What happened
On 9 June 2018, a Cirrus SR22, registration G-SRTT, was conducting a private VFR cross-country flight from North Weald Airfield to Retford Gamcroft Airfield. Approximately six minutes into the flight, while cruising at 1,500 ft agl, the pilot noticed the engine oil pressure had dropped significantly to approximately 9 psi.
Following the drop in pressure, the engine began to run roughly, eventually producing grey smoke and oil spray from the cowlings. As the engine power failed and the aircraft entered a shallow descent, the pilot's forward visibility became obscured by dense smoke. Fearing an engine fire and unable to identify a safe landing field due to the smoke, the pilot followed manufacturer recommendations and activated the Cirrus Aircraft Parachute System (CAPS) at roughly 800 ft agl. The aircraft descended into a field near Benington, resulting in no injuries to the pilot or the passenger.
The investigation
The AAIB examined the wreckage and the engine components. The investigation focused on the engine's oil system and the physical state of the connecting rods. Analysis of the flight data logs showed that while oil temperature initially rose, the oil pressure dropped to red warning levels shortly before the engine ceased rotation.
Physical examination of the engine revealed that the bearing material on the No 3 and No 4 connecting rods had melted. This extreme heat caused the connecting rod cap bolts to fail under overload. The failure of the No 4 connecting rod was violent enough to strike the crankcase, creating a hole that allowed oil to escape during the flight. There was no evidence of an external oil leak or oil burning in the cylinders prior to this mechanical failure.
Findings
- The engine failure was caused by the overheating of the connecting rod cap bolts.
- This overheating occurred because the engine was operating with insufficient oil to provide necessary cooling and lubrication to the bearings.
- The loss of oil led to the melting of bearing material, which increased vibration and eventually caused the mechanical failure of the connecting rod assembly.
- The pilot correctly followed the Pilot's Operating Handbook (POH) and manufacturer guidance by deploying the CAPS when faced with an engine failure at low altitude.