What happened
On April 28, 2018, a Beechcraft 58 Baron, registration HB-GGP, was conducting an IFR navigation flight from Les Eplatures, Switzerland, to Ajaccio Campo dell’Oro, France. While cruising near the RETNO waypoint, the pilot heard an explosion originating from the right engine. The pilot immediately checked the engine parameters, feathered the propeller, and shut down the engine. After notifying air traffic control, the pilot diverted to Avignon Caxton airport, where the aircraft landed with slight damage to the airframe.
The investigation
The investigation focused on the mechanical failure of the right Teledyne Continental Motors TSIO 520 LB engine. Examination of the wreckage revealed that the number 4 cylinder had separated from the engine block. This separation caused engine cowling deformation, an impact on the leading edge of the right wing, and the projection of engine oil onto the tail section.
Inside the engine, the separation of the cylinder led to the failure of the associated piston and the separation of the connecting rod from the crankshaft. Investigators found evidence of fretting on the cylinder bases where they contact the engine halves. Of the cylinder's mounting hardware, two studs were missing, and five of the remaining six studs had failed due to fatigue cracking.
Maintenance records showed the engine had undergone a major overhaul in 1999 and had operated for 1,275 hours since. The engine was operating under a time-limit extension granted by the Swiss Federal Office of Civil Aviation (FOCA) following a specific inspection in July 2015. During that 2015 inspection, cylinders 1, 3, and 5 had been removed and replaced. No other cylinder removal or reinstallation work had been performed since that date.
Findings
- The separation of the number 4 cylinder was caused by fatigue cracking and subsequent brittle fracture of the mounting studs.
- The investigation concluded that the damage was consistent with inadequate tightening of the cylinder fasteners during the 2015 maintenance inspection.
- While cylinders 1, 3, and 5 were removed during the previous inspection, the manufacturer's overhaul manual requires that all cylinders—including those not removed—be retightened to specific torque values to prevent such failures.
- The presence of fretting on the cylinder bases indicated that improper torque application had likely initiated the fatigue process.