What happened
On 4 April 2023, a Jabiru UL-450, registration G-CDFK, was performing a private sightseeing flight from Damyns Hall Aerodrome in Essex. After a successful initial takeoff, the pilot performed a second takeoff at 1105 hrs. During the climb, the pilot noticed the aircraft was not reaching the expected altitude and observed the airspeed dropping rapidly from 60 to 50 knots.
As the aircraft reached approximately 300 ft agl, the engine power failed. With insufficient altitude to return to the runway and no suitable landing sites nearby, the pilot attempted to maintain flight. The engine subsequently stopped, causing the aircraft to stall and enter a right-hand spin. The aircraft struck trees in a wooded area before hitting the ground. The accident resulted in two serious injuries, affecting both the pilot and the passenger.
The investigation
The AAIB examined the wreckage and the engine components. While the engine itself was found to be internally sound, investigators identified a split in the rubber coupling that connects the carburettor to the engine's plenum chamber. Laboratory analysis revealed that this nitrile rubber component showed signs of age-related degradation and embrittlement, with a through-crack present around 65% of its circumference.
Records indicated that the coupling had not been replaced since the aircraft was manufactured in 2006, meaning it was approximately 11 years past the manufacturer's recommended five-year life limit. The investigation also noted that the location of the coupling and its mounting clips made visual inspection difficult. Furthermore, the pilot's personalized pre-takeoff checklist did not fully align with the manufacturer's Pilot's Operating Handbook, lacking specific checks for carburettor heat and target RPM.
Findings
- The loss of engine power was caused by a split in the rubber carburettor coupling, which allowed an incorrect air-fuel mixture to enter the engine.
- The coupling was significantly past its recommended service life, having likely been in use for over 16 years.
- The position of the coupling made it difficult to inspect for cracks during routine maintenance.
- The pilot's airspeed reduced significantly, and the aircraft entered a stall and spin due to the loss of power and subsequent loss of control.
- The pilot may not have responded to the stall warning due to inattentional deafness caused by the high workload of the emergency.