What happened
On a scheduled flight from Saskatoon to Prince Albert, Saskatchewan, a Transwest Air Limited Beech 99A, registration C-FDYF, experienced a sudden and violent loss of pitch control. While the aircraft was at approximately 4,000 feet, the crew deployed the flaps for their approach when an audible bang originated from the rear of the fuselage. Immediately following this, the aircraft pitched up into a near-vertical attitude, stalled, and entered a left-hand spin.
Although the pilots managed to recover the aircraft to a near-horizontal attitude using full up-elevator and power adjustments, the plane was forced into an emergency landing. During the descent, the aircraft struck a knoll, which severed the landing gear and the belly cargo pod. The aircraft then bounced, struck a barbed-wire fence, and slid to a halt. The six occupants (two pilots and four passengers) sustained serious injuries, though none were life-threatening. All passengers and crew evacuated through the rear cabin door.
The investigation
Investigators examined the horizontal stabilizer trim control system and discovered that the stabilizer trim actuator had detached from its upper airframe mounting structure. The investigation revealed that during a recent heavy-maintenance inspection, a replacement actuator had been installed. However, the mounting bolts were not actually passing through the mounting lugs of the actuator. Instead, the bolts were installed through the airframe brackets only, effectively trapping the mounting lug assemblies between the bolt shocks and the rivets.
This improper installation was particularly difficult to detect because the installer had to work in a confined, awkward position, relying on feel rather than sight to align the bolts. The weight of the actuator was held by the improperly placed bolts, creating a false sense of security that the component was secure. Because the actuator is under constant compression while on the ground, standard functional tests and dual inspections failed to identify the error. It was only under the varying compressive and tensile loads experienced during flight that the rivets wore away, allowing the actuator to become unseated.
Findings
- The flight crew lost pitch control because the horizontal stabilizer trim actuator became detached from its mounting structure during flight.
- During the replacement of the actuator, the upper attachment bolts were inserted through the airframe but failed to pass through the actuator's mounting lugs.
- The incorrect bolt placement trapped the lug assemblies in a way that supported the weight of the actuator, providing a false impression of a successful installation.
- Existing maintenance inspections, ground testing, and flight testing were unable to detect the faulty attachment.