# NTSB MKC90LA088: Bell 47, N5120B, Oakdale, MN, April 18, 1990

- Type: NTSB accident investigation
- Classification: Accident · Serious injuries · Limited investigation · Final report
- Date: April 18, 1990, 09:30 local (14:30 UTC)
- Place: Oakdale, MN
- Coordinates: 45.010219, -93.030418 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=45.0102&lon=-93.0304&mi=10
- Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping)
- Highest injury: Serious injuries
- Injuries: 1 serious
- Status: Completed, report September 28, 1992
- Report: Approved by Office Director
- NTSB case key (mKey): 34963
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=MKC90LA088
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/34963/pdf
- Page: https://openairdocs.org/ntsb/MKC90LA088

## Probable cause

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

## Sequence of events

In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf).

- Airframe/component/system failure/malfunction, cruise - normal
  - rotor drive system,clutch assembly - slipped (cause)
  - maintenance - improper, company maintenance personnel (cause)
  - autorotation - attempted, pilot in command
  - object - wire,static (contributing factor)
- Forced landing, descent - emergency
- In flight collision with terrain/water, descent - uncontrolled
  - rotor rpm - not possible, pilot in command

## Analysis

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.

## Aircraft

- Registration: [N5120B](https://openairdocs.org/deregistered/N5120B-1997-02-11), registration cancelled 1997-02-11, S/N 1941 (OpenAirDocs)
- Reported make and model: BELL 47
- Registry model: [Bell 47G-2](https://openairdocs.org/model/bell-47g-2) (registry code 1181008) (OpenAirDocs)
- Category: Helicopter
- Serial number: 1941
- Airframe hours: 4610
- Engines: LYCOMING VO-435-A1F 260 horsepower reciprocating
- Damage: Substantial damage
- Flight: Aerial application, Part 137
- Operator: Twin City Helicopters, Inc.
- Registered owner: Jims Aviation Services, Ltd
- Airworthiness certificate: Special, Restricted (Special)
- Last inspection: Annual, April 12, 1990, 4 hours before
- Airport: NONE

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 43; commercial, flight instructor; airplane ratings: single-engine sea, multi-engine land, single-engine land; instrument ratings: airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 4,100 total, 1,200 in this make and model, 3,900 as pilot in command

## Weather

- Conditions: Visual (VMC), Day
- Wind: 220° at 14 knots
- Visibility: 15 statute miles
- Lowest ceiling: none
- Temperature: 48 °F, dew point 19 °F
- Altimeter: 30.00 inHg
- Observed at: MSP, 4 nm away

## FAA reports on this event (1)

The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs).

- [Accident 19900418010139A](https://openairdocs.org/incident/19900418010139A), Apr 18, 1990: Bell 47G2, Oakdale, MN, Substantial damage. Engine overspeed. Autorotated to hard landing. Just out of annual had worn out clutch assembly and parts.

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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