The NTSB recently issued a Safety Alert regarding the loss of tail rotor effectiveness (LTE) in helicopters. In issuing the alert, the Safety Board noted 55 accidents it investigated between 2004 and 2014 involving LTE. The NTSB noted the propensity for LTE, or uncommanded yaw, to occur suddenly in single-engine, conventional helicopters at speeds below 30 knots and the danger of it leading to loss of control.
Factors contributing to LTE identified in the Alert include: varying airflow from the main rotor blades—particularly at high power settings—or from the environment, which can affect airflow entering the tail rotor; operating at airspeeds below translational lift; operating at high altitudes and high gross weights; operating near large buildings or ridgelines, which can cause turbulence; and the relative wind direction.
Prevention and Recovery
The NTSB notes that LTE training is best done in a simulator but even that type of training sometimes is not optimum for creating the "surprise" factor that accompanies the onset of LTE. The NTSB advises the following precautionary techniques for avoiding and recovering from LTE: evaluate wind speed and direction in preflight planning; know and adhere to the performance limitations imposed by the helicopter's manufacturer; be aware of the helicopter's flight control characteristics, particularly tail rotor pedal forces, to recognize and react quickly to unanticipated yaw; review the FAA's Helicopter Flying Handbook for more specific tips regarding LTE; and train for and know how to recover immediately from LTE, mindful that it can be sudden and violent.
LTE (right yaw) recovery techniques per the Helicopter Flying Handbook include forward cyclic and reduced power and collective pitch (altitude permitting), full left pedal and, if none of that works, autorotation.
By way of basic prevention, the Helicopter Flying Handbook on LTE counsels pilots to abide by weight limitations; maintain wind awareness along ridgelines and near buildings, especially at airspeeds near 8-12 knots when a loss of translational lift can occur; avoid tailwinds or crosswinds when operating at airspeeds below 30 knots; avoid out-of-ground-effect operations and high-power-demand situations below 30 knots; and monitor the amount of pedal being used (if insufficient pedal is available the pilot might not be able to counteract unanticipated right yaw).
The NTSB cited three recent investigations and related accident reports as illustrations of LTE accidents. Two of them were fatal. One fatal accident occurred on Oct. 14, 2014, at Wichita Falls, Texas. According to the NTSB, the pilot of a Bell 206L1 was approaching a hospital helipad when he elected to go around. Wind was less than three knots. From a speed of 12 to 15 knots and with one-quarter to one-inch of left pedal applied, he added power, tipped the nose over and pulled collective. The helicopter immediately entered a violent right spin. "The pilot said he tried hard to get control of the helicopter by applying cyclic and initially 'some' left anti-torque pedal 'but nothing happened.' The pilot said he added more left anti-torque pedal, but not full left anti-torque pedal as the helicopter continued to spin, and he was still unable to regain control. He also said the engine had plenty of power and was operating fine. The pilot recalled the helicopter spinning at least five times before striking the ground inverted" and catching fire. The pilot survived, the patient died, and the EMS crew died later from their injuries.
The second fatal accident occurred on Oct. 4, 2011, in New York. A privately operated Bell 206B with five aboard crashed into the water on a sightseeing flight after a rearward departure from the East 34th Street Heliport (6N5). According to the report, the flight departed without the pilot having performed weight-and-balance calculations or any preflight performance planning. On departure, the helicopter transitioned over the shoreline from in-ground-effect to out-of-ground-effect while climbing 60 feet over the water; the helicopter yawed as the pilot then completed a pedal turn and a low-rotor-rpm warning sounded. The pilot thought he had an engine-out warning and turned back to the heliport, which oriented the tail into adverse wind. When he realized the engine was normal, he increased collective; the helicopter entered an uncommanded right yaw that accelerated into a spin that could not be corrected with full left pedal. The helicopter hit the water, rolled inverted and sank. The three rear-seat passengers died from their injuries.
The NTSB issued a Safety Alert last month regarding the loss of tail rotor effectiveness (LTE) in helicopters. In issuing the Alert, the Board noted 55 accidents it investigated between 2004 and 2014 involving LTE. The NTSB noted the propensity for LTE, or uncommanded yaw, to occur suddenly in single-engine, conventional helicopters at speeds below 30 knots and the danger of it leading to loss of control.
Factors contributing to LTE identified in the Alert: varying airflow from the main rotor blades—particularly at high power settings—or from the environment, which can affect airflow entering the tail rotor; operating at airspeeds below translational lift; operating at high altitudes and high gross weights; operating near large buildings or ridgelines, which can cause turbulence; and the relative wind direction.
Prevention and Recovery
The NTSB notes that LTE training is best done in a simulator but even that type of training sometimes is not optimum for creating the "surprise" factor that accompanies the onset of LTE. The NTSB advises the following precautionary techniques for avoiding and recovering from LTE: evaluate wind speed and direction in preflight planning; know and adhere to the performance limitations imposed by the helicopter's manufacturer; be aware of the helicopter's flight control characteristics, particularly tail rotor pedal forces, to recognize and react quickly to unanticipated yaw; review the FAA's Helicopter Flying Handbook for more specific tips regarding LTE; and train for and know how to recover immediately from LTE, mindful that it can be sudden and violent.
LTE (right yaw) recovery techniques per the Helicopter Flying Handbook include forward cyclic and reduced power and collective pitch (altitude permitting), full left pedal and, if none of that works, autorotation.
By way of basic prevention, the Helicopter Flying Handbook on LTE counsels pilots to abide by weight limitations; maintain wind awareness along ridgelines and near buildings, especially at airspeeds near 8-12 knots when a loss of translational lift can occur; avoid tailwinds or crosswinds when operating at airspeeds below 30 knots; avoid out-of-ground-effect operations and high-power-demand situations below 30 knots; and monitor the amount of pedal being used(if insufficient pedal is available the pilot might not be able to counteract unanticipated right yaw).
The NTSB cited three recent investigations and related accident reports as illustrations of LTE accidents. Two of them were fatal. One fatal accident occurred on Oct. 14, 2014, at Wichita Falls, Texas. The pilot of a Bell 206L1 was approaching a hospital helipad when he elected to go around. Wind was less than three knots. From a speed of 12to 15 knots and with one-quarter to one-inch of left pedal applied, he added power, tipped the nose over and pulled collective. The helicopter immediately entered a violent right spin. "The pilot said he tried hard to get control of the helicopter by applying cyclic and initially 'some' left anti-torque pedal 'but nothing happened.' The pilot said he added more left anti-torque pedal, but not full left anti-torque pedal as the helicopter continued to spin and he was still unable to regain control. He also said the engine had plenty of power and was operating fine. The pilot recalled the helicopter spinning at least five times before striking the ground inverted" and catching fire. The pilot survived, the patient died and the EMS crew died later from their crash injuries.
The second fatal accident cited occurred on Oct. 4, 2011, in New York. A privately operated Bell 206B with five aboard crashed into the water on a sightseeing flight after a rearward departure from the East 34th Street Heliport (6N5). The flight departed without the pilot having performed weight-and-balance calculations or any preflight performance planning. On departure the helicopter transitioned over the shoreline from in-ground-effect to out-of-ground-effect while climbing 60 feet over the water; the helicopter yawed as the pilot then completed a pedal turn and a low rotor rpm warning sounded. The pilot thought he had an engine-out warning and turned back to the heliport, which oriented the tail into adverse wind. When he realized the engine was nominal, he increased collective; the helicopter entered an uncommanded right yaw that accelerated into a spin that could not be corrected with full left pedal. The helicopter hit the water, rolled inverted and sank. The three rear-seat passengers died from their injuries.