What happened
While climbing through flight level 3/10, the crew of a Canadian Airlines Boeing 737 experienced intense airframe vibrations. No other abnormal cockpit indications were noted during the initial phase of the flight. The crew elected to return to Vancouver, British Columbia, and noted that the vibrations subsided as the aircraft decelerated during descent. However, moderate vibrations returned during the approach with flaps extended. The aircraft completed a normal landing with emergency services on standby.
Post-flight inspections identified that a two-foot portion of the right elevator trim tab had broken away. The remaining inboard section of the tab had detached from its first hinge, leaving it held only by the control rod attachment, while the outboard section remained attached to the elevator but was oscillating uncontrollably. This uncontrolled movement of the tab caused the elevator to oscillate, generating the vibration felt by the crew.
The investigation
Investigators examined the fractured composite material and found that the outer layers had debonded from the honeycomb core. The cracks appeared to have progressed from the leading edge toward the trailing edge. The presence of de-icing fluid inside the tab suggested that the internal damage likely occurred prior to the aircraft's most recent de-icing in Winnipeg, which had taken place approximately 24 hours earlier.
Maintenance records for a July 1993 repair to the trim tab were found to be insufficiently detailed. While the records noted the replacement of loose fasteners and a leading-edge repair, the specific nature of the work on the composite structure could not be verified. However, an examination by Boeing concluded that the examined sample showed no evidence of improper prior repairs. Furthermore, the trim tab bearings were inspected and found to be within serviceable limits, ruling out the common cause of flutter related to worn bearings.
Findings
- The structural failure of the trim tab was caused by aerodynamic loads exceeding the component's strength.
- The component's strength had been compromised by recent or existing damage, which may have included spontaneous composite debonding or fatigue.
- It is highly probable that the initial damage was caused by a collision with a ground vehicle or airport structure while the aircraft was on the ground.