# NTSB LAX97LA034: McDonnell Douglas MD-600, N630N, Flagstaff, AZ, November 4, 1996 - Type: NTSB accident investigation - Classification: Accident · No injuries · Limited investigation · Final report - Date: November 4, 1996, 08:23 local (15:23 UTC) - Place: Flagstaff, AZ - Coordinates: 35.22948, -111.72055 (estimated) - Nearby events: https://openairdocs.org/map.md?lat=35.2295&lon=-111.7206&mi=10 - Phase of flight: [Landing](https://openairdocs.org/ntsb-phases/550) (OpenAirDocs' grouping) - Highest injury: No injuries - Injuries: 1 uninjured - Status: Completed, report February 2, 1998 - Report: Approved by Office Director - NTSB case key (mKey): 29678 - NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=LAX97LA034 - NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/29678/pdf - NTSB docket: https://data.ntsb.gov/Docket?ProjectID=29678 - Page: https://openairdocs.org/ntsb/LAX97LA034 ## Probable cause An uncommanded main rotor blade to tail boom contact due to a 'blowback' phenomena after a successful preplanned autorotation by the pilot to a high density altitude airport with a test aircraft designed at a maximum gross weight. ## Sequence of events In NTSB's coding before 2008 (https://www.ntsb.gov/GILS/Documents/codman.pdf). - Miscellaneous/other, landing - roll - misc rotorcraft,main rotor/tail boom contact - design stress limits of aircraft - exceeded, production/design personnel (contributing factor) - inadequate substantiation process,insuff review, manufacturer - rotor system,main rotor - uncommanded (cause) - aircraft weight and balance - selected, production/design personnel (contributing factor) - weather condition - high density altitude (contributing factor) ## Analysis The test pilot made a successful preplanned autorotation to a runway in the test helicopter for certification purposes. The aircraft was operated at a designed maximum gross weight to establish a height velocity curve for future operations in high density altitudes. During the ground slide, the main rotor blades contacted and severed the tail boom. Various combinations of engineering design and the 'blowback' phenomena allowed the retreating main rotor blades to tilt downward and contact the tail boom. ## Factual narrative On November 4, 1996, at 0823 hours mountain standard time, the pilot of a McDonnell Douglas MD-600 (NOTAR) experimental helicopter, N630N, experienced a tail boom separation during a landing at Flagstaff, Arizona. Visual meteorological conditions existed at the time and no flight plan was filed for the local test flight. The aircraft was substantially damaged and the pilot was not injured. The aircraft is owned and operated by McDonnell Douglas Helicopter in Mesa, Arizona, and was being operated under an experimental certificate. The six-bladed main rotor helicopter can seat six to seven persons and is equipped with a 600-shaft horsepower engine. This was a recertification flight test and the pilot was performing a series of height velocity landings. The helicopter was being flown at 4,100 pounds maximum gross weight and was being monitored by onboard instrumentation and telemetry. The pilot indicated this was the 10th landing in the test profile. With a target data entry point of 60 knots indicated airspeed and an altitude of 15 feet, the aircraft touched down at 30 knots. During the 3.5 second and 200-foot slide on the skids, he felt the aircraft shudder, followed by a separation of the tail boom from contact with the main rotor blades. The winds were from 210 degrees at 2 knots and the runway was dry. The pilot reported no mechanical malfunctions or problems with the aircraft prior to the accident. A videotape of the accident sequence was taken by McDonnell Douglas (MD) ground personnel. The tape indicated a normal autorotative approach and touchdown. During the slide down the runway, the main rotor blades contacted the tail boom and severed it. The aircraft came to a full stop and the pilot exited the aircraft. Recorded engineering test data indicated the aircraft touched down at 1.5 g's with a 2.6 foot per second rate of descent. The position of the collective control during the landing and the ground slide was at near the 100 percent up position at touchdown and during the ground slide. In a discussion with MD test engineers, they described the main rotor blade contact with the tail boom as to have been a result of forward velocity and low/decaying main rotor rpm (advanced ratio) due to a full up collective position during the ground run out phase following the autorotative touchdown. In the condition of a high advance ratio, due to the low/decaying main rotor rpm and forward speed, a "blowback" of the main rotor disk occurs. They described this condition as the forward portion of the main rotor disk being displaced upward, while the rear portion of the disk displaces downward. This "blowback" condition is compounded by the high angle pitch setting which causes blade stall over a large portion of the rotor disk. This resulted in an excessive "blowback" that quickly allows tail boom contact by the main rotor blades. In a further discussion with the MD engineers, they explained that this "blowback" condition exists in all helicopters, but is more apparent in this model due a greater gross weight, reduced flare/deceleration capabilities because of tail boom length and installation angle, and the increased surface of the additional main rotor blade resulting in a more rapid decay of main rotor rpm. McDonnell Douglas Helicopter Company has claimed an exemption from public disclosure of the engineering test data and the video associated with this accident as proprietary and confidential information. ## Aircraft - Registration: [N630N](https://openairdocs.org/aircraft/N630N), registered now, S/N 0004F, built 1984 (OpenAirDocs) - Reported make and model: McDonnell Douglas MD-600 - Registry model: [Hughes 369F](https://openairdocs.org/model/hughes-369f) (registry code 4470708) (OpenAirDocs) - Registry engine: [Rolls-Royce 250-C47 series](https://openairdocs.org/engine/rolls-royce-250-c47-series) (registry code 54235, turbo-shaft) (OpenAirDocs) - Category: Helicopter - Serial number: 0004F - Airframe hours: 1961 - Engines: Allison 250-C47M 808 horsepower turbo shaft - Damage: Substantial damage - Flight: Part 91 - Registered owner: Mcdonnell Douglas - Airworthiness certificate: Special, Experimental (Special) - Last inspection: Annual, April 26, 1996, 63 hours before - Departed: FLG - Airport: FLG, Flagstaff Pullium, runway 21, 7,999 × 150 ft, asphalt ## Flight crew As NTSB recorded them. NTSB leaves out names. - Pilot; age 48; airline transport, flight instructor, commercial; airplane ratings: multi-engine land, single-engine land; instrument ratings: helicopter, airplane; medical: class 2, valid medical--w/ waivers/lim; seat: left; flight hours: 10,317 total, 1,169 in this make and model, 8,120 as pilot in command, 88 in the last 90 days, 31 in the last 30 days, 1 in the last 24 hours ## Weather - Conditions: Visual (VMC), Dawn - Wind: 210° at 2 knots - Visibility: 25 statute miles - Lowest ceiling: none - Temperature: 32 °F, dew point 28 °F - Altimeter: 29.00 inHg ## FAA reports on this event (1) The FAA's own accident and incident reports, matched to the aircraft by N-number and date (OpenAirDocs). - [Incident 19961104034519I](https://openairdocs.org/incident/19961104034519I), Nov 4, 1996: Hughes 369FF, Flagstaff, AZ, Minor damage. Narrative: while performing flight tests on a proto-type MD630N at Flagstaff airport, Flagstaff, Arizona, the pilot entered the ninth (9th) auto-rotation of… ## Other investigations on N630N (2) 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. - [NTSB LAX04CA122](https://openairdocs.org/ntsb/LAX04CA122), Feb 12, 2004: McDonnell Douglas 369FF (N630N), Mesa, AZ, substantial damage. Probable cause: the pilot's inadequate compensation for the wind gusts and subsequent failure to maintain directional control of the helicopter, which resulted in a hard landing. A contributing factor was a wind… - [NTSB LAX97LA061](https://openairdocs.org/ntsb/LAX97LA061), Nov 21, 1996: McDonnell Douglas MD-600N (N630N), Flagstaff, AZ, substantial damage. Probable cause: The pilot's inattention to the altitude and inadvertent late entry into an autorotation maneuver below the established minimum test altitude with a helicopter operating at maximum gross weight in a… ## Docket (3 files) The files NTSB released with the investigation on August 13, 1997, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=29678. 1.1 MB in all. - Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2 (item 1, .PDF, 513 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40389995&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1%2F2-Redacted.PDF - MD 630N Information (item 2, .PDF, 376 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40071961&FileExtension=.PDF&FileName=MD%20630N%20Information-Master.PDF - MD vertical horzontal ground data (item 3, .PDF, 178 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40071962&FileExtension=.PDF&FileName=MD%20vertical%20horzontal%20ground%20data-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.