Bell 407 accident near Mountain City, Tennessee, June 3, 2016
On June 3, 2016 at about 5:45 pm local time, a 1998 Bell 407 (helicopter), registered N4999, was substantially damaged in an accident during initial climb near Mountain City, Tennessee. It was flown under charter and air-taxi rules (Part 135). 1 person had minor injuries; 3 others were unhurt. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
An incorrect and unapproved repair of the engine compressor diffuser performed by a machinist at a repair facility, which resulted in failure of the compressor diffuser aft plate and a partial loss of engine power.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- June 3, 2016 · about 5:45 pm local time
- Place
- Mountain City, Tennessee · map
- Type
- Accident
- Injuries
- 1 person had minor injuries; 3 others were unhurt.
- Weather
- visual conditions (good weather)
- Aircraft
- Bell 407 NO SERIES, built 1998 · all 407s on the register
- Registration
- N4999 · no longer on the register · serial 53323
- Damage
- Substantial damage
- Flight
- Flight · charter and air-taxi rules (Part 135)
The NTSB's narrative final · quoted from the NTSB record
During the initial climb, about 125 ft above ground level, the commercial pilot of the helicopter heard a loud noise that was accompanied by a left yaw, rapidly increasing measured gas temperature, and decreasing power turbine rpm. During the subsequent forced landing, the helicopter landed hard, which spread the skids and resulted in substantial damage. Examination of the engine revealed a partial separation of the compressor diffuser, which allowed intake air to leak before combustion. Metallurgical examination of the compressor diffuser revealed that a portion of the aft plate had separated near a braze joint; however, manufacturer specifications indicated that the aft plate should have been a single component, not two components joined together by a brazing process. The compressor diffuser had been overhauled by a repair facility and installed on the accident helicopter about 98 hours of operation before the accident. Information from the repair facility revealed that a machinist had joined a second ring to the compressor diffuser aft plate using a brazing process. There was no approval for that process and it is likely that the machinist performed the procedure to correct a mistake made during a dimensional shim repair, which was an approved procedure.
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
What happened, in order
- Loss of engine power (partial) during initial climb defining event
- Off-field or emergency landing during emergency descent
- Hard landing during landing
The NTSB's findings
- cause Personnel issues › Task performance › Maintenance › Repair › Maintenance personnel
- cause Aircraft › Aircraft handling/service › Maintenance/inspections › (general) › Incorrect service/maintenance
Pilot
- Certificate: commercial pilot
- Ratings: single-engine land; instrument: airplane; instrument: helicopter; rotorcraft: helicopter
- Flight time: 8,550 hours in all; 680 in this make and model; 52 in the last 90 days; 17 in the last 30 days; 6,900 as pilot in command; 0 on instruments
- Last flight review: August 16, 2015
- Medical certificate: Class 1 (with waivers/limitations)
- Seat: rgt
- Injury: minor injuries
The aircraft
- Airframe total time: 8,449 hours
- Last inspection: approved inspection programme, June 1, 2016; 99 hours since
- Maximum gross weight: 5,250 lb
- Seats: 4
- Landing gear: fixed
- Engine: Allison 250-C47B (turboshaft); 6,768 hours total
- Operator: Phi Air Medical
The flight
- Departed from: Mountain City TN at 5:45 pm
- Destination: TN91 Johnson City TN
Weather at the time
- Light: daylight
- Wind: from 310° at 11 knots
- Visibility: 10 statute miles
- Sky: scat at 3,600 ft
- Temperature: 77°F (25°C), dew point 57°F (14°C)
- Altimeter: 30.17 inHg
- Observation at 5:55 pm from TNB, 15 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 | 2 | ||
| Passengers | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
7 documents, released by the NTSB on April 24, 2018. View them here, or download them; the NTSB redacts some personal details before release, and this site shows the NTSB's own titles rather than its file names.
| # | Document | What it is | |
|---|---|---|---|
| 1 | Pilot/operator Aircraft Accident Report, NTSB Form 6120.1 | PDF, 9 pages | View Download |
| 2 | Interview Summaries | PDF, 4 pages | View Download |
| 3 | Engine Data Plot | PDF, 1 page | View Download |
| 4 | Engine Manufacturer Materials Examination Report | PDF, 23 pages | View Download |
| 5 | Correspondence from Repair Facility | PDF, 1 page | View Download |
| 6 | Photo Array | PDF, 3 pages | View Download |
| 7 | Statement of Party Representatives to NTSB Investigation | PDF, 16 pages | View Download |
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
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.