To all,
Please note: I was & still am talking ONLY about the tail DRIVE failure here. The actions for tail rotor CONTROL failure are quite different.
Eden,
No, I was not specifically alluding to your post but I am becoming concerned that general advice on this thread is apparently heading towards favouring experimenting with power on, rather than entering autorotaion in what is probably the most critical emergency a rotary pilot will ever have to deal with, i.e. tail rotor DRIVE failure. Hopefully most of us will never have to deal with it for real. Get it wrong on the day and in many (if not all) aircraft you're possibly in a loss of control situation. I have seen many an occasion (in simulators only - agreed and thank goodness! - but in at least three different ones, two American and one French) where a pilot has elected not shut down the engines following a tail rotor DRIVE failure and has completely lost control well before hitting the ground. I lost control myself on a number of occasions before I had got my own skills up to scratch.
There is an infamous piece of film where a Bell 47 (throttle on collective, please note) on a photographic sortie over Sydney harbour, back in the 1960s, lost its tail rotor drive. The (relatively highly experienced) pilot tried to fly out of it under power and completely lost control. For its finale the aircraft descended in a very rapid flat spiral, finally tumbling over. The aircraft crashed hard onto a factory, killing all on board. I was first shown the film back in the 70s by a military QHI, himself now long since retired, who was a personal friend of, and an ex-colleague of, the pilot involved. The mainstream thinking even back then on the '47 was that the correct action in the event of tail drive failure was to autorotate and shut down the engine, which puts the aircraft in a more controllable and predictable flight regime. If he had done that he might have got wet but still been enjoying his retirement today. Whatever, this unfortunate pilot didn't survive to debrief it.
The yaw from transmission drag in autorotation is much less than from the torque reaction in normal powered flight and is quite controllable by use of opposite cylic, in other words cross-controlled. Many tail rotors have considerably less "negative pitch" range than "positive pitch" for this reason.
I cannot see how varying engine power on throttle will do much else more than cause Nr variations and further problems, especially bearing in mind that on something like the R-22, for example, that Nr can be absolutely critical. The main transmission power to keep the aircraft in balance following loss of tail rotor drive would only be that required to offset transmission drag plus any aerodynamic yawing effects from the tail and insufficient to sustain more than a descent.
I accept that it MAY be possible in a controlled situation for an experienced, well trained and pre-briefed crew to do something different and make a safe landing in some cases (hence my comment on reading the flight manual) BUT often, tail rotor drives fail at critical moments, at low speeds close to the ground and sometimes without much warning. A delay of a second or two before pilot action can make a big difference to the outcome. We used to get pilots to realise this in the sim by feeding in vibration as a pre-warning that something was amiss, then fail the tail drive. After a little coaching, usually the guys would get it right, they offloaded the rotor by entering autorotation and usually could fly the aircraft on cyclic, accepting the out of balance situation that resulted. Having done that, it was relatively straight forward to make a controlled landing PROVIDED that the engines were completely shut down before flaring at the bottom for the EOL. If the engines were left running even at ground idle the residual power caused loss of yaw control during the landing.
However, when we later didn't pre-brief them to expect it and gave no warning to the pilot by missing out the vibration, they tended to be slower with the collective lever and quite often lost control because with power applied the aircraft yawed rapidly and then rolled out of control. These guys were no dummies, believe me.
I do know of one notable exception to the above. A military Super Puma operated in the middle east in the early 1990s had a tail rotor drive failure at a high cruise speed (140 kts). The pilot found that each time he tried to gently slow down the aircraft yawed very badly. He admitted afterwards that he didn't attempt to enter autorotation, possibly as he was not too confident of his ability to carry out a good EOL. He elected instead to find a long runway, made like a fixed wing, and ran the aircraft on at 140 kts! He managed to keep the aircraft upright, but he used an awful lot of tarmac to stop! I would not personally have risked that technique but then I was fairly good at engine offs, having had lots of training and given lots of instruction in them on that type (including in the dark and on instruments to ground level just to make it a bit of a challenge if you like!).
I think following drive failure the tail rotor auto-rotating stuff is a red herring. If you react to yaw with pedal you will rapidly slow and probably stop the tail rotor. An auto-pilot heading hold will do the same. I don't think we should kid ourselves that it is going to start up again!
Sorry for the long drawn out post but I think it's very important for people with not much training on tail rotor malfunctions to get it right in their minds before it catches them out for real. Good topic!