Bell 407 N4999

Summary

On 3 June 2016 at about 17:45 local time, a Bell 407 registered N4999, operated by PHI Air Medical, was involved in an accident near Mountain City, Tennessee, United States. 4 people were on board and one had minor injuries. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

3 June 2016 at 17:45

Classification

Accident

Location

Mountain City, Tennessee, United States

Coordinates

36.4747, -81.8047

Aircraft

Bell 407

Registration

N4999

Category

Helicopter

Year built

1998

Engines

1

Operator

PHI Air Medical

Operating rule

Part 135: Air Taxi & Commuter

Phase of flight

Not recorded

Route

Mountain City → Johnson City

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

ERA16LA202

1 person had minor injuries.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full NTSB narrative

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.

Quoted verbatim from the NTSB record.