Helicopter Autorotations

Helicopter engine failures are probably the number one worry that anyone flying in a helicopter may have. It’s human nature to be concerned about things we don’t fully understand and very often, once any subject of concern is fully understood, it is no longer a source of worry. The subject of helicopter autorotation is one such subject, both for a casual observer or passenger and also prospective helicopter pilots too.

Here’s an in depth explanation by Yorkshire Helicopters founder and chief pilot Oli Nicholls on exactly what happens in a helicopter autorotation and how the landing can easily be a successful event.
We also have a more simple guide on this blog post explaining the basic elements, so if you’re completely new to helicopters and simply have concerns, you may wish to read this first, then return to this page.

Ready to find out more about Helicopter Autorotations? Here We Go

It is a misconception that if the engine of a helicopter fails then it will fall from the sky in dramatic fashion. First of all let me settle your nerves. That doesn’t happen. So what does happen? You need to enter a state of flight known as autorotation. Essentially this is a descent, with enough of a decent rate that the up-flow of air passes through the helicopter blades to continue to drive them round at sufficient speed in the absence of the engine – a bit like a sycamore seed falling from a tree.

I remember looking up autorotations myself before I even stepped into a helicopter for the first time. It can be a bit off-putting because it seems quite a complex and scary manoeuvre particularly when the explanation involves a series of graphs that immediately makes you look elsewhere for a quick answer. The truth is it will take you longer to master the art of hovering than it will to enter, maintain and recover a basic autorotation. Even the advanced ones can be a formality.

When you’re learning to fly, autorotations come after you will already be proficient at controlling the helicopter at altitude, you will be able to alter speed, turn, climb and descend. Then comes the unknown autorotation. I must say at this point that very, very few pilots will ever have to use this well practised exercise in anger. And I hope that it remains just that – a well practised exercise.

So back to the more technical elements. Like I’ve already said if the engine fails there has to be a way of controlling the helicopter to the ground safely and the first and arguably most important part of this exercise is the initial entry into the autorotation.

So how do you enter an autorotation?

Simple. Just descend. At least in theory it is almost the exact same control inputs required for a normal descent except larger, done quicker and with an exaggerated flare which can seem more abrupt at first.

The rate at which you enter this descent is something that is built up gradually. Your first autorotation will be a slow and steady initiation into a descent until eventually the collective lever is all the way down and you cannot descend any more.

At this point the air travelling up through the blades is enough to drive them round and the engine RPM can be reduced using the throttle safely while the rotor RPM remains in an acceptable range. The process is similar to riding a bike, if you stop pedalling (the engine stops) the wheels (the blades) keep turning.
It is important to note however that for practising we ‘enter the autorotation’ before we reduce engine rpm – this is for safety. In a real engine failure the engine RPM have already reduced as a result of a fault and therefore the autorotation is now a necessity.

Nothing is more certain than the fact your altitude will be reducing as a result of the vastly reduced thrust. Thrust is the force produced by the blades that is holding you up and weight the one that pulls you down. If weight is greater than thrust (like it is in autorotation) then you are descending. For basic autorotations (those flown at a steady speed and direction of travel) that really is the most dramatic part over.

So what do we do now? I must remember to mention rotor RPM at this point. As you probably already know, the collective lever changes the angle of the blades and therefore the amount of lift and drag they experience. When you raise the lever in normal powered flight the angle of the blades increases which means you get more lift. Thanks to clever engineering the engine delivers more power to overcome the effects of drag thus keeping the blades at the same speed and so we go up.

In autorotation there is no engine and so when you raise the lever, the angle of the blades increases and the effects of drag slow them down. When you lower the lever the opposite will occur; drag reduces and the blades speed up. So the collective controls rotor RPM. Everything else you will be glad to know remains the same: the cyclic controls speed and direction while the pedals control yaw and balance.

So why the mention of RPM? During a basic autorotation rotor RPM remains quite stable. However slight adjustments may need to be made to keep the rotor RPM high enough to maintain a controlled descent but not so high that we damage the blades and bearings.

From now on in this article, we’re going to assume that RPM means the rotor RPM, not engine, as obviously we’re assuming engine failure.

For practising sake this is important. RPM varies with weight, so if the helicopter is heavy at the time of practise then the helicopter falls quicker, more air passes through the rotor, and the RPM sits higher. You might need to raise the lever a fraction in this case. If the RPM becomes too low you would lower the lever slightly to increase RPM. On any given day and conditions RRPM will vary and so it’s important we have some control over it.

At high density altitudes, autorotative descent, and therefore RRPM are also higher. Imagine a pebble falling quickly through the air then slowing down as it hits the surface of  water and slowly sinking from that point onwards. The water can be likened to more dense air closer to the ground.

When you move onto advanced autorotations the objective is to fly the helicopter in autorotation safely into a suitable landing area which might involve changing the helicopter’s speed, direction, and rotor RPM. If you fly slower then you will travel less distance in a given time thus reducing the helicopter’s range in autorotation. If you carry out a series of turns, or one prolonged turn, you can keep the helicopter in a small radius of position over the ground. Reducing rotor RPM (while keeping it within acceptable limits) can help the helicopter reach its maximum range by increasing lift.

It is much more common to see fluctuations in rotor RPM during advanced autorotation practise, in particular during turns and changes in speed because you are altering the amount of air that is passing through the rotor.
More technically you are actually changing the helicopters ‘effective weight’. We already said that RPM increases with increasing weight, and falls with decreasing weight, but that was the real weight of the helicopter.

So what is ‘effective weight’? It’s the amount of weight the helicopter appears to have as a result of a particular manoeuvre. In a steep banked turn, for example, the ‘effective weight’ increases. Not only do you have the weight of the helicopter pulling the aircraft down, you also generate a centrifugal force acting away from the centre of the turn making it ‘feel’ heavier. As well as this; during the turn the aircraft will fall faster as a result of the thrust being used to turn the helicopter as opposed to acting directly upwards holding us in the air.

These effects can be short lived and wear off once you have resumed a normal level autorotation. During this time you need to control RPM by raising or lowering the lever accordingly to change the amount of drag.

The flare (a rearward movement of the cyclic forcing the rotor disc and nose of the aircraft upwards) is a manoeuvre used to restore rotor rpm on the entry and slow you down a bit later. If you were unfortunate enough to suffer a real engine failure, then unless you saw it coming you will inevitably experience a loss of rotor RPM until the moment you establish autorotation.

We teach the initial entry to autorotation to include a flare because the effect of this is to increase rotor rpm by momentarily increasing the flow of air into the rotor system from the underside which is achieved in a nose up position.

As a side note, forward cyclic has the opposite effect and decreases RPM momentarily (until the increased speed you get by holding the nose down causes rate of descent to build and thus eventually RPM goes up because you fall quicker.

As a rule of thumb: flaring or turning increases RPM. Rolling out of the turn and forward cyclic decreases RPM. Knowing this can help you be one step ahead of the RPM and proactively control it rather than reactively chasing it.
So after all that hard work, you have changed speed and direction whilst simultaneously controlling rotor RPM and you are now within a whisker of a field to land in. Well done.

You will preferably be into the wind as the point now is to slow the helicopter right down and reduce the rate of descent enough to land it. The wind has a huge influence on your groundspeed. It is actually your groundspeed you want to reduce to almost zero. Ideally your indicated speed is somewhere between 60-70 knots (this depends on type) so whatever happened before to get you to this point you need to be conscious of ensuring your speed is somewhere within this region ready for the next part.

This might mean having to force the nose down to recover any lost speed. Being into a wind of say 30 knots means you groundspeed is actually only 30-40 knots leaving less work to be done during the final flare. If you got it wrong and found yourself downwind then in reality you have 90-100 knots of groundspeed to lose. A tall order. During practise you will always do this into the wind and often an instructor will want to see at least 10 knots to make life easier.
The flare to land should be ‘progressive’ meaning you may gradually increase the amount of flare depending on what happens. At around 40-60 feet above ground level and 60-70 knots, you will begin to apply rearward (also called ‘aft’) cyclic in an attempt to flare the helicopter.

The nose up position (or attitude as it might be called) does a few things: first of all the small amount of thrust you do have during autorotation acts in the opposite direction to that you are travelling and starts to slow you down. As the increased amount of air passes through the rotor disc this gives you a little extra lift and begins to slow the descent rate. You will also experience an increased rotor RPM but you don’t want to over correct and raise the lever too much. The last thing you want is to slow the blades down excessively at this point.

Depending on how aggressive the initial flare was and the effect of the wind you will either not slow down enough and need to flare more, or you might start to slow down too quickly resulting in stopping with too much height in which case you want to hold off for a few seconds longer until closer to the ground.

It’s all about judgement and practise. At some point one of two things will happen: you have flared so much there is little else to give, or you are still in a flare position but close to the ground in which case the tail might contact the ground (it does happen). In either scenario your action is the same, to level the helicopter usually by gently using forward cyclic. If you do this too aggressively you can cause the helicopter to generate some forward speed again so smoothly does it.

The helicopter is now level at a reasonable height off the ground and hopefully minimal speed for the conditions. Just before your skids contact the ground there is one last thing to do: smoothly raise the lever and generate any last remaining lift they have left, bearing in mind the blades will start to slow down as a result of the drag this creates. This lift has the effect of ‘cushioning’ the landing.

At that stage it is important to keep the helicopter as straight as possible using the pedals while allowing the friction between the skids and landing surface to slow the helicopter until it comes to a complete stop. Tip here: don’t pull back on the cyclic, that doesn’t have the effect of slowing you down quicker. If you needed to decelerate quicker once on the ground then you could, in theory, and cautiously, begin lowering the lever.

This may sound like a very complex set of skills to learn and try and memorise, though in fact, it takes far longer to describe it here than it does to actually do it for real.

The overwhelming majority of helicopter pilots will never, ever need to use these skills for real, though we practice them all the time so that, should the need arise, we have the skills and confidence in place to instinctively make the correct decisions that will bring us to a successful landing. If you’re just starting out in helicopter flying and would like to know more, then hopefully we’ve inspired you to try a helicopter trial lesson and dip a toe into the fascinating world of being a helicopter pilot.