# NTSB FTW96TA222: McDonnell Douglas MD-520N, N520DD, Bethany, OK, May 16, 1996 - Type: NTSB accident investigation - Classification: Accident · No injuries · Final report - Date: May 16, 1996, 15:00 local (20:00 UTC) - Place: Bethany, OK - Coordinates: 35.500148, -97.640922 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=35.5001&lon=-97.6409&mi=10 - Phase of flight: [Standing](https://openairdocs.org/ntsb-phases/150) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 2 uninjured - Status: Completed, report May 23, 1997 - Report: Approved by Office Director - NTSB case key (mKey): 19999 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=FTW96TA222 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/19999/pdf - Page: https://openairdocs.org/ntsb/FTW96TA222 ## Probable cause The main rotor blades' contact with the tail boom due to gusting winds during rotor deceleration. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Miscellaneous/other, standing - idling rotors - rotor system,main rotor blade - blade strike (cause) - weather condition - gusts (cause) - weather condition - tailwind ## Analysis Prior to the landing, the instructor pilot verified the wind from the tower (180 degrees at 19 knots, gusting to 22 knots). With the helicopter stable on the ground, downwind from a nearby building, the flight controls were frictioned down and normal shut down procedures were started. Passing through 19% rotor RPM, the rotor brake was applied. To demonstrate that the rotor would continue turning without rotor brake application, the instructor released the brake at 10% RPM and allowed the rotors to turn down to 9% RPM. The rotor brake was then re-applied, and while the rotor was decelerating, 'two or three thumps were heard', and the instructor observed the rotors 'jerk and come to a stop.' The instructor stated that wind being redirected around the building may have caused the rotors to flap down into the tail boom. Examination of the aircraft revealed that at least two of the three main rotors contacted the tail boom. No mechanical defects were found in the flight control and rotor systems. According to the manufacturer's flight manual, the shut down sequence and rotor brake application were conducted in accordance with current published procedures. ## Factual narrative On May 16, 1996, approximately 1500 central daylight time, a McDonnell Douglas MD-520(NOTAR) helicopter, N520DD, owned by U.T.E. Management Corporation, and operated by the Federal Aviation Administration (FAA) as a Title 14 CFR Part 91 public use training flight, sustained substantial damage while executing shut down procedures at Wiley Post Airport, Bethany, Oklahoma. Visual meteorological conditions prevailed and no flight plan was filed. The airline transport rated pilot-in-command and the pilot receiving the training were not injured. Both occupants were FAA Aviation Safety Inspectors. The flight was conducted locally and had just landed for final shut down and parking. At 1400 the helicopter departed to conduct an evaluation/currency flight for an FAA Aviation Safety Inspector and to familiarize him with the NOTAR helicopter. The pilot-in-command (instructor) was an FAA Aviation Safety Inspector qualified in the helicopter. Subsequent to one hour of uneventful flight, the aircraft landed at the east ramp of the airport. During the approach to landing, the Wiley Post tower reported (to the pilots) winds from 180 degrees at 19 knots, gusting to 22 knots. After the landing, the instructor lifted the helicopter into a hover and demonstrated an autorotation from a hover with no difficulty. Upon completion of the maneuver, the instructor hover taxied to near the original parking spot from which the flight commenced. The nose was oriented to 360 degrees and the helicopter was landed in front of a hangar building (tail facing toward the building). The instructor stated that he selected the landing spot in front of the building because he thought it would block much of the wind. Prior to shut down the instructor demonstrated how to center the rotor disc utilizing outside reference points. With the helicopter stable on the ground, the flight controls were frictioned down and normal shut down procedures were started. Passing through 19% rotor RPM, the rotor brake was applied. To demonstrate that the rotor would continue turning without rotor brake application, the instructor released the brake at 10% RPM and allowed the rotors to turn down to 9% RPM. The rotor brake was then re-applied, and while the rotor was decelerating, "two or three thumps were heard", and the instructor observed the rotors "jerk and come to a stop." The instructor stated that wind being redirected around the building may have caused the rotors to flap down into the tail boom. According to the manufacturer's flight manual, the shut down sequence and rotor brake application were conducted in accordance with current published procedures. Examination of the aircraft by a FAA inspector revealed that at least two of the three main rotors had struck the tail boom, resulting in structural damage to the rotors. No mechanical anomalies or pre-existing defects were found in the flight control system or rotor system (including the rotor brake system). ## Aircraft - Registration: [N520DD](https://openairdocs.org/deregistered/N520DD-1997-06-02), registration cancelled 1997-06-02, S/N LN030 (OpenAirDocs) - Reported make and model: McDonnell Douglas MD-520N - Registry model: [Mcdonnell Douglas Helicopter 500N](https://openairdocs.org/model/mcdonnell-douglas-helicopter-500n) (registry code 3027374) (OpenAirDocs) - Category: Helicopter - Serial number: LN0030 - Airframe hours: 185 - Engines: Allison 250-C20R/2 450 horsepower turbo shaft - Damage: Substantial damage - Flight: Public aircraft, Part 91 - Registered owner: FAA - Airworthiness certificate: Standard, Normal - Last inspection: Annual, September 1, 1995 - Departed: PWA - Airport: PWA, Wiley Post ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 55; flight instructor, airline transport; airplane ratings: multi-engine land, single-engine sea, multi-engine sea, single-engine land; instrument ratings: helicopter, airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 10,000 total, 7 in this make and model, 9,500 as pilot in command, 33 in the last 90 days, 10 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Day - Wind: 180° at 19 knots, gusting 22 - Visibility: 15 statute miles - Lowest ceiling: none ## 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 19960516011459A](https://openairdocs.org/incident/19960516011459A), May 16, 1996: Hughes 500N, Oklahoma City, OK, Substantial damage. Evaluation FAA check flight. During spool down and parking. Wind gusts caused rotors to come down. Struck tailboom. (-23) the pilot had just completed a one… 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.