What happened
During a routine training flight from Winnipeg, Manitoba, to Prince Albert, Saskatchewan, the crew of a Beech King Air C9-0A, registration C-FGXU, experienced a sudden in-flight emergency. While cruising at flight level 220, the crew heard a loud bang followed by intense airframe vibrations and a significant upward pitch of the aircraft.
To regain control, the captain disconnected the autopilot, reduced engine power, and applied heavy nose-down pressure and trim. As the aircraft slowed to below 150 knots, the vibrations ceased, though elevator control remained limited. The crew diverted to Dauphin, Manitoba, for an emergency landing. During the descent, the crew extended the landing gear to increase the descent rate and stabilize the aircraft. At approximately 200 feet above the ground, a second brief vibration and sudden pitch-down occurred, necessitating aggressive control inputs. The crew successfully completed a flapless landing without further incident.
The investigation
Investigators examined the elevator trim system, which utilizes a pushrod attached to a trim tab via stainless steel bushings. Although pre-flight checks had indicated normal operation, a recent inspection 150 hours prior to the event had revealed that the inner bushing was already corroded and seized within the outer bushing.
Technical analysis of the hardware revealed that the left elevator trim-tab pushrod had failed due to fatigue cracking in the threaded section of the rod end. The investigation found that the fit between the inner and outer bushings was improper, involving an interference fit. Furthermore, the through bolt had been tightened beyond the manufacturer's prescribed torque, which prevented the clevis from rotating once the bushings seized.
Findings
- The fatigue cracking of the pushrod was caused by increased bending loads resulting from a progressively stiffening attachment.
- The seizure of the bushings was likely caused by galling and wear, driven by the ill-fitting nature of the bushings and excessive installation torque on the through bolt.
- The stainless steel-on-stainless design of the bushings increases the risk of seizure if lubrication is inadequate.
- There was evidence that out-of-tolerance bushings might have been manually reamed to facilitate installation, which can lead to dimensional errors and subsequent seizure.
Safety action
- Transport Canada issued a Service Difficulty Alert (AL-2003-03) advising operators of Raytheon/Beech 90 series aircraft to inspect elevator trim-tab hardware for free rotation.
- The manufacturer, Raytheon/Beech, conducted quality-control checks on its bushing inventory and issued a Safety Communiqué to alert operators to the risk of seized bushings.
- Maintenance manual revisions were implemented to provide clearer instructions on bushing installation and inspection.