NTSB Identification: LAX97LA034.
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Accident occurred Monday, November 04, 1996 in FLAGSTAFF, AZ
Probable Cause Approval Date: 02/02/1998
Aircraft: McDonnell Douglas MD-600, registration: N630N
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

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.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

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.

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