What happened
On 28 March 2023, a Piper PA-31-350, registration G-FCSL, was conducting a private flight from Cardiff to Shoreham. While cruising at 2,000 ft agl south of Salisbury, the aircraft experienced a loud bang and significant adverse yaw, indicating a failure of the right engine. The crew observed that the right engine cowling was covered in oil, the dipstick access flap had opened, and a dent was visible on the cowling.
As the pilot performed shutdown checks and feathered the propeller, flames were seen emerging from the right engine. The crew activated the firewall fuel shut-off valve, which extinguished the fire after approximately 10 seconds. The pilot declared an emergency with Boscombe Down and diverted to Bournemouth Airport, the nearest suitable airfield. The aircraft landed on Runway 26 without further incident. There were no injuries to the two crew members or the single passenger on board.
The investigation
The AAIB examined the right engine and found that the number 2 cylinder had detached from the crankcase. This failure caused the breaking of all six attachment studs and both through-bolts. The investigation revealed that a section of the crankcase had also broken away. The damage was extensive, including a jammed piston due to ring damage and a separation of the connecting rod from the big end bearing shell. The engine and its accessories showed evidence of both oil spray and fire damage around the turbocharger.
Technical analysis of the hardware involved examining the remaining studs and bolts using extraction tools. The investigators focused on the fatigue characteristics of the metal components to determine how the cylinder became unseated.
Findings
- The primary cause of the engine failure was the fatigue failure of the cylinder flange attachment studs and through-bolts.
- The failure sequence likely began with a loss of preload in three of the front attachment studs.
- This loss of tension created non-uniform, high-cycle tensile bending loads on the remaining studs and through-bolts, eventually leading to their failure.
- While the fatigue characteristics of certain studs suggested a loss of tightness, the specific reason for the initial loss of preload could not be determined.