# NTSB ERA19LA206: Bell OH58C, N911GV, Gainesville, FL, June 25, 2019 - Type: NTSB accident investigation - Classification: Accident · No injuries · Class 3 · Limited · Final report - Date: June 24, 2019, 22:00 local (Jun 25, 2019, 02:00 UTC) - Place: Gainesville, FL - Coordinates: 29.693301, -82.277253 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=29.6933&lon=-82.2773&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Defining event: [Landing-flare/touchdown: Miscellaneous/other](https://openairdocs.org/ntsb-events/900) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed - Report: Approved by Office Director - Investigation class: Class 3 - NTSB office: Eastern Region Aviation - Parties: FAA FSDO - NTSB case key (mKey): 99742 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ERA19LA206 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/99742/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=99742 - Page: https://openairdocs.org/ntsb/ERA19LA206 ## Probable cause The flight instructor's failure to maintain control of the helicopter during a simulated forced landing autorotation, which resulted in pylon whirl that substantially damaged the helicopter. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Performance/control parameters: landing flare](https://openairdocs.org/ntsb-findings/01062041): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Bell OH-58C, 1 case](https://openairdocs.org/ntsb-findings/01062041/cases.md?model=1181580), [All aircraft, 1,285 cases](https://openairdocs.org/ntsb-findings/01062041/cases.md) - [Use of equip/info: aircraft control](https://openairdocs.org/ntsb-findings/02063040): Personnel issues › Task performance · Instructor/check pilot (probable cause); cases with it: [Bell OH-58C, 4 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md?model=1181580), [All aircraft, 9,295 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md) ## Sequence of events The events NTSB coded, in order, by phase of flight. The defining event best describes what happened. - [Landing-flare/touchdown: Miscellaneous/other](https://openairdocs.org/ntsb-events/900) (defining event); cases with it: [Bell OH-58C, 2 cases](https://openairdocs.org/ntsb-events/900/sequence.md?model=1181580), [All aircraft, 1,556 cases](https://openairdocs.org/ntsb-events/900/sequence.md) ## Analysis During an night instructional flight in a helicopter, the flight instructor was demonstrating a straight-in autorotation to landing following a simulated engine failure. At touchdown, the heels of the landing skids touched first and the helicopter “rocked” forward, which was followed by several more “throws” in a counterclockwise motion that became progressively more “violent.” The instructor described that the helicopter was experiencing pylon whirl and continued to manipulate the flight controls during the ground run until helicopter came to rest. The helicopter’s tailboom was substantially damaged during the accident sequence. ## Factual narrative On June 24, 2019, about 2200 eastern daylight time, a Bell OH-58C, N911GV, was substantially damaged when it was involved in an accident at Gainesville Regional Airport (GNV), Florida. The pilot receiving instruction and flight instructor were not injured. The helicopter was operated as a public aircraft instructional flight. The pilot receiving instruction and instructor planned to perform standard maneuvers in unaided and aided (with night vision goggles [NVG]) night visual meteorological conditions. According to the pilot receiving instruction, both pilots were wearing NVGs when the instructor took control of the helicopter to demonstrate a standard, straight-in autorotation for landing on the parallel taxiway. After entry, the instructor made trim and alignment adjustments during the descent and announced “100 feet” above ground level prior to initiating the decelerative flare. The pilot receiving instruction said the radar altimeter displayed 75 feet at the announcement and described the flare, the remainder of the descent, and the leveling of the helicopter prior to touchdown. The pilot receiving instruction stated that, at touchdown, “I immediately felt like we were driving down a pothole filled dirt road and after several quick bumps, up and down while skidding across the taxiway, I felt a massive whip forward and to the left followed by several more throws in a counterclockwise motion. The throws became more intense and violent and … braced myself for the aircraft to come apart or tip over.” During the ground run, the instructor announced the helicopter was experiencing “pylon whirl.” The instructor continued to adjust the flight controls until the ground run ended and the “throws” subsided. The instructor described the descent and the “initial pitch pull” to slow the descent and level the helicopter for touchdown. At touchdown, the heels of the landing gear skids touched “first”, and the helicopter “rocked” forward, “grabbed,” and began to oscillate “uncontrollably.” The instructor stated that he adjusted the flight controls to keep the helicopter “straight and level” and lowered the collective control “to prevent pylon whirl.” The instructor repositioned, shut down, and inspected the helicopter after the landing and said that the “effects of pylon whirl were readily apparent.” A Federal Aviation Administration (FAA) examined the helicopter after the accident and noted damage to the tail boom, isolation mount, and the upper cowling around the rotor mast of helicopter. According to the OH-58A/C Technical Manual, “Pylon whirl is a condition which occurs after blade flapping and mast bumping. The resultant motion of the pylon is elliptical, and spike knock is apt to occur. If the frequency of motion coincides with a particular natural frequency of the helicopter, and the amplitude and direction of the force is large enough, damaging vibrations can occur in the aft section tailboom of the helicopter. Motion of this type could occur duringtouchdown autorotations, if operational limits are exceeded. The Transportation Safety Board of Canada cited in Aviation Investigation Report A16P0161: The fuselage of the Bell 206B is suspended from the main-rotor transmission by 2 A-frame pylons. Spherical bearings allow the pylons, and hence the main-rotor transmission and mast, to move in relation to the fuselage, with the motion dampened by the isolation mount. Main-rotor thrust and dynamic forces normally maintain the main-rotor transmission and fuselage in alignment; however, certain maneuvers can cause unbalanced pylon motion. This phenomenon, known as pylon whirl, can lead to spike knock, which is described as follows: Spike knock is the contact of the transmission drag pin with the roof mounted static stop plate as a result of pylon motion or pylon whirl. This results in an audible knocking noise. Low rotor RPM, extreme asymmetric loading, poor execution of an auto-rotational landing, and low G maneuvers are factors that may contribute to the occurrence of spike knock. ## Aircraft - Registration: [N911GV](https://openairdocs.org/deregistered/N911GV-2022-12-03), registration cancelled 2022-12-03, S/N 70-15212 (OpenAirDocs) - Reported make and model: BELL OH58C - Registry model: [Bell OH-58C](https://openairdocs.org/model/bell-oh-58c) (registry code 1181580) (OpenAirDocs) - Category: Helicopter - Serial number: 70-15212 - Year built: 1970 - Airframe hours: 11444.1 - Engines: Allsion 250-C20C 420 horsepower turbo shaft - Damage: Substantial damage - Flight: Instructional, Part 91 - Operator: City Of Gainesville Police Dept - Registered owner: City Of Gainesville Police Dept - Airworthiness certificate: Standard, Normal - Last inspection: 100 hour, May 23, 2019 - Departed: GNV, Gainesville, FL - Destination: GNV, Gainesville, FL - Airport: GNV, Gainesville Rgnl, runway Taxiway A, 4,785 × 40 ft, asphalt - Equipment: Electronic primary flight display, ADS-B ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 30; injuries: none; commercial; instrument ratings: helicopter; medical: class 2, without waivers/limitations; last flight review January 23, 2019; seat: right; restraint: 4-point; flight hours: 327 total, 327 in this make and model, 293 as pilot in command, 61 in the last 90 days, 17 in the last 30 days, 2 in the last 24 hours - Pilot; age 50; injuries: none; airline transport, commercial, flight instructor; airplane ratings: single-engine sea, single-engine land; instrument ratings: airplane; medical: class 2, with waivers/limitations; seat: left; restraint: 4-point; flight hours: 5,788 total, 3,602 in this make and model, 44 in the last 90 days ## Weather - Conditions: Visual (VMC), Night - Wind: 260° at 7 knots - Visibility: 10 statute miles - Lowest ceiling: none - Temperature: 82 °F, dew point 73 °F - Altimeter: 30.04 inHg - Observed at: KGNV ## 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 20190624007929A](https://openairdocs.org/incident/20190624007929A), Jun 24, 2019: Bell OH-58C, Gainseville, FL, Substantial damage. On 6/24/19 at approx 1720 hrs a member of the gainesville police department and a civilian instructor took off for an NVG training flight. The training began… ## Docket (6 files) The files NTSB released with the investigation on May 26, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=99742. 8.8 MB in all. - Pilot/operator aircraft accident report, NTSB form 6120.1 (item 1, PDF, 11 pages, 5.5 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11720954&FileExtension=pdf&FileName=NTSB%20Form%206120_Redacted-Rel.pdf - Pilot's statement (item 2, PDF, 2 pages, 99 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11747972&FileExtension=pdf&FileName=Pilot%20Statement-Rel.pdf - FAA inspector's statement (item 3, PDF, 1 page, 95 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11747915&FileExtension=pdf&FileName=FAA%20Inspector%20Statement-Rel.pdf - Photos (item 4, PDF, 3 pages, 2.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11747905&FileExtension=pdf&FileName=Photo%27s-Rel.pdf - Report from foreign agencies (item 5, PDF, 20 pages, 381 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11748131&FileExtension=pdf&FileName=TSB%20A16P0161-Rel.pdf - Technical manual excerpt (item 6, PDF, 4 pages, 54 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11748369&FileExtension=pdf&FileName=OH-58AC%20Spike%20Knock%20abd%20Pylon%20Whirl-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.