Cessna 404 Titan
Maintenance Inspection Authority
Troubleshooting
Aircraft Equipment Problem Critical
Aircraft Aircraft Damaged; Flight Crew Inflight Shutdown; General Maintenance Action
Aircraft
During approach to ZZZZ; pilot reports first strong vibration then an oil pressure decrease on engine left. Emergency checklist performed; propeller put in idle position and engine shutoff. ATC is aware of the issue and the pilot asked for priority landing. One engine landing is performed and the aircraft is parked.The engine GTSIO-520-M have TSN/TSO 952FH and was rebuilt in 20XX. Since rebuilt; no cylinder change or removal occurs. Per EASA (European Aviation Safety Agency) and French regulation our company reported to BEA (French NTSB) and engine manufacturer. We replaced the engine in order to return the aircraft to service and decided to inspect the broken engine. We also contacted two engine shops which indicate that the most probable cause is 'counterweight detuning.' Mechanics disassembled the engine in order to find root cause. The inspection report stated one counterweight is broken which could break [a] lot of internal part.If the root cause is the counterweight; the inspection also showned that a lot of connecting rod nuts (locknut 'spiralock') which should be torqued to 550-575 in/lbs was easily removed. The lock system was inoperative on most of them. They could be fit and unfit by hand. This remark should be taken in account because we incriminate the counterweight for many years; but it's possible that the poor quality of these locknuts with age is the root cause (Locking system failure could cause loosening; vibrations; then connecting bolt broke; which broke the counterweight).
Maintenance Manager reported that a Cessna 404 pilot experienced strong vibrations; an oil pressure decrease; and subsequent engine failure on the left engine while on approach.