NTSB Identification: LAX97LA034 .
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14 CFR Part 91: General Aviation
Accident occurred Monday, November
04, 1996 in FLAGSTAFF, AZ
Probable Cause Approval Date: 2/2/1998
Aircraft:
McDonnell Douglas MD-600, registration: N630N
Injuries: 1 Uninjured.

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 as follows:
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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