What happened
On 31 March 1998, a Piper PA-3/0, registration G-BAWN, was conducting a flight near Kexby, Lincolnshire. The aircraft had recently undergone a 'Star Annual' inspection and subsequent test flights to verify the functionality of the fuel quantity gauging system. During a flight intended to investigate a slight tremor felt by the pilot, the aircraft's handling characteristics deteriorated rapidly.
While cruising at approximately 1,500 feet, the initial light vibration transitioned into a violent oscillation in pitch. The vibration became so intense that the crew required two pairs of hands on the control yoke to maintain any semblance of control. Due to the severity of the instability, the pilots determined that a standard approach to the runway was impossible. They instead opted for a forced landing in a field of winter wheat, approximately 2 to 2.5 miles southeast of the runway. The aircraft performed a wheels-up, engines-off landing. While the two persons on board escaped with only minor injuries, the aircraft sustained extensive damage, including fractured wing spars, rendering it damaged beyond economic repair.
The investigation
Investigators examined the aircraft's stabilator and identified severe damage consistent with aeroelastic flutter. The investigation established that the trim/anti-balance tab elements were free to move independently because the essential hardware—including an AN3-11A bolt, an MS2CO364-1032C nut, washers, and an inner plain bearing—was missing from the control rod.
Evidence of contact between the bolt threads and the bellcrank plates, as well as a bronze bush, suggested that the bolt had progressively migrated out of the assembly rather than fracturing or being improperly installed. The investigation found no evidence that the holes in the bellcranks had been enlarged by over-tightening or a loose fit, nor was there evidence of bearing seizure.
Findings
- The catastrophic pitch oscillations were caused by the disconnection of the trim/anti-balance tabs.
- The initial tremor experienced by the pilot likely indicated the assembly was becoming loose, which escalated to violent flutter once the bolt departed.
- The maintenance organization could not determine why the nut had backed off the bolt, as the annual inspection did not require disassembly of this specific component.
- The loss of the bolt and nut allowed the control surfaces to move erratically, leading to the loss of effective pitch control.