What happened
On July 12, 2010, a Bombardier CRJ700, registration F-GRZN, operated by Brit Air, experienced a series of technical malfunctions during a flight rotation involving Bilbao and Paris. During the initial climb from Bilbao, the crew noticed the autopilot could not maintain the engaged vertical mode, accompanied by an "AP PITCH TRIM" alert. The crew transitioned to manual flight and continued to the destination without further incident.
Following maintenance checks in Paris that failed to identify any faults, the aircraft performed a subsequent leg back to Bilbao. During this flight, the autopilot failure recurred. Upon the landing flare at Bilbao, the crew experienced unusual physical effort required to control the aircraft. After touchdown, the crew discovered that the elevator control had become stuck in a nose-up position during taxiing. Subsequent maintenance inspections revealed that the autopilot pitch-up control cable had snapped.
The investigation
The investigation focused on the mechanical failure of the control chain and the effectiveness of previous maintenance inspections. Investigators determined that the cable breakage occurred during an ILS approach at Bilbao, prior to the crew taking manual control. Analysis of the flight data recorder and a cockpit video recorded by the crew confirmed the occurrence of the "AP PATCH TRIM" alert.
Technical examination of the broken cable showed that the failure was caused by progressive fatigue. This fatigue was initiated by a manufacturing defect where the cable end was improperly positioned within the quadrant groove. This misalignment introduced abnormal bending stresses at the swaged end of the cable. Furthermore, the investigation looked into why the maintenance task (22-11-24-710-801) failed to detect the issue, noting that the task's instructions were potentially imprecise regarding how to verify elevator movement.
Findings
- The fatigue failure of the autopilot pitch-up cable was caused by the improper installation of the cable end in the quadrant groove, likely occurring during the aircraft's initial construction.
- The broken cable formed a loop that eventually jammed the autopilot servo-motor, leading to the observed stiffness in the elevator controls during the landing flare.
- Maintenance testing performed in Paris was unable to detect the degradation because the system was still capable of partial movement, and the specific maintenance task did not provide sufficient guidance for verifying full-range elevator travel.
- The cockpit video provided by the crew was instrumental in confirming the specific EICAS alert messages that were not explicitly detailed by the flight recorders.