NTSB Identification: LAX00FA061.
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Accident occurred Tuesday, December 28, 1999 in AVALON, CA
Probable Cause Approval Date: 10/23/2002
Aircraft: Aerospatiale AS-350D, registration: N3593S
Injuries: 1 Serious,6 Minor.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

The helicopter collided with terrain following a loss of engine power while maneuvering at low level on a sightseeing flight. As the helicopter passed about 250 feet above ground level (agl) over a ridgeline, the pilot began a descending and decelerating right turn to observe a herd of buffalo. About midway down the ridge, the low rotor rpm warning horn sounded. The pilot lowered collective and the horn stayed on. He noted the rotor speed was down between 300 and 320 rpm and concluded that he would have to make an immediate landing. The helicopter landed downslope at an estimated 10 to 15 mph, and the tail rotor struck the ground first. The helicopter pitched forward, continued to slide down the hill, collided with trees, and rolled onto its right side. An 80-psi check of the pneumatic system revealed two calibrated leaks and an additional leak. The attachment fitting for the P3 air accumulator on the governor pressure (Pg) line was 3/4 of a turn loose. The engine was removed and shipped to the manufacturer for testing. Several discrepancies due to damaged hardware were corrected during preparation for the test run. The engine was placed in a test stand and started normally. The engine completed a test protocol with no discrepancies noted. The P3 accumulator fitting was loosened and the engine immediately went to flight idle. The engine would not accelerate above idle power with the P3 accumulator fitting loose. Once the fitting was loosened less than finger tight, it continued to loosen to 3/4 to a full turn loose.

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

A loose pneumatic fitting caused the engine to go to flight idle power at a low altitude, which resulted in a forced landing in hilly terrain.

Full narrative available

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