# NTSB WPR20LA235: Mcdonnell Douglas 600, N208MP, Thompson Falls, MT, July 21, 2020 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: July 21, 2020, 05:58 local (11:58 UTC) - Place: Thompson Falls, MT - Coordinates: 47.559688, -115.43952 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=47.5597&lon=-115.4395&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-hover: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Western Pacific Region - Parties: Rolls Royce, FSDO, MD Helicopters, Inc - NTSB case key (mKey): 101660 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=WPR20LA235 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/101660/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=101660 - Page: https://openairdocs.org/ntsb/WPR20LA235 ## Probable cause A partial loss of engine power for reasons that could not be determined based on available evidence. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Aircraft](https://openairdocs.org/ntsb-findings/01050000): Aircraft › Aircraft power plant · Unknown/Not determined (probable cause); cases with it: [Mcdonnell Douglas Helicopter 600N, 1 case](https://openairdocs.org/ntsb-findings/01050000/cases.md?model=3027377), [All aircraft, 151 cases](https://openairdocs.org/ntsb-findings/01050000/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Maneuvering-hover: Loss of engine power (partial)](https://openairdocs.org/ntsb-events/342) (defining event); cases with it: [Mcdonnell Douglas Helicopter 600N, 1 case](https://openairdocs.org/ntsb-events/342/sequence.md?model=3027377), [All aircraft, 1,858 cases](https://openairdocs.org/ntsb-events/342/sequence.md) ## Analysis While on an approach to the landing zone, the pilot lowered the collective to start a vertical descent to land. At that time, the engine and rotor speeds suddenly decreased, the helicopter yawed to the left and started to settle. Because the helicopter was approaching the ground fast, the pilot increased the collective to use the remaining engine power to cushion the landing. The helicopter impacted the ground, and the pilot rolled the throttle to idle, but noticed that engine speed and rotor rpm gauges were below the normal idle parameters. When he rolled the throttle to off, he heard a grinding "metal-to-metal" noise, and the rotor blades stopped spinning almost immediately. Data downloaded from the onboard incident recorder captured a “low rotor RPM” event during the flight, followed by “Flameout” and “Ng Low” triggers. According to the data, the full authority digital engine control system (FADEC) attempted to re-light the engine for the next 53 seconds until the throttle was rolled to cut-off; however, the engine did not restart. A postaccident examination of the engine revealed no evidence of any preimpact mechanical malfunctions or failures that would have precluded normal operation, and the reason for the low rotor rpm triggering event could not be determined. ## Factual narrative On July 21, 2020, at 0558 Pacific daylight time, a McDonnell Douglas 600N helicopter, N208MP, was substantially damaged when it was involved in an accident near Thompson Falls, Montana. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 133 rotorcraft external load operation. The pilot stated that, as he was on an approach to the landing zone, he lowered the collective to start a vertical descent to land. At that time, the engine and rotor speeds suddenly decreased, the helicopter yawed to the left and started to settle. As the helicopter was approaching the ground fast, the pilot increased the collective to use the remaining engine power to cushion the landing. The helicopter impacted the ground, and the pilot rolled the throttle to idle, but noticed that engine speed and rotor rpm gauges were below the normal idle parameters. When he rolled the throttle to off, he heard a grinding "metal-to-metal" noise, and the rotor blades stopped spinning almost immediately. A postaccident examination revealed the N1 system turned smoothly and was continuous from the compressor to the starter/generator and first-stage turbine wheel. The N2 system turned smoothly and was connected to the aircraft power train. Foreign object damage (FOD) was not observed on the compressor impeller blades or compressor inlet and the inlet barrier filter system was intact and free of debris. The fourth-stage turbine wheel was normal in appearance when viewed from the exhaust collector. The first-stage turbine nozzle and blades were viewed via borescope with no abnormalities observed. The engine mounted fuel filter bowl retained about 1/3 bowl of fuel, which was normal in smell and appearance. The fuel filter element presented no obvious contamination. All the fuel lines from the airframe to the engine were flow-checked with no obstructions noted. The engine electronic control unit (ECU), which retains data in nonvolatile memory, was downloaded during the wreckage examination. The ECU includes an incident recorder function, which begins capturing engine and input parameters upon trigger actuation and records a line of data (“record”) every 1.2 seconds. The initial incident recorder triggering event was “low rotor RPM” (Nr Droop), which fell below the 92% threshold. Then, within 3 seconds, the incident recorder captured the “Flameout” and “Ng Low” triggers. According to the data, the FADEC system attempted to re-light the engine for the next 53 seconds until the throttle was rolled to cut-off; however, the engine did not restart. The engine was transported to an inspection facility and installed on a calibrated test stand. After the engine was warmed up for about 5 minutes, the power was gradually increased to the takeoff setting, where the observed power was 680 shaft horsepower (shp). No unusual vibration was noted. Several accelerations and decelerations were achieved followed by testing of the anti-ice system and bleed valve,all of which were within specification. A full power calibration protocol was achieved resulting in a maximum predicted take-off rating of 664 shp at standard day conditions, with no anomalies noted. The fuel nozzle was removed and inspected. The inlet screen was normal in appearance and not obstructed. The fuel hose was installed in a test bench and 600 pounds per hour of fuel was flowed through the hose to check for leaks. No leaks were observed. ## Aircraft - Registration: [N208MP](https://openairdocs.org/deregistered/N208MP-2021-04-13), registration cancelled 2021-04-13, S/N RN042, built 1998 (OpenAirDocs) - Reported make and model: MCDONNELL DOUGLAS 600 (N) - Registry model: [Mcdonnell Douglas Helicopter 600N](https://openairdocs.org/model/mcdonnell-douglas-helicopter-600n) (registry code 3027377) (OpenAirDocs) - Registry engine: [Allison 250-C47 series](https://openairdocs.org/engine/allison-250-c47-series) (registry code 03029, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: RN042 - Year built: 1998 - Airframe hours: 2984.6 - Engines: Rolls Royce M250-C47M 505 horsepower turbo shaft - Damage: Substantial damage - Flight: External load, Part 133 - Airworthiness certificate: Standard, Normal - Last inspection: Annual, May 7, 2020 - Departed: Thompson Falls, MT - Airport: NA - Equipment: ADS-B ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 37; injuries: none; commercial, flight instructor; airplane ratings: single-engine sea, multi-engine land; instrument ratings: airplane, helicopter; medical: class 2, without waivers/limitations; seat: left; restraint: 4-point; flight hours: 6,450 total, 150 in this make and model, 6,300 as pilot in command, 140 in the last 90 days, 65 in the last 30 days ## Weather - Conditions: Visual (VMC), Day - Wind: 280° at 3 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 52 °F, dew point 50 °F - Altimeter: 29.95 inHg - Observed at: SZT ## 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 20200721011279A](https://openairdocs.org/incident/20200721011279A), Jul 21, 2020: Mcdonnell Douglas Helicopter 600N, Thompson Falls, MT, Substantial damage. The pilot had just finished an external load operation and was in the process of landing when the helicopter expieance a loss of power. When the pilot noticed… ## Docket (6 files) The files NTSB released with the investigation on June 30, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=101660. 14.2 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 11 pages, 5.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13373509&FileExtension=pdf&FileName=6120_Redacted-Rel.pdf - Pilot's statement (item 2, PDF, 1 page, 71 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13305369&FileExtension=pdf&FileName=pilot%27s%20statement_Redacted-Rel.pdf - Rolls-royce engine report (item 3, PDF, 16 pages, 537 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14119980&FileExtension=pdf&FileName=Rolls-Royce%20Engine%20Report%20CAE847850%20a_Redacted-Rel%20(1)-Rel.pdf - Ecm values (item 4, PDF, 3 pages, 29 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13305953&FileExtension=pdf&FileName=ECM%20values-Rel.pdf - Statement of party representatives to NTSB investigation (item 5, PDF, 2 pages, 6.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=13305548&FileExtension=pdf&FileName=Signed%20party%20forms_Redacted-Rel.pdf - Release of aircraft wreckage, NTSB form 6120.15 (item 6, PDF, 1 page, 1.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=14119843&FileExtension=pdf&FileName=Wreckage%20Release%20signed_Redacted-Rel%20(5)-Rel.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.