Back to home pageZS-SJD Stab out of Trim Warning

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06 Dec 2020 - The South African CAA have issued their final report into the Stab out of trim incident on 2 Sep 2019 to 737-800, ZS-SJD (28829/582), FF 1/6/2000 (20 Years old), operated by Mango.

The crew reported that just before the aircraft levelled off at Flight Level (FL) 360 overhead Bothaville (Free State Province), the “stab out of trim” light illuminated in the instrument panel and remained on. The crew actioned the recommendation stated in the Quick Reference Handbook (QRH) by disengaging the autopilot. Following the disengagement of the autopilot, the aircraft’s nose pitched down, indicating a significant horizontal stabiliser out of trim condition. Again, the crew referenced the QRH and followed the instructions to override the horizontal stabiliser electric motor clutch by applying force to the trim wheels to keep it in the set position; thereafter, manual horizontal stabiliser trim was achieved. The crew stated that they were able to control the aircraft, but the automated horizontal stabiliser electrical trim was not responsive. The pilot-incommand (PIC) advised the Johannesburg radar controller that the autopilot had been disengaged and that they were no longer compliant with the requirements of the Reduced Vertical Separation Minimum (RVSM). The radar controller then cleared the aircraft to descent to FL280 (28000 feet). After considering that FL280 will have an impact on fuel planning, the crew declared an emergency by broadcasting a “PAN PAN PAN” on frequency 128.3 megahertz (MHz) and requested to return to FAOR. The radar controller approved their request and provided the crew with vectors back to FAOR. Emergency services were requested to be on standby during the landing phase as the maximum flap setting was limited to 15°. The crew carried out an instrument landing system (ILS) approach with the aircraft trimmed manually. The aircraft landed safely on Runway 03L at 0649Z. The aircraft was not damaged, and no occupants were injured during the incident.

The investigation revealed that whilst levelling off at top of climb, the elevator continued to move in the nose up direction without the stabiliser moving in unison, causing the “stab out of trim” light to illuminate. After the crew had disconnected the autopilot, the elevator moved further to a nose up position (original position) in response to the manual column input. The cause of the stab trim not moving with the elevator could not be determined as all tests conducted on the system were satisfactory.

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*** Updated 23 Nov 2020 ***

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