ANC04LA087

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.

History of Flight

On August 6, 2004, about 0855 Alaska daylight time, a skid-equipped Hughes OH-6A helicopter, N243D, sustained substantial damage when it impacted the ground following a loss of directional control while landing at Merrill Field, Anchorage, Alaska. The helicopter was being operated as a visual flight rules (VFR) local area personal flight under Title 14, CFR Part 91, when the accident occurred. The helicopter was operated by the pilot. The private rotorcraft-rated pilot and the sole passenger received serious injuries. Visual meteorological conditions prevailed. The flight originated at Merrill Field, about 0840. No flight plan was filed, nor was one required.

Numerous witnesses reported that the helicopter was near midfield over runway 24 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 the helicopter reached about 100 feet above the ground. The helicopter then descended to the ground while rotating, and came to rest on the turf along the south edge of runway 24. 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.

During an interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) on August 6, the pilot reported that he was preparing to land at a hangar, located south of runway 24. He said he was returning to land at the conclusion of a short local flight. He indicated that about 15 feet 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.

The accident sequence was recorded on an on-board video camera mounted in an airplane that was taxiing eastbound on taxiway "November." The wide angle lens of the camera was recording a view of the rear seat passenger, but also captured images of the accident helicopter as it was landing. A copy of the video was reviewed by the NTSB IIC, and it contained a view of the accident helicopter coming into view at the right edge of the video screen. The helicopter appeared to be in a stable, slow speed hover about 15 to 20 feet above the ground. The helicopter then began to rotate to the right and ascend before descending. It spun about 7 turns before ground contact.

Crew Information

The pilot holds a commercial pilot certificate with airplane single-engine land, multiengine land, and instrument airplane ratings. He also holds a private pilot certificate with a rotorcraft helicopter rating, which was issued on June 6, 2004. According to the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1) submitted by the pilot, the pilot reported that he had accrued over 10,000 hours in all aircraft, 101.5 hours in rotorcraft, 58.5 hours in the accident aircraft make and model, and 19.2 hours as pilot-in-command in the accident aircraft.

Aircraft Information

The helicopter was a previous military observation aircraft that was issued a standard airworthiness certificate by the Federal Aviation Administration (FAA) on September 26, 2003. The most recent annual inspection was October 3, 2003.

Meteorological Information

At 0853, an Aviation Routine Weather Report (METAR) at Merrill Field was reporting in part: Wind, 250 degrees (true) at 4 knots; visibility, 10 statute miles; clouds and sky condition, clear; temperature, 63 degrees F; dew point, 48 degrees F; altimeter, 29.97 inHg.

Wreckage Information

The helicopter was examined in the pilot/owner's hangar at Merrill Field on August 10, 2004, by the NTSB IIC. The examination revealed drive-train continuity from the main rotor system, through the transmission, to the tail rotor gearbox. The tail rotor pedals had control continuity from the cockpit to the tail rotor blades.

The lower end of the vertical stabilizer was bent to the right. The tail boom had a counterclockwise twist about 2 inches forward of the tail rotor gearbox attach point. The tail rotor blades were separated about midspan, and each exhibited tearing at the point of separation. The tail rotor gearbox rotated freely by hand. The tail rotor gearbox chip detector was free of any metal contaminants. The tail rotor drive shaft was slightly bent about midspan, with rotational scuffing visible about 18 inches forward of the aft end of the shaft. The midspan hanger bearing surface was shiny and clean. Slight rotational scuffing was visible just aft of the bearing surface. The tail rotor shaft was separated about 20 inches aft of the forward end. The point of separation had counterclockwise rotational twisting and compression on the aft end of the separated segment.

No preaccident malfunction of the anti-torque system was observed.

Additional Information

A loss of directional control that produces a right yaw, other than a mechanical malfunction, may be attributed to several factors. The FAA discusses unanticipated right yaw, also called loss of tail rotor effectiveness (LTE), in Advisory Circular 90-95. The advisory circular notes that a loss of control 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.

The U.S. Army, the primary operator of the OH-6 helicopter, includes a discussion of LTE in the aircraft's operators manual (TM 55-1520-214-10). Chapter 8-31 of the Army's flight manual contains the following: "a. Loss of tail rotor effectiveness is the occurrence of an uncommanded and rapid right yaw which does not subside of its own accord and which, if not quickly reacted to, can result in loss of aircraft control. However, the term 'loss of tail rotor effectiveness' is misleading. The tail rotor on this aircraft has exhibited the capability to produce thrust during all flight regimes. Under varying combinations of wind azimuth and velocity, tail rotor thrust variations can occur. When this occurs, the helicopter will attempt to make a sudden quick yaw to the right. This yaw is usually correctable if immediate additional left pedal is applied. Correct and timely pilot response to an uncommanded right yaw is critical. If the response is incorrect or slow, the yaw rate may rapidly increase to a point where recovery may not be possible in the terrain flight regime. The pilot must anticipate these variations, concentrate on flying the aircraft, and not allow a yaw rate to build."