The RAF Puma flew for 20 years plus in a condition likely to give a hardover in yaw due to the lack of a centre-ing device in / on the servo mechanism. It would happen if you lost one of the 2 pull cables to the tail rotor hydraulic servo valve. On this type there is no manual reversion and no way of isolating the hydraulics.
We looked at this type of failure in the sim, and discovered it was probably irrecoverable, as I intimated earlier. I did some personal research in the mid 1980's and discovered that the Norwegian aircraft had a centre-ing device fitted. Myself and other QHIs afterwards bleated long and hard about this and I think we eventually got through to the powers-that-were, as all the RAF aircraft are, I believe now modded with a centre-ing device. If not, they Bl**dy should be!
Even with such a device fitted, a broken pitch control spider can still give a hardover. Check the UK AAIB site for the report on the S.Wales police helicopter that landed on the roof of a house last year.
It's worth all heli pilots training for this in a sim. It doesn't matter which type as long as the main blades go round in the same direction as your helicopter (otherwise the yaw directions can get a bit confusing). Many heli pilots believe that closing down the engines is the answer to all tail rotor malfunctions. It ain't!
ShyT.
[This message has been edited by ShyTorque (edited 24 February 2001).]