US investigators have disclosed that a Boeing 767-300 freighter, which experienced a jammed horizontal stabiliser en route to Hawaii, had undergone maintenance on the stabiliser trim actuator a month earlier.
The incident occurred on 25 April as the Amazon Prime Air aircraft was operating a service from Ontario, California, to Lihue.
As it descended towards Lihue, the autopilot “repeatedly disconnected”, says the US National Transportation Safety Board, and the crew could not resolve the issue.
The pilots took manual control at around 10,000ft and noticed that the considerable forward pressure on the yoke was required to maintain descent.
Having observed that the horizontal stabiliser remained at a position of 3.4 units, the crew attempted to use alternate electric trim, but this was unsuccessful.
The pilots levelled the aircraft at 3,000ft and deployed flaps to reduce the yoke force required.
Running stabiliser trim checklists failed to improve the situation, says the safety board in preliminary findings, and the crew opted to divert to Honolulu where the jet landed without further incident.
“The crew reported that they did not receive a [stabiliser trim alert]…message or the associated cockpit warning light at any point during the flight,” the safety board adds.
Neither of the two crew members was injured.
Investigators have determined that the freighter underwent scheduled maintenance, about a month beforehand, during which an overhauled horizontal stabiliser trim actuator was installed.
This actuator had been active on the aircraft for 11 cycles – a total of 31h – at the time of the event. It was removed after landing and sent to the manufacturer for examination.
“Flight data from the incident is being reviewed, and the investigation is ongoing,” says the safety board which, earlier this month, upgraded the significance of the occurrence.
