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RIGHT ACP NOT WORKING AND STUCK MIC DURING CRUISE, FLIGHT CREW RETURNED TO BASE. R & R REMOTE ELECTRONIC UNIT IAW WITH SAAB 340 AMM 23-50-05-04, OPS TEST SATISFACTORY.

FAILURE OF ACTUATOR ASSEMBLY. THE BRACKETS DISCONNECTING THAT CAUSED THE UPPER BREAK.” MR. WEBSTER ALSO STATED THIS WAS A “MAINTENANCE ISSUE THAT SHOULD HAVE BEEN ADDRESSED. WE HAVE A SERVICE BULLETIN OUT JUST FOR THAT.” HOWEVER, UNDER THE CURRENT SERVICE BULLETIN 94 THE INSPECTION REQUIREMENT WAS ONLY DUE TO BE INSPECTED “AT THE NEXT 100-HOUR INSPECTION OR ANNUAL INSPECTION, WHICHEVER OCCURS FIRST.” IT IS IMPORTANT TO NOTE THAT WHEN THIS SERVICE BULLETIN WAS PUBLISHED THE HELICOPTER WAS NOT YET DUE FOR ITS NEXT 100-HOUR INSPECTION. THE FAILURE OCCURRED PRIOR TO THE NEXT 100 HOUR INSPECTION WHICH WAS DUE IN 13 HOURS.

REMOVAL OF THE NR 1 CYLINDER FOR VISUAL INSPECTION REVEALED A FAILURE OF THE INTAKE VALVE. ENGINE MONITOR SHOWED MASSIVE INGESTION OF ENGINE OIL INTO REMAINING CYLINDERS CAUSING LOSS OF POWER FROM OIL FOULING OF UNAFFECTED SPARK PLUGS. AFFECTED CYLINDER SHOWED CATASTROPHIC DAMAGE TO PISTON AND COMBUSTION AREA FROM MULTIPLE PHYSICAL CONTACT. VALVE FURTHER FRACTURED AND PARTS BECAME WEDGED IN THE INDUCTION TUBE AS WELL AS THE COMMON MANIFOLD AREA. WHEN THE INTAKE ROCKER ASSEMBLY WAS REMOVED ALONG WITH THE CAP, DISCOVERED THE TOP OF VALVE, ABOVE VALVE KEYS WAS THE AREA OF SEPARATION AND FAILURE. VISUAL INSPECTION WITH A 30X MICROSCOPE, MECHANIC'S CONSIDERED OPINION THAT THE FAILURE OF THE INTAKE VALVE MAY HAVE BEEN CAUSED FROM A STRESS RISER CREATED DURING MANUFACTURING AT EITHER THE MACHINING, HEAT TREATING PROCESS, OR PERHAPS WAS AT A MOLECULAR LEVEL DURING THE CREATION OF THE RAW MATERIAL. IT SHOULD BE DULY NOTED THAT THE AIRCRAFT WAS IN COMPLIANCE WITH SERVICE BULLETIN SB388C EXACTLY 85.8 HOURS EARLIER AND NO DEFECT OR ABNORMALITIES WERE NOTED AT THE TIME OF THE INSPECTION.