Fenestron Stall
On early versions of the fenestron, as fitted to the Gazelle, there have been
instances of loss of tail rotor effectiveness during low speed manoeuvres. Extensive trials failed to pinpoint a precise flight regime that would produce this and with modern generations of fenestron, where the direction of rotation is reversed, the problem is not considered likely to occur. However if, during hovering, an undemanded yaw to the left occurs (not associated with loss of TR drive), the correct recovery action is to apply full right pedal, and maintain it until the yawing stops.
From a well known operators EC155 Part B.