# NTSB ERA16LA202: Bell 407, N4999, Mountain City, TN, June 3, 2016 - Type: NTSB accident investigation - Classification: Accident · Minor injuries · Limited investigation · Final report - Date: June 3, 2016, 13:45 local (17:45 UTC) - Place: Mountain City, TN - Coordinates: 36.474723, -81.804725 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=36.4747&lon=-81.8047&mi=10 - Phase of flight: [Initial climb](https://openairdocs.org/ntsb-phases/350) (OpenAirDocs' grouping) - Defining event: [Initial climb: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) - Highest injury: Minor injuries - Injuries: 1 minor, 3 uninjured - Status: Completed, report September 11, 2018 - Report: Approved by Office Director - NTSB office: Eastern Region Aviation - Parties: Rolls Royce, PHI Air Medical, FAA/FSDO, Aero Propulsion Support - NTSB case key (mKey): 93300 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ERA16LA202 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/93300/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=93300 - Page: https://openairdocs.org/ntsb/ERA16LA202 ## 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. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Maintenance: repair](https://openairdocs.org/ntsb-findings/02062015): Personnel issues › Task performance · Maintenance personnel (probable cause); cases with it: [All aircraft, 99 cases](https://openairdocs.org/ntsb-findings/02062015/cases.md) - [Maintenance/inspections](https://openairdocs.org/ntsb-findings/01010500): Aircraft › Aircraft handling/service · Incorrect service/maintenance (probable cause); cases with it: [All aircraft, 103 cases](https://openairdocs.org/ntsb-findings/01010500/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Initial climb: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) (defining event); cases with it: [All aircraft, 1,858 cases](https://openairdocs.org/ntsb-events/342/sequence.md) - [Emergency descent: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) - [Landing: Hard landing](https://openairdocs.org/ntsb-events/092); cases with it: [All aircraft, 2,102 cases](https://openairdocs.org/ntsb-events/092/sequence.md) ## Analysis 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. ## Factual narrative On June 3, 2016, about 1345 eastern daylight time, a Bell 407, N4999, operated by PHI Air Medical, was substantially damaged during a forced landing to a parking lot, following a partial loss of engine power during initial climb near Mountain City, Tennessee. The commercial pilot incurred minor injuries. The two crewmembers and one patient were not injured. The on-demand air medical flight was conducted under the provisions of 14 Code of Federal Regulations Part 135. Visual meteorological conditions prevailed and a company flight plan was filed for the flight that originated from the parking lot; destined to Johnson City Medical Center Heliport (TN91), Johnson City, Tennessee.The pilot stated that during initial climb, about 125 feet above ground level, he heard a loud "bang" that was accompanied by a left yaw and rapidly increasing measured gas temperature (MGT). The pilot advised the crewmembers that he was rejecting the climb and landing immediately. The pilot further stated that he had to maneuver the helicopter right to avoid ground personnel, level the helicopter with cyclic control, and used all available collective to cushion the landing. The helicopter was equipped with a Rolls Royce (Allison) 250-C47B, 650 horsepower turboshaft engine. Examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that it landed hard, which spread the skids and resulted in substantial damage to the fuselage. Review of data downloaded from an engine monitor revealed that during the accident takeoff, the MGT increased beyond maximum for takeoff while the power turbine rpm decreased. Examination of the engine at the operator's facility revealed a partial separation of the compressor diffusor, which allowed intake air to leak prior to combustion. The compressor diffusor was forwarded to the engine manufacturer's facility for teardown examination under the supervision of an FAA inspector. Examination of the compressor diffuser revealed that a portion of the aft plate had separated near a braze joint. Review of engine manufacturer specifications revealed that the aft plate should have been a single component and not two components joined together by a brazing process. Maintenance records revealed that the compressor diffusor had been overhauled by a repair facility and installed on the accident helicopter about 98 hours of operation prior to the accident. According to the FAA principle maintenance inspector (PMI) of the repair facility, a machinist had joined a second ring to the compressor diffuser aft plate using a brazing process. There was no approval (neither through the engine manufacturer nor the FAA) for that process and it is possible that the machinist performed the procedure to correct a mistake he had made during a dimensional shim repair, which was approved through the FAA designated engineering representative (DER) program. The machinist no longer worked for the repair facility and had left the company prior to theaccident. Further review of the repair facility by the FAA PMI revealed that they had authorization through the FAA DER program for approximately 4 years to repair compressor diffusers. During those 4 years, the repair facility had performed repairs on 52 other compressor diffusers. Most of those were common repairs to vanes, with 17 being dimensional shims to the aft plates. Review of the other 17 similar repairs revealed that the machinist in question had not worked on any of those. According to the Quality and Engineering Manager at the repair facility, ultrasonic testing also failed to identify the unapproved repair. An immediate corrective action for the repair facility was to suspend dimensional shim repairs indefinitely. The repair facility also ensured that their three-tier approval process (engineering, quality and management) was required for any fixture, tooling, drawing, or process before it was performed and that all necessary personnel were trained in that method. ## Aircraft - Registration: N4999 (not matched to a registry record) - Reported make and model: BELL 407 (NO SERIES) - Category: Helicopter - Serial number: 53323 - Year built: 1998 - Airframe hours: 8449 - Engines: ALLISON 250-C47B 650 horsepower turbo shaft - Damage: Substantial damage - Flight: Part 135 - Operator: Phi Air Medical - Registered owner: Phi Inc - Airworthiness certificate: Standard, Normal - Last inspection: AAIP, June 1, 2016, 99 hours before - Departed: Mountain City, TN - Destination: TN91, Johnson City, TN - Flight plan: Company VFR - Equipment: None ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 53; injuries: minor; commercial; airplane ratings: single-engine land; instrument ratings: airplane, helicopter; medical: class 1, with waivers/limitations; last flight review August 16, 2015; seat: right; restraint: 4-point; flight hours: 8,550 total, 680 in this make and model, 6,900 as pilot in command, 52 in the last 90 days, 17 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 310° at 11 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 77 °F, dew point 57 °F - Altimeter: 30.17 inHg - Observed at: TNB, 15 nm away ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Accident 20160603008559A](https://openairdocs.org/incident/20160603008559A), Jun 3, 2016: Bell 407, Mountain City, TN, Substantial damage. This was a helicopter air ambulance flight departing from the parking lot of a manufacturing plant. The intended destination of the flight was johnson city… ## Docket (7 files) The files NTSB released with the investigation on April 24, 2018, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=93300. 6.4 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 2.8 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462412&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Interview Summaries (item 2, .PDF, 55 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462413&FileExtension=.PDF&FileName=Interview%20Summaries-Master.PDF - Engine Data Plot (item 3, .PDF, 30 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462414&FileExtension=.PDF&FileName=Engine%20Data%20Plot-Master.PDF - Engine Manufacturer Materials Examination Report (item 4, .PDF, 18 photos, 2.4 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462415&FileExtension=.PDF&FileName=Engine%20Manufacturer%20Materials%20Examination%20Report-Master.PDF - Correspondence from Repair Facility (item 5, .PDF, 90 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462416&FileExtension=.PDF&FileName=Correspondence%20from%20Repair%20Facility-Master.PDF - Photo Array (item 6, .PDF, 3 photos, 299 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462417&FileExtension=.PDF&FileName=Photo%20Array-Master.PDF - Statement of Party Representatives to NTSB Investigation (item 7, .PDF, 680 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40462418&FileExtension=.PDF&FileName=Statement%20of%20Party%20Representatives%20to%20NTSB%20Investigation-Master.PDF From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.