South African investigators are probing the service history of a horizontal stabiliser trim motor after a serious out-of-trim incident occurred to a Boeing 737-800 approaching cruise altitude.

After taking off from Johannesburg bound for Cape Town on 2 September, the Mango aircraft was preparing to level off at 36,000ft in the vicinity of Kimberley, some 250nm southwest of its departure point.

Preliminary findings by the South African Civil Aviation Authority show that an out-of-trim stabiliser indicator illuminated, and the crew responded by disengaging the autopilot after consulting the quick-reference handbook.

But the aircraft then pitched nose-down, suggesting that a “significant” out-of-trim situation had developed, says the inquiry.

The electric stabiliser trim “did not work”, according to the crew, and the pilots had to control the aircraft by trimming the stabiliser manually – although the trim wheel would “jump back” to its original position after being turned.

Boeing’s 737 crew operations manual states that “grasping” the trim wheel will stop stabiliser motion, and that certain flight conditions might require more effort to rotate the wheel.

The pilots overrode the electric motor by applying more force to the trim wheels, and managed to restore stabiliser trim.

Owing to the disengagement of the autopilot and the trim problem the aircraft could no longer comply with reduced vertical separation minima criteria, which would have allowed it to operate in airspace with 1,000ft intervals between cruise altitudes.

It had to descend to 28,000ft and this lower altitude affected its fuel calculations, prompting the crew to decide to return to Johannesburg, where emergency services were put on standby. The aircraft landed safely on runway 03L some 55min after departure.

Investigators have determined that the aircraft (ZS-SJD) had been experiencing in-flight oscillations and underwent maintenance activities during May-July this year.

The inquiry adds that, in early August, the stabiliser trim motor was removed in order to service another Mango 737-800 (ZS-SJA) and a replacement was installed two days later.

It states that the service history of this replacement trim motor, at this preliminary stage of the inquiry, “could not be determined with certainty”.

“We will be looking into the history of the horizontal stabiliser trim motor and other aspects which may have contributed to the failure of the motor, which may or may not have safety implications,” it adds.

None of the 141 passengers and six crew members was injured. The aircraft involved – originally delivered to South African Airways in 2000 – also underwent repairs to its horizontal stabiliser in April 2017 after suffering damage during an apparent large bird-strike.