Behaviour of tail rotors under certain flight conditions

Important operational safety information concerning the behaviour of tail rotors under certain flight conditions

FOCA SAND-2009-001


Various incidents and accidents that have occurred due to the incorrect operation of the tail rotor control or delayed action by the pilot have prompted the Swiss Federal Office of Civil Aviation (FOCA) to issue an information bulletin on this topic. The aim here is to ensure that pilots will be able to avoid critical situations in the future, which can frequently be attributed to a lack of familiarity with this particular behaviour of tail rotors.


The points listed below primarily take the form of operational safety information concerning potentially dangerous situations that may arise due to incorrect operation of the tail rotor control:

Hover flight or very slow speed flight with helicopters.

  • Wherever possible, rotation around the vertical (yaw) axis of the fuselage in hover flight should always take place in the direction of the main rotor rotation (power pedal). This requires more power, but can readily be stopped again without the need for additional power.
  • If an unintended rotation in the opposite direction to that of the rotor should occur due to cross or tail wind, or as a result of a sudden increase in the collective pitch, the following action should be taken immediately: The power pedal of the tail rotor control should be applied, if necessary to its maximum extent (mechanical stop), and held in this position until the rotation stops.
  • When a helicopter is being operated at the limit of its performance capability, for example in the mountains, it is important to operate the power pedal at an early stage, especially on approach or when rapidly increasing collective pitch, in order to ensure that no directional changes in direction around the vertical (yaw) axis can occur. This action prevents an unintended rotation opposite to the direction of the rotor rotation, which can only be stopped by applying additional power and usually results in a decrease in the main rotor RPM, since in certain circumstances the required additional power may no longer be available. A decrease in the main rotor RPM leads to a corresponding decrease in the tail rotor RPM, which may, in certain circumstances, result in a complete loss of tail rotor efficiency.
  • Hover and very slow speed flights out of ground effect should be carried out with sufficient reserve power and only by pilots who have been trained for this kind of operation (CPL level) and have the necessary practical experience.

Note: Eurocopter has published a service letter ("Lettre-Service") dealing with this problem, in which the aerodynamic behaviour of various types of tail rotor, including "Fenestron", is described in a clear and comprehensible manner.

This FOCA publication will not guarantee any revisions and new publications on this topic

Further information



Federal Office of Civil Aviation

Helicopter flight operations
3003 Bern
Fax: +41 58 465 96 01

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