Synopsis
Shortly after takeoff, the pilot of a Hughes 369D helicopter experienced yaw control difficulties, resulting in a loss of control and subsequent crash landing. The pilot sustained minor injuries. With no mechanical abnormalities found, the most probable cause is loss of tail rotor effectiveness (LTE).
History of the Flight
The pilot arrived to reposition the helicopter to its normal base after annual maintenance. Weather was good with surface wind from 250° at 15 kt. After resolving outstanding maintenance items, the flight commenced. The helicopter faced into wind and lifted into a high hover. The pilot reported lacking directional control via the yaw pedals; after a few seconds, the helicopter began spinning. As the spin accelerated, the pilot became disorientated and lowered the collective lever to land. The helicopter drifted right into a wooded area beside the apron, colliding with trees, and came to rest on its side partly submerged in a pond. The pilot was briefly unconscious, freed himself from the harness, and escaped through the shattered windscreen. Witnesses offered assistance.
Background Information
During the annual inspection, the only work on the yaw control system was routine replacement of the tail rotor gearbox oil. Post-accident inspection confirmed continuity between yaw pedals and tail rotor blades with no defects. Damage indicated a right turn.
Loss of Tail Rotor Effectiveness
LTE is a critical low-speed aerodynamic characteristic causing rapid, uncommanded yaw. If not corrected, it can result in loss of control. It is not a technical malfunction and can occur in conventional single main rotor helicopters below 30 kt. LTE occurs when airflow through the tail rotor is disturbed, altering thrust. Disturbances can come from main rotor downdraft, blade tip vortices, or natural turbulence/wind. Conditions inducing LTE include high power settings and slow forward airspeeds, especially where translational lift is changing and relative airflow is within ±15° of the 10 o'clock position for anticlockwise main rotor helicopters, making right turns more susceptible.
Discussion
A high power setting was needed for the vertical climb into a high hover. The 15 kt windspeed could trigger translational lift in hover; a shift in relative wind toward the 10 o'clock position could cause main rotor tip vortices to impinge on tail rotor airflow. Given these conditions and no mechanical abnormalities, LTE most likely caused the loss of control. The pilot stated he was unaware of LTE; his training in 1984 and subsequent tests did not cover it. Following a 2003 fatal accident attributed to LTE, the AAIB made Safety Recommendation 2003-126 for the CAA to publish LTE information. The CAA did so in training materials, GASIL 1/2004, and FODCOM 1/2004.
Current Education and Training on LTE
The CAA advises that current training addressing LTE includes: EASA Part-FCL syllabus (PPL(H) Ex 18(d) and theoretical knowledge); AIC Pink 066/2013 recommending flight and theoretical awareness for light helicopter pilots; type-rating training covering specific LTE characteristics; and EHEST materials including a training leaflet with an LTE section.
Safety Action
The CAA plans to review and expand Safety Sense Leaflet 17, 'Helicopter Airmanship', to include LTE information and avoidance/recovery techniques.