NTSB Identification: MKC90LA088.
The docket is stored on NTSB microfiche number 42101.
Accident occurred Wednesday, April 18, 1990 in OAKDALE, MN
Probable Cause Approval Date: 09/28/1992
Aircraft: BELL 47, registration: N5120B
Injuries: 1 Serious.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

WHILE FERRYING THE HELICOPTER TO CONDUCT AN AERIAL APPLICATION OPERATION, THE PLT SAID THAT THE ENGINE BEGAN TO OVERSPEED. THE PLT ENTERED AN AUTOROTATION TO REDUCE ENGINE RPM, BUT HAD TO CLEAR POWER LINES. USING THE ROTOR RPM TO CLEAR THE POWER LINES, THE ROTOR RPM DECAYED AND THE ACFT DROPPED IN FROM ABOUT 50 TO 75 FEET AGL. POST ACCIDENT INVESTIGATION REVEALED A MALFUNCTIONING MAIN TRANSMISSION CENTRIFUGAL CLUTCH. THE POWERPLANT TEST FAILED TO REVEAL ANY MALFUNCTION. AN ANNUAL INSPECTION HAD BEEN PERFORMED 6 DAYS AND 4 HOURS OF FLIGHT TIME PRIOR TO THE ACCIDENT.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A SLIPPING ROTOR DRIVE CLUTCH ASSEMBLY DUE TO IMPROPER MAINTENANCE BY COMPANY PERSONNEL. CONTRIBUTING FACTORS WERE STATIC WIRES IN THE PATH.

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