When I was learning to fly helicopters, we practised all sorts of emergencies. We performed simulated engine failures numerous times; we talked about what to do if you had an engine fire, if a control stuck, or if a warning light came on, and we also discussed tail rotor problems, but only a very little. This is not surprising really, as tail rotor failures are extremely rare. But there is one tail rotor problem that is probably not discussed enough – and should be. This is loss of tail rotor effectiveness, or LTE.
What is LTE?
LTE is typically defined as ‘a critical low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate, which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control’. What that means in practice is that the pedals seem to stop working properly and the helicopter starts turning in circles. It has been incorrectly called ‘Tail Rotor Stall’, a phenomenon, which according to experts, doesn’t actually exist!
Why Does LTE Happen?
A loss of tail rotor effectiveness occurs when the tail rotor is not able to produce enough thrust to stop the yawing of the helicopter. The tail rotor is not stalled; it is simply being asked to do too much, and it can’t cope. Accidents caused by LTE typically occur at low altitude and low airspeed, usually when the pilot is distracted and doesn’t make corrections early enough. But why exactly does LTE happen occur?
Pilots use tail rotor thrust to counteract the torque produced by the turning of the main rotor. The pilot controls the heading when hovering by using the pedals to vary the thrust generated by the tail rotor. This normally works well. But helicopters fly in many different environmental situations, and a number of factors may affect the amount of tail rotor thrust required. This means that sometimes not enough power is available for the tail rotor, and this is when you can experience LTE.
Decoding Science. One article at a time.