What happened
On 2 September 2019, a Boeing 737-800, registration ZS-SJD, was performing a scheduled domestic service from O.R. Tambo International Aerodrome to Cape Town International Aerodrome. The aircraft was carrying 141 passengers and 6 crew members.
As the aircraft was leveling off at Flight Level 360 near Bothaville, the crew observed the "stab out of trim" warning light illuminate. Following established procedures in the Quick Reference Handbook, the pilots disengaged the autopilot. Immediately after disengagement, the aircraft experienced a significant nose-down pitch. The crew managed to regain control by manually overriding the horizontal stabilizer electric motor clutch, which allowed them to achieve manual trim.
Due to the loss of automated trim and the inability to maintain RVSM-compliant altitude, the pilot-in-command declared a "PAN PAN" emergency and requested an immediate return to Johannesburg. Because the flight control issue limited the maximum flap setting to 15 degrees, emergency services were requested to be on standby. The aircraft landed safely on Runway 03L at 0649Z with no injuries to the 147 people on board.
The investigation
SACAA AIID investigators examined the flight control systems and the aircraft's maintenance history. The investigation focused on why the horizontal stabilizer failed to move in synchronization with the elevator. Testing was performed on the recovered stabilizer trim motor (STM), but the results were satisfactory, with no specific fault identified in the component itself.
Investigators also reviewed the maintenance records of the operator's fleet. It was discovered that several other aircraft in the fleet were fitted with STMs that had not yet undergone a specific required modification (status level 9) as outlined in a service letter. Following the incident, the operator removed these unmodified units from other aircraft to undergo the necessary upgrades.
Findings
- The primary cause of the incident was that the elevator continued to move in a nose-up direction without the horizontal stabilizer moving in unison, which triggered the out-of-trim warning.
- Upon disengaging the autopilot, manual column inputs caused the elevator to move further into a nose-up position, exacerbating the trim imbalance.
- The specific mechanical or electrical reason why the stabilizer failed to track the elevator movement could not be determined, as all subsequent system tests were successful.
Safety action
- The operator has since implemented a fleet-wide inspection and removed all STMs that had not reached the required modification status level 9.
- A recommendation was made to ensure maintenance personnel accurately record all required data fields following the testing of critical components and systems.