What happened
On 17 December 2021, a Piper PA34-220T (registration G-OBUC) was conducting a general aviation flight from Bournemouth, UK, to Weston Airport, Ireland. While cruising at approximately 1,000 feet over the Irish Sea, near the coast of County Wicklow, the pilot reported engine roughness to Dublin Air Traffic Control. Shortly thereafter, the pilot issued a MAYDAY call, reporting that the aircraft was operating on only one engine following a loud bang and a sudden drop in manifold pressure.
The pilot observed oil smoke trailing from the right-hand engine cowlings and noted significant aircraft yaw. After securing the right-hand engine and feathering the propeller, the pilot continued the flight toward Weston Airport. Despite the emergency, the pilot successfully performed a visual approach and landed on Runway 25. There were no injuries to the pilot or the three passengers on board, and no fire occurred during the landing.
The investigation
The AAIU examined the aircraft and discovered that the number three cylinder had detached from the right-hand engine's crankcase, causing substantial damage to the crankcase and surrounding components. Metallurgical analysis of the engine hardware revealed that seven of the eight bolts used to secure the cylinder had failed due to high-cycle fatigue. This failure was likely caused by a loss of preload, meaning the bolts were no longer tight enough to hold the components securely.
Investigators also found evidence that one bolt's nut had been missing for some time, as indicated by rust on the threads. Additionally, the investigation looked into the engine's maintenance history, noting that the engines had been overhauled in 2010. While the use of certain sealants and silk thread on the crankcase surfaces was noted, the investigation could not definitively link these materials to the specific failure of the cylinder studs.