What happened
On the evening of 10 September 2013, a Boeing 737-300, registration ZS-SMJ, was conducting a scheduled cargo flight from Cape Town International Airport to Port Elizabeth Airport. While climbing through flight level 170, the captain experienced a failure of several primary flight instruments. Shortly after, the crew detected a burning odor and observed smoke emerging from the circuit breaker panel located behind and to the left of the captain's seat.
Upon noticing that circuit breakers C498 and C425 had tripped, the crew declared a mayday and initiated a diversion back to Cape Town. Although the smoke began to dissipate within minutes, the captain's primary instruments remained non-functional for the duration of the flight. The crew performed a monitored approach, with the first officer handling the approach to minimums before the captain took control for a visual landing. The aircraft landed safely without damage, and no injuries were sustained by the crew.
The investigation
Following the incident, technicians performed extensive troubleshooting on the aircraft. Initial efforts focused on the central air data computer auto-transformer T98, as it was suspected that a short circuit or low resistance might have caused the component to overheat. However, replacing the transformer did not restore the failed instruments, and subsequent testing of both the original and the replacement unit showed no signs of overheating or malfunction.
Investigators also examined the wiring associated with the digital flight data recorder (DFDR) upgrade. A potential issue was traced to plug D4449P, which involved wires that had been previously disconnected and stowed during a supplemental type certificate installation. While this area was inspected, the defect could not be replicated during ground testing.
Findings
- The electrical system failure was the primary reason for the diversion, though the specific origin of the smoke and the cause of the failure could not be determined.
- The crew acted promptly by declaring an emergency and following the 'Smoke, fire or fumes' checklist.
- The aircraft was fully airworthy and maintained according to all regulatory requirements at the time of departure.
- The primary flight instruments remained inoperative for the remainder of the flight, necessitating a monitored approach technique.