History of the Flight
The aircraft, a Mooney M20J registered G-BIWP, was owned by a syndicate of four pilots. Two pilots planned a day trip from Sherburn-in-Elmet, Yorkshire, to Texel, Holland, accompanied by two passengers. Two days before the flight, the pilot in command had the aircraft refuelled to 50 gallons US, about 14 gallons less than full. On the morning of 29 April 1999, he filed a flight plan by telephone with Manchester ATC, which was activated upon departure from Sherburn by the airport duty manager.
Witnesses saw four people wearing lifejackets around the aircraft. The aircraft taxied towards Runway 11, stopped for power checks, and backtracked before departure. Several witnesses described the takeoff and initial climb as normal. Later, two witnesses saw the aircraft flying east about one mile west of Selby, just below cloudbase estimated at 600 to 800 feet. The pilot in command, seated in the right seat, made radio contact with Church Fenton Approach and received clearance to climb to FL 55. The flight proceeded under radar advisory, climbing to 2,000 feet before being handed over to Humberside ATC. The handover was without incident.
Two people working in a shed about one mile west of the accident site heard engine trouble. Two others near the site heard the engine running but cutting out four or five times, followed by an impact. Several witnesses saw the aircraft descending out of cloud in a steep nose-down attitude, turning slowly right, then impacting the ground. First responders encountered intense fire and small explosions; emergency services arrived within minutes.
Meteorological Conditions
It could not be determined what forecast the pilots obtained. The forecast for the destination was CAVOK with north-easterly winds at 12 kt. At Sherburn on departure, conditions were overcast at 700 to 800 feet with an easterly wind of 10 kt. At the accident site 30 minutes after the accident, police helicopter reported cloudbase at 500 feet, consistent with other witness observations. Pilot reports gave cloud tops of 1,800 to 2,000 feet.
Radar Data
Recorded radar returns from Great Dunn Fell and Claxby were plotted. The data showed a significant reduction in groundspeed followed by a loss of climb rate, indicative of power loss or reduction. The first radar contact was near where the aircraft entered cloud; the final contact was approximately over the accident site.
Pilot Experience
The pilot in command, aged 51, held a Private Pilot's Licence obtained in 1991, an IMC rating, and had 437 hours total (205 on type). His most recent flight was six weeks prior. The pilot flying, seated left, had 295 hours (65 on type) and an IMC rating that lapsed two months prior. He had one hour flight time in the 28 days before the accident.
Pathology
There was no evidence of any medical factor influencing the accident. All four persons suffered fatal injuries at impact.
Engineering Investigation
The aircraft struck the ground on a southerly heading and came to an immediate halt. Aileron mass balance horns were inclined at 42° to horizontal, giving the impact angle. Extensive disruption indicated moderate impact speed. The rear fuselage showed compressive buckling; the left stabiliser tip contacted the ground, suggesting some right rotation. Intense post-impact fire destroyed the cabin, inboard wings, and engine accessory gearbox. The engine buried in a 1-metre crater filled with water, protecting it from fire. The most probable cause of the fire was electrical.
The three-bladed propeller showed one blade virtually undamaged, one bent aft, and one broken off in bending overload. Blades had virtually no leading-edge damage or rotational scuffing, indicating low power, low RPM at impact. Blades appeared at fine pitch, consistent with low power. The aircraft was complete at impact; flaps and landing gear were retracted.
Detailed Examination
General
Post-impact fire destroyed cockpit instrumentation. Propeller RPM, mixture, and throttle control knobs were found fully forward (maximum RPM, rich mixture, throttle open). However, the throttle butterfly in the fuel injector was closed. There was no evidence of pre-impact cable disconnect; the disagreement was likely due to impact disruption. The cowl flap lever was pushed fully forward (flaps closed). Flying controls showed no pre-impact disconnect. The horizontal stabiliser trim screwjack had 11 threads exposed; reference to a similar aircraft indicated approximately neutral trim.
Engine and Fuel System
The engine (1,870 operating hours) was examined at a UK overhaul agency. No pre-impact mechanical failure was found. No faults in fuel injector, boost pump, or engine-driven pump, though oil on the diaphragm suggested some valve guide wear. The vacuum pump and accessory drives appeared intact. The dual-drive magneto was burned and untestable.
The most significant finding was severe wear on both inlet valve cam lobes, with about 0.140 inches removed from the cam peaks. This would reduce inlet valve lift and power output. The overhaul agent stated such wear is occasionally seen on engines returned for reasons other than power loss. The problem is associated with infrequent flying; after inactivity, oil drains from the camshaft, leading to corrosion pitting and wear. Inlet cams are more affected as they operate two valves each.
The inboard wing fuel tanks were destroyed; fuel lines downstream of the selector were examined. Some pipe unions were only finger-tight, possibly due to impact distortion. The fuel selector was found selected to the left tank. The fuel strainer was free of debris; no blockages were found. Injector, boost pump, engine-driven pump, and flow divider had no defects.
Autopilot
The aircraft was fitted with a King KFC 200 Autopilot, most often used in heading and attitude mode. It had altitude hold and NAV mode capability. In attitude mode, it maintains a pilot-selected attitude.