What happened
On 26 February 2007, a BAE Systems Jetstream 4100, registration G-MAJI, was preparing for a commercial passenger flight at Durham Tees Valley Airport. While waiting to enter Runway 23 for departure, the co-pilot moved the flight controls gust lock lever to the unlocked position to perform a 'full and free' movement check. During this procedure, the co-pilot noticed that the control column movement was restricted when pulled aft of the neutral position. The commander subsequently verified that the control column consistently hit an apparent obstruction approximately three inches behind neutral. The crew taxied the aircraft back to the stand and handed the matter over to the engineering department.
The investigation
Investigators examined the aircraft's gust lock mechanism, which utilizes a lock pin that engages into a slot within a cable quadrant in the rear fuselage. The investigation established that the movement of the flight control lever caused the pin to remain engaged in the slot, effectively keeping the elevators in a locked state. This was attributed to a lack of sufficient lubrication in the mechanism. The investigation also noted that the existing maintenance schedule required lubrication of these components every 6,000 flight hours, which for this specific operator equated to roughly every four years.
Findings
- The primary cause of the control restriction was inadequate lubrication of the gust lock mechanism associated with the elevator circuit.
- The lack of lubrication caused the locking pin to 'hang up' in its slot, preventing the elevators from moving freely.
- The maintenance interval of 6,000 flight hours was insufficient to ensure the mechanism remained functional for this operator's usage patterns.
- The operator had previously experienced 19 similar occurrences involving the gust lock system, though many were related to the locks failing to engage rather than failing to disengage.