Synopsis
On 31 March 1998 at 1402 hours UTC, a Piper PA-30, registration G-BAWN, was involved in an incident near Kexby, Lincolnshire. The aircraft, which had recently undergone a 'Star Annual' inspection and several test flights, was on a test flight with the owner and the Chief Engineer on board. Both occupants sustained minor injuries. The aircraft was damaged beyond economic repair, principally due to fracture of both wing spars.
Flight History
The aircraft had just received an annual inspection during which a persistent fuel quantity gauging defect was rectified. The Chief Engineer air-tested the aircraft for renewal of the Certificate of Airworthiness and to check the fuel gauging system; no abnormalities were noted. Two further test flights checked the fuel gauging system, again with no handling problems. Four days later, the owner collected the aircraft and took off to return to his base at Granston. During the flight, he noticed a slight tremor on the control yoke when light pressure was applied. Although not alarmed, he returned to Sturgate to have it checked. A conversation with the Chief Engineer suggested the vibration might be engine-related due to a magneto drop. The engine was run, and they took off for a test flight to see if the problem had cleared. At 1,500 feet in the cruise, the tremor was reproduced and appeared speed-related. The Chief Engineer decided to return immediately to Sturgate.
While still about 2 miles north of Runway 27/09, the tremor abruptly changed to a violent vibration so severe that two pairs of hands were required on the yoke to prevent intolerable pitch oscillations. Reducing airspeed slightly improved the situation. The pilots decided against a right turn of about 120° to land on the runway, feeling they lacked sufficient control. Instead, they continued on a heading of 150°, descending for a forced landing in fields 2 to 2.5 miles southeast of the runway. A wheels-up, engines-off landing was made in a field of winter wheat. Although the landing was carried out without severe difficulty and the pilots evacuated normally, the aircraft sustained sufficient damage to be beyond economic repair.
Examination
Examination revealed severe damage to the stabilator, typical of control surface aeroelastic flutter. There were clear signs of gross overtravel of the two-piece trim/anti-balance tab, whose elements were free to move independently of the stabilator because the AN3-11A bolt, MS20364-1032C nut, washers, and inner plain bearing were missing from the control rod. The assembly should have a nut and bolt locking the bellcranks together with the control rod between them. Signs of contact with the thread of the bolt were discerned on both bellcrank plates and in one bronze bush in the rod end. There was no evidence of distortion or oversizing of the holes in the bellcranks indicating the bolt had been wrenched or loose. All indications were that the bolt progressively migrated out of the assembly. There were no signs of distress suggesting seizure of the bearings. Consideration was given to the possibility of bolt or nut fracture, but no previous history of this existed, and no evidence of abnormal loading was found.
Discussion
It is beyond doubt that the severe oscillations in pitch control were caused by the disconnected tabs. The Chief Engineer expressed surprise at the dramatic effect, leading to the possibility that some pilots and engineers may not be aware of the catastrophic potential of such an event. The Certificate of Airworthiness renewal flight had included a Vne dive; had the disconnection occurred during that phase, complete loss of control and/or structural failure may have occurred. The initial tremor felt by the owner was probably the whole assembly becoming loose, changing to violent oscillation as the bolt departed.
Conversations with the maintenance organisation failed to discover why the nut backed off. The annual check required lubrication and inspection of flying controls, which did not require disassembly. The last work in that area was several years ago when the stabilator was replaced. It is unlikely an engineer would have missed a loose or missing nut during an annual inspection, especially when the assembly was lubricated (a Piper Service Manual requirement every 100 hours). No explanation was offered. However, the organisation issued maintenance requirements for aircraft with all-moving tailplanes, calling for inspections for security of the assembly and conformance with manufacturer's specification, and renewal of bolts every 500 hours. The CAA published a resume of the circumstances in its General Aviation Safety Information Leaflet (GASIL) with a warning to pilots and engineers to pay particular attention to this area.