What happened
On 14 May 2017, a Silence Twister, registration G-JINX, was performing a formation aerobatic display at the Abingdon Air and Country Show at MOD Abingdon, Oxfordshire. The aircraft was flying as the number two position in a two-ship formation. During the second manoeuvre of the sequence, a barrel roll, the pilot noted a lack of performance and requested the formation leader reduce power to maintain position.
Following a subsequent stall turn, the pilot identified a significant power loss and decided to discontinue the display to attempt a forced landing on the runway. While attempting a tight descending turn to align with the runway, the engine stopped. The aircraft struck the grass area east of the runway with its landing gear and flaps retracted. The impact caused severe damage to the aircraft and engine, and the pilot sustained serious injuries.
The investigation
The AAIB investigation focused on the engine's failure and the circumstances of the landing. Examination of the UL260iSA engine revealed that the number 3 piston had been badly damaged, with a significant section broken away. The investigation found that the engine had effectively seized due to a loss of lubrication and cooling.
Data from the Engine Monitoring System (EMS) showed that while the engine was running, oil pressure dropped below 20 psi, triggering a low-pressure warning. However, the engine continued to rotate for approximately 90 seconds after the pressure reached zero before finally stopping. The investigation also noted that the pilot's attention was focused externally on maintaining formation, which likely prevented the detection of the declining oil pressure. Additionally, emergency responders faced difficulties accessing the cockpit because there were no visible instructions on how to operate the canopy from the outside.
Findings
- The engine seized because the pistons expanded and gripped the cylinder walls due to a lack of lubrication and cooling.
- A loss of oil likely occurred through the exhaust system or via the damaged number 3 piston, which would not have been visually detectable from the ground because oil was being injected into the exhaust to produce smoke.
- The pilot did not notice the low oil pressure warnings due to the high level of concentration required for formation flying and the absence of an enabled audio alert.
- The aircraft's cockpit safety cell and energy-absorbing seats contributed to the pilot's survival despite the severity of the impact.