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NTSB Report
April 18, 1990 · Bell 47 · N5120B
Oakdale, MN
Overview
Report Status
Probable Cause
Event Type
Accident
Date
April 18, 1990
Injury Outcomes
1 injury (1 serious injury)
Location
Oakdale, MN
Aircraft
Bell 47
Tail Number
N5120B
Operator
Twin City Helicopters
Injuries
Serious
Aircraft Damage
Substantial
NTSB Number
MKC90LA088
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.
NTSB Probable Cause
A SLIPPING ROTOR DRIVE CLUTCH ASSEMBLY DUE TO IMPROPER MAINTENANCE BY COMPANY PERSONNEL. CONTRIBUTING FACTORS WERE STATIC WIRES IN THE PATH.
Full Narrative
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.
Flight
Event TypeAccident
Event Time09:30 CDT
Nearest Airport0nm N
Runway0
Runway Length0
Runway Width0
Flight Plan FiledNone
Aircraft FireNo
Aircraft ExplosionNo
Aircraft
AircraftBell 47
Tail NumberN5120B
Aircraft CategoryHeli
DamageSubstantial
OperatorTwin City Helicopters
Operating RulePart 137
Engines1
Engine TypeReciprocating
Engine DetailsLycoming · VO-435-A1F · Reciprocating · 260
Seats3
People & Injuries
Injury Summary1 injury (1 serious injury)
Fatal0
Serious1
Minor0
Uninjured0
Total1
Crew 1Age 43 · Second Class
Conditions
ConditionsVMC
LightDaylight
Wind220° @ 14 kt
Visibility15 sm
CeilingNone
Temperature / Dew Point48 / 19 C
Altimeter30.00 inHg