# NTSB GAA15LA296: Bell 206, N49660, Ozark, AL, September 24, 2015 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: September 24, 2015, 14:45 local (19:45 UTC) - Place: Ozark, AL - Coordinates: 31.432222, -85.571662 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=31.4322&lon=-85.5717&mi=10 - Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping) - Defining event: [Maneuvering-hover: Loss of control in flight](https://openairdocs.org/ntsb-events/240) - Highest injury: No injuries - Injuries: 5 uninjured - Status: Completed, report June 1, 2016 - Report: Approved by Office Director - NTSB office: General Aviation Accidents - Parties: FAA Alabama and Northwest Florida FSDO - NTSB case key (mKey): 92136 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=GAA15LA296 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/92136/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=92136 - Page: https://openairdocs.org/ntsb/GAA15LA296 ## Probable cause The pilot's failure to maintain clearance from terrain while hovering, which resulted in the helicopter's skid impacting terrain, subsequent abnormal airframe vibrations, and a hard landing during the subsequent hovering autorotation. ## Findings NTSB's coded findings; those marked are part of the probable cause. - [Use of equip/info: aircraft control](https://openairdocs.org/ntsb-findings/02063040): Personnel issues › Task performance · Pilot (probable cause); cases with it: [Bell 206B, 45 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md?model=1181511), [All aircraft, 9,295 cases](https://openairdocs.org/ntsb-findings/02063040/cases.md) - [Rotorcraft flight control: main rotor control](https://openairdocs.org/ntsb-findings/01046710): Aircraft › Aircraft propeller/rotor · Incorrect use/operation (probable cause); cases with it: [Bell 206B, 2 cases](https://openairdocs.org/ntsb-findings/01046710/cases.md?model=1181511), [All aircraft, 28 cases](https://openairdocs.org/ntsb-findings/01046710/cases.md) - [Attention/monitoring: monitoring environment](https://openairdocs.org/ntsb-findings/02021545): Personnel issues › Psychological · Pilot (probable cause); cases with it: [Bell 206B, 25 cases](https://openairdocs.org/ntsb-findings/02021545/cases.md?model=1181511), [All aircraft, 809 cases](https://openairdocs.org/ntsb-findings/02021545/cases.md) - [Performance/control parameters: altitude](https://openairdocs.org/ntsb-findings/01062012): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Bell 206B, 20 cases](https://openairdocs.org/ntsb-findings/01062012/cases.md?model=1181511), [All aircraft, 1,310 cases](https://openairdocs.org/ntsb-findings/01062012/cases.md) - [Terrain](https://openairdocs.org/ntsb-findings/03021000): Environmental issues › Physical environment · Awareness of condition (probable cause); cases with it: [Bell 206B, 2 cases](https://openairdocs.org/ntsb-findings/03021000/cases.md?model=1181511), [All aircraft, 267 cases](https://openairdocs.org/ntsb-findings/03021000/cases.md) - [Performance/control parameters: prop/rotor parameters](https://openairdocs.org/ntsb-findings/01062052): Aircraft › Aircraft oper/perf/capability · Not attained/maintained (probable cause); cases with it: [Bell 206B, 11 cases](https://openairdocs.org/ntsb-findings/01062052/cases.md?model=1181511), [All aircraft, 182 cases](https://openairdocs.org/ntsb-findings/01062052/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 control in flight](https://openairdocs.org/ntsb-events/240) (defining event); cases with it: [Bell 206B, 32 cases](https://openairdocs.org/ntsb-events/240/sequence.md?model=1181511), [All aircraft, 5,002 cases](https://openairdocs.org/ntsb-events/240/sequence.md) - [Maneuvering-hover: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Bell 206B, 57 cases](https://openairdocs.org/ntsb-events/470/sequence.md?model=1181511), [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md) - [Maneuvering-hover: Attempted remediation/recovery](https://openairdocs.org/ntsb-events/060); cases with it: [Bell 206B, 14 cases](https://openairdocs.org/ntsb-events/060/sequence.md?model=1181511), [All aircraft, 1,422 cases](https://openairdocs.org/ntsb-events/060/sequence.md) - [Maneuvering-hover: Abrupt maneuver](https://openairdocs.org/ntsb-events/270); cases with it: [Bell 206B, 2 cases](https://openairdocs.org/ntsb-events/270/sequence.md?model=1181511), [All aircraft, 346 cases](https://openairdocs.org/ntsb-events/270/sequence.md) - [Maneuvering-hover: Off-field or emergency landing](https://openairdocs.org/ntsb-events/440); cases with it: [Bell 206B, 21 cases](https://openairdocs.org/ntsb-events/440/sequence.md?model=1181511), [All aircraft, 3,365 cases](https://openairdocs.org/ntsb-events/440/sequence.md) - [Autorotation: Hard landing](https://openairdocs.org/ntsb-events/092); cases with it: [Bell 206B, 28 cases](https://openairdocs.org/ntsb-events/092/sequence.md?model=1181511), [All aircraft, 2,102 cases](https://openairdocs.org/ntsb-events/092/sequence.md) ## Analysis According to the commercial pilot, he was hovering the helicopter in a field to position it for departure on a personal flight. He reported that the helicopter "moved out of ground effect," lost lift, and then began to settle. The left skid subsequently contacted terrain, and the helicopter began to roll left. The pilot arrested the rolling motion with flight control inputs, and the helicopter climbed to about 10 to 15 ft above the ground. However, the ground contact had placed the helicopter in an unusual attitude and generated a "rocking motion." Although the pilot attempted to correct for the unusual attitude, the main rotor system contacted portions of the upper cowling and the transmission, and the transmission driveshaft contacted its mounting system, which induced abnormal vibrations throughout the airframe. Due to these vibrations and the possible rotor system damage, the pilot initiated a hovering autorotation. The helicopter subsequently landed hard, which resulted in substantial damage to the tailboom. A postaccident examination of the helicopter revealed substantial damage to the transmission drive system. The pilot reported that there were no preimpact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. ## Factual narrative On September 24, 2015, about 1445 central daylight time, a Bell 206B helicopter, N49660, landed hard during a hovering autorotation in a field, about 3 miles to the east of Blackwell Field (71J) in Ozark, Alabama. The commercial pilot and four passengers were not injured, and the helicopter sustained substantial damage. The helicopter was registered to SAF Aviation Services and Leasing LLC, Midland City, Alabama and operated by Eagle Aviation Academy LLC, Ozark, under the provisions of 14 Code of Federal Regulations Part 91 as a day, visual flight rules personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from Blackwell Field (71J), Ozark. According to the pilot, he was hovering in a field to position the helicopter for departure. The helicopter "moved out of ground effect" and a "loss of lift" occurred allowing the helicopter to settle. The left skid contacted terrain and the helicopter began to roll to the left. The pilot arrested the rolling motion with the flight control inputs and the helicopter climbed 10 to 15 feet above the ground. Flight control inputs were made to correct for the unusual attitude and a "rocking motion" that was induced as the helicopter departed the ground. The pilot reported that the unusual attitude resulted in the main rotor system contacting portions of the upper cowling, and the transmission and the transmission driveshaft contacting its mounting system, which induced abnormal vibrations throughout the airframe. The pilot reported that due to the abnormal vibrations and the unknown damage at the time, he initiated a hovering autorotation. The helicopter landed hard resulting in substantial damage to the tail boom. A postflight inspection also revealed substantial damage to the transmission drive system. The pilot reported there were no pre-impact mechanical failures or malfunctions with the airframe or engine that would have precluded normal operation. WRECKAGE AND IMPACT INFORMATION The damage photos supplied by the operator displayed buckling on the right side and a perforation on the top of the tail boom in the sheet metal skin aft of where the tail boom attaches to the fuselage. Additionally, the photos displayed damage sustained to the transmission stop mount, consistent with impact from the transmission drag pin assembly. The photos also displayed damage to the transmission isolation mount, consistent with impact from the transmission driveshaft coupling. ADDITIONAL INFORMATION Pylon Whirl Condition During the course of the accident investigation, it was identified that the Bell 206 series flight manuals were deficient with the identification and reporting of the condition known as "pylon whirl." The manufacturer has agreed to make the Bell 206 community aware of this condition through either an operations safety notice and/or a change to the flight manuals. The operator's manual for the U.S. Army OH-58A/C (a single engine, observation type militaryhelicopter produced by Bell, similar to the Bell 206 series), Technical Manual 55-1520-228-10, discusses the pylon whirl condition. This manual states in part; "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 during touchdown autorotations, if operational limits are exceeded." Spike Knock Condition During the course of the accident investigation, it was identified that the Bell 206 series flight manuals were deficient with the identification and reporting of the condition known as "spike knock." The manufacturer has agreed to make the Bell 206 community aware of this condition through either an operations safety notice and/or a change to the flight manuals. The operator's manual for the U.S. Army OH-58A/C (a single engine, observation type military helicopter produced by Bell, similar to the Bell 206 series), Technical Manual 55-1520-228-10, discusses the spike knock condition. This manual states in part; "Spike knock occurs when the round pin in the drag-pin fitting contacts the side of the square hole of the pylon stop, which is mounted to the roof. It creates a loud noise and will occur during a rocking of the pylon. The following factors can cause a spike knock: low rotor revolutions per minute (RPM), extreme asymmetric loading, poor execution of an autorotational landing and low G maneuvers below +0.5 Gs." "Spike knock will be more prevalent during zero ground run autorotational landings than for sliding autorotational landings and running landings." "Spike knock in itself is not hazardous but is an indicator of a condition that could be hazardous. If spike knock is encountered, an entry must be made on Department of Army Form 2408-13-1, to include the flight conditions under which the spike knock has occurred. An inspection will be performed by maintenance personnel before continuing." "During landing, starting, and rotor coastdown, spike knock could also occur, especially if there are high winds and/or the elastomeric damper is deteriorated. This type of spike knock is not considered damaging to the aircraft." Bell 206A/B Series Maintenance Manual The Bell 206A/B series maintenance manual does briefly discuss the pylon whirl condition and the spike knock condition in a pylon whirl inspection flowchart in Chapter 63 – Main Rotor Drive System. This manual states in part; "Perform pylon whirl inspection following pilot report or evidence of abnormal landing, excessive slope landing, operation in severe turbulence, low rotor RPM during flight (power on or off), rapid and extreme cyclic input, excessive spike knock, or main driveshaft contact with isolation mount." AbnormalVibrations The Federal Aviation Administration's Helicopter Flying Handbook (FAA-H-8083-21A) discusses abnormal vibrations associated with helicopters. This handbook states in part; "With the many rotating parts found in helicopters, some vibration is inherent. A pilot needs to understand the cause and effect of helicopter vibrations because abnormal vibrations cause premature component wear and may even result in structural failure. With experience, a pilot learns what vibrations are normal and those that are abnormal, and can then decide whether continued flight is safe or not. Helicopter vibrations are categorized into low, medium, or high frequency." "Low-frequency vibrations (100–500 cycles per minute) usually originate from the main rotor system. The main rotor operational range, depending on the helicopter, is usually between 320 and 500 revolutions per minute." "Medium-frequency vibrations (1000–2000 cycles per minute) range between the low frequencies of the main rotor (100–500 cycles per minute) and the high frequencies (2100 cycles per minute or higher) of the engine and tail rotor." ## Aircraft - Registration: [N49660](https://openairdocs.org/aircraft/N49660), registered now, S/N 1825, built 1975 (OpenAirDocs) - Reported make and model: BELL 206 (B) - Registry model: [Bell 206B](https://openairdocs.org/model/bell-206b) (registry code 1181511) (OpenAirDocs) - Registry engine: [Allison 250 SER 400HP](https://openairdocs.org/engine/allison-250-ser-400hp) (registry code 03013, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 1825 - Year built: 1975 - Airframe hours: 4775.8 - Engines: Rolls Royce 250-C20B 400 horsepower turbo shaft - Damage: Substantial damage - Flight: Personal, Part 91 - Airworthiness certificate: Standard, Normal - Departed: 71J, OZARK, AL - Destination: 71J, OZARK, AL - Equipment: None ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 40; injuries: none; commercial; instrument ratings: helicopter; medical: class 1, without waivers/limitations; last flight review April 26, 2015; seat: left; restraint: 4-point; flight hours: 1,627.5 total, 260.1 in this make and model, 1,200 as pilot in command, 58.9 in the last 90 days, 30.2 in the last 30 days, 1.7 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 40° at 8 knots - Visibility: 10 statute miles - Lowest ceiling: 2,500 ft broken - Temperature: 79 °F, dew point 66 °F - Altimeter: 30.00 inHg - Observed at: KHEY, 7 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 20150924017459A](https://openairdocs.org/incident/20150924017459A), Sep 24, 2015: Bell 206B, Ozark, AL, Substantial damage. Came to hover, translated left and snagged the LH skid. Recovered, felt vibration , pilot decided to attempt a hovering autorotation , resulting in a hard… ## Docket (3 files) The files NTSB released with the investigation on March 7, 2016, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=92136. 1.1 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 437 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40437338&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF - Accident Photographs (item 2, .PDF, 586 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40437339&FileExtension=.PDF&FileName=Accident%20Photographs%20-Master.PDF - OH-58 A/C Spike Knock and Pylon Whirl Information (item 3, .PDF, 66 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40437340&FileExtension=.PDF&FileName=OH-58%20A%2FC%20Spike%20Knock%20and%20Pylon%20Whirl%20Information%20-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.