety action
Following the investigation, the Director of Civil Aviation was advised to consider implementing a requirement for non-destructive testing (NDT) on the GCU lever latch mechanism whenever the unit is removed for bench testing. In response to the incident, the operator inspected the GCU latch mechanisms across its entire fleet and confirmed all were serviceable and free of cracks.