What happened
On 26 July 1998, a Kolb Twinstar Mk III, registration G-MZKB, was performing a private flight near Louth, Lincolnshire. The pilot, accompanied by a passenger, had departed from Manby Airfield with the intention of visiting a nearby farm strip.
Witnesses observed the aircraft taking off from a grass area using full power. Shortly after crossing the airfield boundary, the aircraft was seen flying at a low speed. An observer noted that the engine noise momentarily decreased before increasing again, followed by a visible drop in the left wing. The aircraft then entered a steep, nose-down spiral to the left, impacting a field of oil-seed rape with the left wing tip first. The impact destroyed the aircraft, and both the pilot and passenger sustained fatal injuries.
The investigation
Investigators examined the wreckage and found the fuselage structure had been disrupted and the tail boom buckled, but there was no evidence of any pre-impact structural failure or disconnection of the flying controls. The engine, a Rotax 582, remained largely intact and was capable of running during subsequent tests.
During engine testing, investigators identified a fuel leak at the mechanical fuel pump inlet. It was discovered that the 'O' clips used to secure the rubber fuel hoses were oversized, creating gaps that allowed air to be drawn into the fuel stream when the engine was running. This air induction could potentially cause a reduction in engine power.
Additionally, the investigation noted the pilot's flying background. While the pilot held a microlight licence, his previous experience was almost entirely on weight-shift control aircraft. The G-MZKB utilized a 3-axis control system, which operates with fundamentally different control movements for pitch and roll.
Findings
- The aircraft entered a steep spiral following a loss of control.
- The pilot had very limited experience (only 9 hours) on the 3-axis control system, having previously flown weight-shift type microlights.
- Air induction into the fuel flow due to oversized hose clips may have caused a momentary reduction in engine power.
- The aircraft's stall characteristics lacked a natural pre-stall buffet, making the onset of a stall difficult to detect.
Safety action
- It is recommended that the CAA consider making the conversion training guidance for pilots moving between weight-shift and 3-axis control systems a mandatory requirement.