What happened
On 18 August 2018, a Thruster T600N aircraft, registration G-CCUZ, was performing a type-refamiliarisation flight at Foston Airfield in Lincolnshire. During the takeoff roll, the pilot noted that the RPM gauge appeared to be under-reading and that the magneto test did not produce a noticeable drop in RPM. After a high-power taxi and subsequent takeoff, the aircraft reached an altitude of approximately 100 ft AGL.
As the aircraft passed the end of the runway, it experienced a loss of thrust, causing the climb to stagnate. Shortly after, a further loss of power occurred, leading to a rapid descent. Faced with obstacles including houses and gardens, the pilot opted to fly into a nearby Leylandii hedge to absorb the impact energy. The aircraft struck the hedge just below its top, resulting in the destruction of the aircraft. Both the pilot and the passenger sustained minor injuries and were able to exit the wreckage unaided.
The investigation
The AAIB examined the Jabiru 2HM engine to identify any mechanical failures. The investigation found no evidence of component failure, malfunction, or issues within the ignition, carburettor, or choke systems. While a slight leak was noted in one exhaust valve, cylinder compression remained within manufacturer limits.
Investigators also reviewed video evidence of the takeoff, which confirmed the loss of climb performance and noted a change in engine tone and a brief misfire during the descent. The investigation also considered the reliability of the aircraft's RPM indication system, noting that the Hall-effect sensor used in this setup can be unreliable, which likely made the magneto drop test difficult to discern.
Findings
- The primary cause of the power loss was an overly rich fuel-air mixture, evidenced by a significant build-up of carbon soot on the cylinders, spark plugs, and exhausts.
- This rich mixture likely caused the engine to be unable to maintain a satisfactory climb.
- The pilot's decision not to attempt a turnback towards the airfield likely prevented a more serious loss of control.
- The impact severity was mitigated by the use of a Leylandii hedge as an energy absorber and the use of four-point safety harnesses by the occupants.