ERA14LA099 · Bell 47D1
Milam Bell 47D1 · Accident: system/component malfunction/failure (non-power) on approach
From Birmingham-Shuttlesworth International Airport (BHM) to Pell City, AL (no airport)
Event
- NTSB case
- ERA14LA099
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Accident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- Unknown
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- General aviation
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Bell 47D1
- Aircraft type
- Bell 47
- Category
- Helicopter
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N93067
- Operator
- Milam
- Onboard fatalities
- Unknown
- Route
- From Birmingham-Shuttlesworth International Airport (BHM), Birmingham, ALTo Pell City, AL (no airport)
- Aircraft age
- About 49 years (built 1965)
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 91: General Aviation
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- Approach
- Defining eventThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- System/component malfunction/failure (non-power)
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Substantial
Cause areas
- Aircraft › Aircraft propeller/rotor
- Personnel issues › Action/decision
- Personnel issues › Task performance
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
The commercial pilot was completing a business flight in the helicopter. He reported that, while descending the helicopter for a confined off-airport landing, at 25 to 35 ft above ground level, he heard a "snap/pop" and that the engine suddenly began "over-revving." The helicopter started descending and then touched down hard, and one of the main rotor blades struck the helicopter's tail. After touchdown, the engine was still running, but the rotor blades were not moving.
Examination of the transmission revealed that it incorporated a light-weight clutch assembly, which was not authorized for use in the installed transmission model. Further examination revealed that the clutch shoe running surface on the inner diameter of the clutch drum exhibited a series of abnormal wear grooves along the entire surface of clutch shoe contact and that the clutch lining of each shoe exhibited glazing, which occurs when the clutch is improperly engaged. The contact surface of each clutch shoe also contained numerous metallic slivers that were atypical of a normal in-service clutch assembly and appeared to be material that had transferred from the drum to the shoes.
During the transmission examination, the pilot was asked to describe his normal engine start and clutch engagement procedure. The clutch engagement procedure described by the pilot was not in accordance with the published procedure. The pilot's procedure likely resulted in a lengthy clutch engagement and an extended period of clutch slippage during each start that led to the glazed condition of the clutch assembly. Once the glazed condition began to develop, the period of clutch slippage during each start lengthened, further contributing to the glazed condition. Ultimately, the glazed condition resulted in the in-flight disengagement of the clutch and a loss of rotor rpm.
The pilot reported that, about 15 flight hours before the accident, he had a mechanic install an engine in the helicopter that produced 25 more horsepower than the previously installed engine. The installation of the higher horsepower engine would have increased the tendency for slippage of a glazed clutch assembly.
The pilot also reported that the transmission had accumulated about 615 hours since overhaul. According to the helicopter's maintenance and overhaul instructions, the transmission should be disassembled every 600 hours to inspect all parts, including the clutch, for damage or excessive wear. Given that the engine had been installed about 15 flight hours before the accident, the 600-hour inspection could have been accomplished at that time. However, review of maintenance records did not indicate that the 600-hour transmission inspection had occurred, and the mechanic who installed the higher horsepower engine reported that he did not disassemble the transmission.
Probable cause
The pilot’s use of an improper clutch engagement procedure, which resulted in clutch glazing, slippage, and, ultimately, the in-flight disengagement of the clutch and a loss of rotor rpm. Contributing to the accident were the installation of a light-weight clutch assembly and a higher horsepower engine, both of which increased the tendency for clutch slippage, and inadequate inspection of the clutch assembly.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record ERA14LA099
- Event ID
- 20140121X71107
- Case number
- ERA14LA099
- Dataset
- full-current
- Source SHA-256
- 5cf380f0061817c0331a6b2d8cc7e0ee3a79bea469a1001dc5c10e56f35f5ab3
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: investigation status. Values present in the bulk record are kept.
- The date is the local date, which is the same as the UTC date the NTSB stores.
- API snapshot SHA-256: f5ad40b5484abeb17b17b6c0766e26738ee86665c8259d67e6802cdc4fa5ca94; retrieved 2026-09-15T23:16:10.930Z.