Merrrill Field Airport
Aircraft Accident/Incident Report

Anchorage, Alaska 99502
Friday, August 6, 2004 8:55 AKD

The commercial certificated airplane pilot was making a landing approach in an ex-U.S. Army military observation helicopter. The pilot holds a private rotorcraft helicopter rating, and had accrued 58.5 hours in the accident helicopter make and model, with 19.2 hours as pilot-in-command. The pilot reported that about 15 feet above ground level (agl), and about 20 knots, he began adding engine power at the conclusion of the landing approach. The nose of the helicopter began to rotate to the right, which quickly became very rapid. The pilot reported that he thought the helicopter made about 5 revolutions. He said that because of the rotation, he was unable to roll the engine throttle to idle. Witnesses reported that the helicopter was near midfield in about a 20 foot-high hover when the nose of the helicopter began to rotate to the right and the helicopter began to climb. The rotations became increasingly rapid as it reached about 100 feet agl. The helicopter then descended to the ground while rotating, and came to rest on the turf along the south edge of a runway. The landing gear collapsed, and the tail rotor blades fractured about mid-span. The main rotor blades continued to turn, and the engine remained running for a short period of time. The accident sequence was recorded on an on-board video camera mounted in an airplane that was taxiing for takeoff as the helicopter was landing. The camera captured images of the accident helicopter coming into view at the right edge of the camera screen. The helicopter appeared to be in a stable, slow speed hover about 15 to 20 feet agl. The helicopter then began to rotate to the right and climb before descending. It spun about 7 turns before ground contact. The helicopter was examined after recovery by the NTSB investigator-in-charge. No preimpact mechanical anomalies were discovered. The reported wind at the airport was from 250 degrees (true) at 4 knots. The FAA and the U.S. Army have published information about the loss of directional control that produces a right yaw, other than a mechanical malfunction, which is also called loss of tail rotor effectiveness (LTE). LTE may be attributed to operations conducted in a high power, low airspeed environment, including left crosswind conditions, wind conditions that may produce weather cocking or weathervaning, a tail rotor vortex ring state, or during the loss of translational lift. Tail rotor effectiveness may be influenced by gross weight, density altitude, low airspeed flight where the tail rotor is required to produce 100 percent of the directional control, and any droop in main rotor rpm.

A loss of tail rotor effectiveness during aerial taxi, and the pilot's delayed remedial action to counteract a rapid right yaw, which resulted in an in-flight loss of directional control, and in-flight collision with terrain. A factor contributing to the accident was the pilot's lack of total experience in helicopters.