South African investigators found an engine generator control unit to be unlatched after a Boeing 737-400 suffered a series of system failures on departure from Johannesburg last month.
The Safair aircraft (ZS-JRM), with 130 passengers and six crew members, had taken off for Cape Town on 16 September.
But while all systems had operated normally before departure, master caution lights indicating multiple failures illuminated on rotation.
Neither autopilot would engage which meant the first officer had to fly the aircraft manually while the crew sought to diagnose the problem, says the South African Civil Aviation Authority. It adds that the collision-avoidance system was also inoperative.
The pilots accepted a climb to 11,000ft – to escape turbulence at low altitude – and they assessed that, although both engines were running normally, the left-hand engine generator and its associated transfer bus were not functioning.
They believed that all other failures had resulted from the malfunction of the transfer bus.
No circuit-breakers had triggered and the crew was unable to remedy the situation through quick-reference handbook procedures.
Pressurisation had switched to the standby system and the crew opted to return to Johannesburg. The aircraft subsequently landed safely on runway 03L, after 28min airborne, but all the captain's flight instruments and a communications system failed during the landing roll.
Subsequent inspection of the aircraft found that the right-hand generator control unit had not been properly latched.
Disconnection of this generator would have resulted in loss of the right-hand AC power main bus, its associated transfer bus, and other systems.
The inquiry attributes the initial master caution on rotation to the improperly-latched right-hand generator control unit.
But it says the investigation is continuing and will look into "other aspects of this occurrence". None of the occupants was injured.
Cirium fleets data shows the aircraft involved was originally delivered to Alaska Airlines in 1998 and leased in by Safair in 2016.
Swedish-UK freight specialist West Atlantic experienced a similar multiple failure on a 737-400 in October last year, which was traced to incorrect racking of the right-hand generator control unit.
This aircraft had departed Leipzig will an inoperative left-hand generator – under deferred defect procedures – but encountered problems when the right-hand generator control unit disconnected during flight.