A Safair Boeing 737-400, registration ZS-JRM performing flight FA-202 from Johannesburg to Cape Town (South Africa) with 130 passengers and 6 crew, had taxied, line up and accelerated for takeoff with all systems working normally. When the aircraft rotated for takeoff, the crew received a master caution and multiple system failure indications, the autopilots could not be engaged so that the first officer continued flying manually. The crew stopped the climb at 8000 feet, later climbed the aircraft to FL110 to escape low level turbulence. The crew found the #1 engine (CFM56) generator had become inoperative, however, the associated bus off light was not illuminated. The #1 engine transfer bus light illuminated. None of the circuit breakers had popped. The TCAS was found inoperative. The crew assessed all of the system failures were the result of the transfer bus being inoperative. The crew decided to return to Johannesburg. The crew did not declare emergency, emergency services were not put on stand by. The aircraft landed safely on Johannesburg's runway 03L about 25 minutes after departure.
South Africa's CAA (SACAA) reported a post flight inspection revealed the Generator Control Unit 2 had not been properly latched. The SACAA opened an investigation into the occurrence rated a serious incident.
A similiar occurrence had happened in the UK in 2018, see Report: West Atlantic B734 near East Midlands on Oct 12th 2018, electrical failures.
The SACAA released their final report concluding the probable cause of the incident was:
The investigation revealed that during rotation, the GCU of the No.1 engine had moved out of its rack, causing its electrical connectors to disconnect, resulting in multiple electrical system failures because of the broken lever latch of the GCU of the No.1 engine.
The SACAA reported the first officer (38, ATPL, 7,266 hours total, 137 hours on type) was pilot flying, the captain (50, ATPL, 17,673 hours total, 709 hours on type) was pilot monitoring.
The SACAA summarized the sequence of events:
During pre-flight, start-up, taxi and take-off roll, all systems operated normally. However, during rotation on Runway 03L, the master caution lights illuminated, showing multiple system failures. Also, the auto pilot (AP) 1 and 2 did not engage.
The crew retracted the landing gears and flaps before reading the after-take-off checklist. Thereafter, they advised air traffic control (ATC) that they were experiencing a problem and requested to maintain 8000 feet (ft), which the ATC approved. Shortly thereafter, the ATC offered a climb to flight level (FL) 110, which the crew accepted to clear low-level turbulence. To diagnose the problem, the crew consulted the Quick Reference Handbook (QRH) to follow and action its recommendations. The crew then checked all circuit breakers to see if they had popped, but they were all in order. They checked the auxiliary power unit (APU) generator and found that it was not latching onto the left alternating current (AC) bus. The crews diagnosis of the problem revealed that the number 1 engine generator failure and the transfer bus failure are the ones that had caused multiple system failures. The crew decided to use the systems still available to them to fly back to FAOR. The ATC cleared the aircraft for landing on Runway 03L. Emergency services were not dispatched as the crew felt that the aircraft was flying normally. The landing was uneventful until the landing roll when the captains flight instruments and communications 1 (COMM 1) failed. After shutdown, it was discovered that the lever latch of the generator control unit (GCU) of the No.1 engine was broken, and that the unit had moved out of its position.
The damage during the incident was only limited to the lever latch of the GCU of the No.1 engine; and none of the passengers were injured.
The SACAA analysed:
On 16 September 2019 during rotation, the aircraft experienced multiple electrical system failures because of the broken lever latch of the GCU of the No.1 engine. The electrical failures that occurred during rotation at FAOR were caused by the GCU of the No.1 engine moving out of its rack, disconnecting the electrical connectors. The disconnection of the GCU of the No. 1 engine resulted in the Gen 1; Gen Bus 1; Transfer Bus 1; 115V AC Electronic Bus 1; 28V DC Bus 1; and 28V DC Electronic Bus 1 becoming inoperative. Electrical System 2 would have still been powered by the right engine through Gen Bus 2.
The lever latch of the GCU of the No. 1 engine had developed a fatigue crack on its hook which was perpendicular to the loading direction, resulting in a shear lip before its final failure. The shear lip caused the unit to move out of its rack, resulting in electrical system 1 disconnection and multiple system failures controlled by the unit.
During multiple electrical system failures caused by the GCU of the No.1 engine moving out of its rack, the crew was still able to fly the aircraft back to FAOR without further incidents because the electrical system 2 was able to supply electrical power to the aircraft.
The GCU of the No.1 engine was fitted to the aircraft on 21 February 2019, seven months prior to the incident, and had accumulated 167 hours since its installation on the aircraft. There were no records of previous snags reported since the fitment of the GCU of the No. 1 engine to the aircraft.
It was not possible to determine when exactly did the latch lever hook failure occur as it had operated for 167 hours over a period of seven months. Also, there was no record of any maintenance work conducted around the area of the GCU of the No 1 engine post the maintenance carried out on 8 August 2019.