Aircraft Information
The aircraft, a Kolb Twinstar Mk III registered G-MZKB, was a side-by-side two-seat microlight with a single Rotax 582 piston engine mounted behind the wing and cockpit. It was built from a partially constructed kit purchased by the pilot in May 1997 and completed over five months. A Permit to Fly was issued on 20 November 1997 after test flying by an approved certification pilot. The aircraft had flown a total of 21 hours at the time of the accident.
History of Flight
On the morning of 26 July 1998, the pilot and his father drove the aircraft on a trailer to Manby Airfield, a disused aerodrome two miles from their home. After assembly and a walkround inspection, the pilot took off from a grass area into wind. The weather was fine with scattered cloud above 2,500 ft, a southerly wind, good visibility, and a surface temperature of 17°C.
A witness observed the takeoff as normal with full power and some flap. The aircraft crossed the airfield boundary at 40–50 ft, appearing slow (estimated 40 mph). Another witness saw it one mile from the airfield at 400–600 ft, climbing more slowly than usual. The engine noise briefly reduced, then increased; the left wing dropped, and the aircraft entered a steep nose-down spiral, impacting the ground left wing tip first.
On-site Examination
The aircraft impacted a field of ripe oil-seed rape in a steep nose-and-left-wing-low attitude, then cartwheeled. The tubular fuselage structure was disrupted, the tail boom buckled, and both wing leading edges severely damaged. The engine was lightly damaged; the propeller showed minimal damage, suggesting low power at impact. Fuel tanks: one was empty from a gash, the other half-full. No evidence of pre-impact structural failure was found.
Subsequent Engine Examination
The engine was removed and tested at the UK Rotax agent. After minor repairs, it ran successfully at full power using both test fuel and fuel from the accident aircraft. However, a fuel leak was detected at the mechanical fuel pump inlet when the engine was off. The leak was due to an incorrectly sized 'O' clip (9/16 inch) used on a hose intended for 1/2 inch. The oversize clip caused an oval seal, allowing air induction when the engine was running (visible as air bubbles in the fuel stream). Despite extensive running, no misfiring occurred, but air ingestion was confirmed.
Pilot's Background
The pilot held a Private Pilot's Licence (Microlight) issued in May 1993, with training on weight-shift control microlights. He had 160 hours on a Gemini Flash II (weight-shift). He logged nine hours on the 3-axis Kolb Twinstar, including five hours of familiarization with the certification pilot. There was no requirement for conversion training between control types, though CAA guidance recommended it.