What happened
On a morning departure from St. John's, Newfoundland, an Air Labrador DeHavilland Dash-8 experienced significant flight control difficulties immediately following rotation. While the pilot flying was executing the take-off, they noted unusual back pressure on the control column and an excessive pitch-up attitude. Efforts to counteract this by applying nose-down trim were unsuccessful, as the elevator trim control wheel became jammed in the full nose-down position.
The captain took control of the aircraft and, after leveling off at 5,000 feet, applied heavy force to the trim wheel. This force freed the mechanism, which was accompanied by a sudden nose-down pitch and the return of normal control column forces. The crew notified Gander ACC and subsequently requested and received priority for a return to St. John's, landing safely without further incident.
Prior to departure, the aircraft had been de-iced using Type I fluid. Although the crew performed a pre-take-off inspection of the wings and spoilers, the aircraft had been exposed to freezing rain and fog during its time on the ramp and during the return flight.
The investigation
Investigators examined the aircraft's elevator control system and found no mechanical deficiencies. Testing performed in a hangar demonstrated that the trim wheel could indeed jam when moved to the full nose-down position, but could be freed using the high breakout force applied by the captain, consistent with existing manufacturer safety supplements.
Technical analysis focused on the spring-tab mechanism, which is designed to reduce the physical effort required by the pilot to move the elevator. The investigation explored whether the increased back pressure was caused by the spring tab being unable to move. While the de-icing process was found to be performed correctly by qualified personnel, the investigation considered that residual ice or snow might have remained in the gap between the spring tab and the elevator, or that new contamination accumulated after de-icing.
Findings
- The movement of the elevator spring tab was restricted, likely due to ice or frozen snow contamination obstructing the gap between the tab and the elevator.
- This obstruction caused the abnormal back pressure and subsequent jamming of the elevator trim control wheel.
- The pilot's use of high breakout force to release the jammed trim was in accordance with established safety procedures.