Bell 47 accident near Oakdale, Minnesota, April 18, 1990
On April 18, 1990 at about 2:30 pm local time, a Bell 47 (helicopter), registered N5120B, was substantially damaged in an accident near Oakdale, Minnesota. It was an aerial application (crop spraying) flight under agricultural flying rules (Part 137). 1 person was seriously injured. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
A SLIPPING ROTOR DRIVE CLUTCH ASSEMBLY DUE TO IMPROPER MAINTENANCE BY COMPANY PERSONNEL. CONTRIBUTING FACTORS WERE STATIC WIRES IN THE PATH.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- April 18, 1990 · about 2:30 pm local time
- Place
- Oakdale, Minnesota · map
- Type
- Accident
- Injuries
- 1 person was seriously injured.
- Weather
- visual conditions (good weather)
- Aircraft
- Bell 47 · all 47s on the register
- Registration
- N5120B · no longer on the register · serial 1941
- Damage
- Substantial damage
- Flight
- Aerial application (crop spraying) flight · agricultural flying rules (Part 137)
The NTSB's narrative final · quoted from the NTSB record
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 complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
Pilot
- Certificate: flight instructor, commercial pilot
- Ratings: multi-engine land; single-engine land; single-engine sea; instructor: airplane multi-engine; instructor: airplane single-engine; instructor: helicopter; instructor: instrument airplane; instrument: airplane; rotorcraft: helicopter
- Flight time: 4,100 hours in all; 1,200 in this make and model; 3,900 as pilot in command
- Medical certificate: Class 2 (valid medical--w/ waivers/lim.)
- Seat: left
The aircraft
- Airframe total time: 4,610 hours
- Last inspection: annual inspection, April 12, 1990; 4 hours since
- Maximum gross weight: 2,450 lb
- Seats: 3
- Landing gear: fixed
- Engine: Lycoming VO-435-A1F (piston); 0 hours total
- Operator: Twin City Helicopters, INC.
The flight
- Flight plan: none
- Runway 0
Weather at the time
- Light: daylight
- Wind: from 220° at 14 knots
- Visibility: 15 statute miles
- Sky: clear
- Temperature: 48°F (9°C), dew point 19°F (-7°C)
- Altimeter: 30.00 inHg
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
The docket, the folder of records gathered during an investigation (maintenance records, photographs, witness statements, examinations), has not been read from the NTSB for this case yet; the list appears here once it has. Open the docket at the NTSB.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.