Dealing with Tail Rotor Problems

When I was learning to fly helicopters, I practised all sorts of emergencies with my instructor. We performed simulated engine failures numerous times – into wind, downwind, and in the hover. We talked about what you would do if you had an engine fire, if the governor failed, or if a warning light came on. We simulated vortex ring, sometimes known as ‘the helicopter stall’. And we also discussed tail rotor problems… but only a very little. Indeed, I think I finished the PPL(H) course with only a very vague idea about possible malfunctions of the tail rotor, and perhaps even less knowledge of what to do about them.

Of course, it’s quite possible that my early training left something to be desired, but I don’t think I was unique in knowing woefully little about this. One of the reasons for this is that, thankfully, it is very rare for something to go wrong with a helicopter’s tail rotor. So maybe it is not surprising that the PPL(H) course concentrates to a greater extent on more common emergencies. Also, it is rather difficult to simulate tail rotor failure. It follows that students and low hours pilots can easily end up knowing very little about it. This is not often an issue; in fact, I don’t think I know anyone who’s had a serious tail rotor problem… yet! However, in rotary aviation as in everything else, if something can go wrong, some day it probably will. So, it’s as well to have a pretty good idea in advance of what you would do about it.

Almost always, in practice, the tail rotor starts to malfunction because it hits something

Tail rotor failures can be caused by a variety of malfunctions – complete stoppage of the working of the tail rotor for some reason, loss of a tail rotor blade, or separation of the tail rotor itself from the aircraft. But in a well-maintained aircraft, these are all very unlikely to happen. They are even less likely to happen unannounced, without a warning light or vibration giving you time to land before the situation becomes too bad. Almost always, in practice, the tail rotor starts to malfunction because it hits something. There is fantastic visibility in helicopters, but in most of them you can’t see behind you. It is easy to forget just how long your aircraft is, and to hit the tail rotor against a fence, building, vehicle or something similar. This doesn’t do it any good at all! Actually, now I come to think of it, I do know someone who wrecked the tail of his R22 when trying to land in the mountains of mid-Wales, and although both he and the helicopter survived, the incident did a fair amount of damage to both.

So, what are the symptoms of tail rotor failure or damage? In just about every case, no matter what the cause, there will be a rapid and uncontrollable right yaw (or left yaw in French and Russian helicopters). In flight it is the tail rotor which prevents the helicopter turning in circles due to torque reaction to the main rotor blades, and without a working tail rotor, this rotation can no longer be prevented. Therefore, you need to get rid of the power to the main rotor, and you do this by immediately entering autorotation. I have read of other ways of dealing with the situation, but without lots of practice, I wouldn’t suggest trying them for real – better to enter autorotation, and do it quickly before the helicopter starts revolving too fast!

According to the Robinson R22 Pilot’s Operating Handbook, if a suitable landing site is not available, the vertical fin may permit limited controlled flight at very low power settings and airspeeds above 70 knots. This is worth bearing in mind, though I’ve heard from other sources that you need to get the power off immediately in order to prevent the helicopter spinning out of control. I daresay if you had enough height you could experiment… for a few seconds at least. Either way, prior to reducing airspeed you would need to enter full autorotation and perform an engine-off landing.

A surprising number of students aren’t taught about LTE during the PPL course

Some of you may be surprised to hear that tail rotor failure is actually so rare. Haven’t you read about accidents caused by a helicopter spinning around out of control? Wouldn’t this have been caused by a tail rotor problem? Yes, it most probably would. But these types of accidents are most frequently caused not by an actual failure, but by what is known as Loss of Tail Rotor Effectiveness (LTE). Never heard of it, or not sure what it is? You’re in good company; a surprising number of students aren’t taught about LTE during the PPL course. But if you fly helicopters, or think you might someday, it’s something you need to know about, so read on…

LTE is typically defined as “… a critical lowspeed 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 your 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! LTE 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 what exactly causes them?

As stated above, tail rotor thrust is used 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.

Typically, this problem occurs when a great deal of power is being applied to the main rotor. Thus, LTE is most common at stages of flight with high power requirements, such as flying at low airspeed and out of ground effect, perhaps on a photo sortie where the helicopter is being operated in a high hover. The situation will be exacerbated by high altitude or hot weather, since again, more power will be required. Since so much power is being generated by the main rotor, a similar amount of thrust will be required from the tail rotor, and sometimes there is simply not enough there.

I typically tell my students, “If you turn too fast to the right you can run out of pedal to stop the turn”

The other main factor affecting the situation is the wind direction. Hovering with a tail wind, or with a wind from the left, is difficult; and pedal turns to the left are much easier than those to the right. In fact, in a strong wind you have to be very careful when turning to the right, or things can get out of control. I typically tell my students, “If you turn too fast to the right you can run out of pedal to stop the turn”. What I actually mean is that you can easily encounter LTE. In fact, wind from different directions actually affects the tail rotor in different ways, but the aerodynamic details of that are too complicated to go into here, and really not necessary.

It is usually possible to deal with any of these scenarios, and many others. The problems occur when you leave things too long and the situation begins to get out of control. Although it may be possible to prevent a rapid yaw fairly easily, much more power is required to stop it once it has begun, and you can quite literally run out of left pedal.

So, what should be done about LTE? As with so many things, prevention is better than cure, so avoid it if at all possible. In relatively strong winds, try not to operate at slow airspeed – particularly below translational lift speed – with a tailwind. Avoid out-of-ground-effect hovering and situations demanding high power, such as low speed downwind turns. Always be aware of the wind direction and strength, and stay alert to changing conditions generally.

If you should be unlucky enough to encounter LTE, the general advice on recovery technique is firstly, apply full left pedal. Simultaneously, move the cyclic forward to increase the airspeed. This will move air past the vertical stabiliser, which will help to stop the rotation. If you have the altitude, lower the collective and reduce the power, since this will reduce the tail rotor thrust requirement. Then, as you start to recover, adjust the controls for normal fight… and start to breathe again! Needless to say, check the Pilot’s Operating Handbook for the helicopter you are flying for specific guidance.

Now, before you start to worry and vow that you’ll never get into a helicopter again, remember that knowledge is power. If you know about the forces involved in LTE, you are halfway to preventing it. Staying out of low-airspeed highpower situations when possible, and concentrating 110% if you really need to fly in these conditions, should mean that you never encounter this problem. And if you recognise it, correcting action is quite possible and not all that difficult, at least in the very early stages. So, do enjoy your flying, but remember that there’s a good reason why most experienced helicopter pilots are cautious and don’t take unnecessary risks.

Helen Krasner

 

Author: FTN Editor

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