Maintenance manager reported that a cessna 404 pilot experienced strong vibrations; an oil pressure ...

Narrative:

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).

Original NASA ASRS Text

Title: 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.

Narrative: 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).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

Attributes
ACN	1525728
Time
Date	201712
Local Time Of Day	1801-2400
Place
Locale Reference	ZZZZ.Airport
State Reference	FO
Environment
Light	Night
Aircraft 1
Make Model Name	Cessna 404 Titan
Flight Phase	Initial Approach
Flight Plan	IFR
Component
Aircraft Component	Engine
Person 1
Function	Other / Unknown
Qualification	Maintenance Inspection Authority
Events
Anomaly	Aircraft Equipment Problem Critical

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.