ASN Wikibase Occurrence # 187848
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| Date: | Friday 3 June 2016 |
| Time: | 13:45 LT |
| Type: |  Bell 407 |
| Owner/operator: | Phi Air Medical |
| Registration: | N4999 |
| MSN: | 53323 |
| Year of manufacture: | 1998 |
| Total airframe hrs: | 8449 hours |
| Engine model: | Allison 250-C47B |
| Fatalities: | Fatalities: 0 / Occupants: 4 |
| Aircraft damage: | Substantial |
| Location: | Laurel Bloomery, Johnson County, TN -
 United States of America
|
| Phase: | Take off |
| Nature: | Ambulance |
| Departure airport: | Mountain City, TN |
| Johnson City, TN (TN91) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
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:
 |
|
Sources:
http://aerossurance.com/helicopters/engine-unapproved-repair/
NTSB
Location
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 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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