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ASN Wikibase Occurrence # 187848
Last updated: 2 September 2021
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Time:13:45 LT
Type:Silhouette image of generic B407 model; specific model in this crash may look slightly different
Bell 407
Owner/operator:Phi Air Medical
Registration: N4999
MSN: 53323
Fatalities:Fatalities: 0 / Occupants: 4
Other fatalities:0
Aircraft damage: Substantial
Location:Laurel Bloomery, Johnson County, TN -   United States of America
Phase: Take off
Departure airport:Mountain City, TN
Destination airport:Johnson City, TN (TN91)
Investigating agency: NTSB
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.

Probable Cause: 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.


Accident investigation:
Investigating agency: NTSB
Status: Investigation completed
Duration: 2 years and 3 months
Download report: Final report

Revision history:

03-Jun-2016 19:40 Aerossurance Added
03-Jun-2016 22:28 Geno Updated [Registration, Cn, Total fatalities, Total occupants, Other fatalities, Location, Departure airport, Destination airport, Source, Narrative]
06-Jun-2016 09:32 Aerossurance Updated [Location, Departure airport, Narrative]
09-Jul-2016 16:58 Aerossurance Updated [Time, Operator, Source, Narrative]
21-Dec-2016 19:30 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
16-Sep-2018 17:30 Aerossurance Updated [Time, Total occupants, Narrative]
16-Sep-2018 19:09 ASN Update Bot Updated [Time, Operator, Departure airport, Destination airport, Source, Narrative, Accident report, ]
07-Oct-2018 09:47 Aerossurance Updated [Time, Source, Narrative]

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