Dynamic rollover

Every so often the accident reports include an account of a helicopter that crashed before it had even taken off. Usually what happened was that the pilot started up the machine, did all the necessary checks and prepared to depart, and then began to lift the helicopter into a hover as usual. But in this instance the helicopter didn’t raise itself into a hover; instead, it suddenly and for no apparent reason turned over on to its side. And, since the rotors were running, it then proceeded to rather expensively thrash itself to bits.

So what causes this? If you listen to conversations around the airfield after such an incident, the words being bandied about – even before the AAIB have considered a visit – are “Dynamic Rollover”, usually accompanied by head shaking and glum expressions. Indeed, dynamic rollover is the most common reason – possibly the only one – for helicopters turning over in such a situation.

Surely, even the most ham-fisted and uncoordinated helicopter pilot could not make an error of that magnitude

So what actually is Dynamic Rollover? If you look carefully at a helicopter, it would seem that it would be quite difficult to turn it over, so long as you had some idea of how to fly it. Helicopters sitting on the ground appear to be quite stable: their skids are normally spaced fairly far apart, forming a wide and apparently stable base. Surely, you would think, the only way to turn such a helicopter over on take-off would be to lift off at such an angle that the C of G fell outside of the skids. And wouldn’t this involve some pretty serious mishandling? In fact, for one fairly typical helicopter type – the Bell 206 JetRanger – the angle from the vertical which would displace the C of G to a sufficient extent to cause the helicopter to turn over has been estimated to be about 31 degrees. And in some helicopters, it is even larger – around 40 degrees. Surely, surely, even the most ham-fisted and uncoordinated helicopter pilot could not make an error of that magnitude, except possibly when taking off from a very steep slope….and even then, that would be quite a slope!

You would be quite correct in your thinking, if the displacement of the helicopter’s C of G were the only factor to be considered. If the helicopter were sitting on the ground without its engine turning this would indeed be the case; it could only overturn if it were on a very steep slope – a situation known as ‘Static Rollover’. But additional factors are involved in dynamic rollover. Power is being applied, and this means that the whole situation becomes completely different.

To understand this, we need to take a look at the forces involved in lifting a helicopter off the ground. When the pilot raises the collective and power is applied, the total rotor thrust acts perpendicular to the rotor blades, so as to raise the helicopter upwards. However, this total rotor thrust also has a horizontal component, which acts about the point of ground contact of the skids. Normally this is not a problem, as this component is very small, and is opposed by the weight of the helicopter. But if one skid lifts off the ground first, the remaining skid now acts as a pivot point about which the horizontal component acts. This component increases as more power is applied and/or the angle of bank becomes steeper, causing the helicopter to tend to roll about the skid which is in contact with the ground, to a greater and greater extent. Beyond a certain critical angle it is impossible to stop the helicopter entering dynamic rollover … and turning over. The angle of bank at which this can happen is fairly small, often less than 10 degrees, and far, far less than that required for static rollover.

If at all possible, the pilot should ensure that both skids lift off at the same time

The phenomenon of dynamic rollover is the reason why take-offs in a helicopter must be undertaken with such care. The pilot raises the collective very slowly until the helicopter is light on the skids, then positions the cyclic to ensure that take-off will be precisely vertical, before raising the collective further. If at all possible, the pilot should ensure that both skids lift off at the same time, so that there is no pivot point to induce dynamic rollover. Students practise this extensively before going solo in a helicopter, since the take-off is one of the most difficult things they will have to do. In addition, great care should be taken to ensure that one skid is not stuck in mud, in long grass, or on ice, since any of these can very easily induce dynamic rollover.

If the pilot realises that the helicopter is not taking off absolutely vertically, what should he do? Most people’s instinctive reaction is likely to be to raise the collective further, to get away from the ground as quickly as possible – after all, no-one has ever collided with the sky. However, in the case of dynamic rollover this makes the situation worse. Raising the collective increases the total rotor thrust, which increases the horizontal component as well; thus it will simply exacerbate the situation. The only thing to do is to lower the collective, get back on the ground, and then try again.

However, by far the most likely time for dynamic rollover to occur is when taking off on sloping ground. Even though a slope may be very shallow, total rotor thrust will already be at an angle to the perpendicular, so a rolling horizontal component will be present. It is also necessary in this situation to lift off one skid at a time in order for the helicopter to take off vertically – there is just no other way to do it. Thus a pivot point is created about the skid on the ground, and with power applied, all the conditions for dynamic rollover are present, unless the pilot is extremely careful. The danger may well not be noticed by the pilot, since the helicopter still seems to be within normal limits of operation – and dynamic rollover can happen very suddenly. If it does occur, it is likely to be about the skid which remains on the ground, ie the uphill skid! Again, this is not what the pilot is likely to be expecting.

Sloping ground take-offs are considered to be one of the most difficult exercises in the PPL(H) syllabus

For this reason, sloping ground take-offs are considered to be one of the most difficult exercises in the PPL(H) syllabus, and one that can make instructors extremely nervous. They are usually practised on very shallow slopes, rarely more than about 5 degrees. The student is taught to raise the collective very slowly, so that the lower skid lifts off the ground, but in an absolutely controlled fashion. At the same time, the student must look outside the helicopter, and position the cyclic so as to keep the machine exactly horizontal. If the collective is raised too quickly, dynamic rollover is likely to occur around the uphill skid. If this looks likely the collective must be immediately lowered…but not too fast, or the helicopter could well bounce and roll around the other skid. The student must on no account try to lift off quickly, to get the helicopter away from the ground; as explained before, this is likely to aggravate the situation. The whole thing is not easy, and is a definite test of coordination and concentration for the new helicopter pilot…and a trial of nerves for his or her instructor!

Although dynamic rollover can happen in still air, sometimes the wind may also contribute to it. Side and rear winds further complicate matters by adding side forces to the fuselage, changing the power required to hover, and so on. Tail rotor thrust may also contribute to the situation. Dynamic rollover can also happen during water take-offs and landings, on helicopters equipped with floats, since water can create a drag force and therefore a pivot point. It may also occur during running landings, if a yaw is allowed to develop. It is a little more likely in helicopters with teetering rotor heads as opposed to fully articulated rotors, but can occur in any helicopter. As with all helicopter operations, every machine and every situation is different.

If you are unfortunate enough to encounter dynamic rollover, it is often already too late to start thinking about what you are going to do about it. The only way to guard against it is to plan for it during every take-off, whether on level ground or a slope. During the walk-around, carefully check the skids; are either of them stuck, or is anything preventing them from moving? It is amazing how many cases of dynamic rollover occur because a skid is stuck under a stray cable or some other obstacle that wasn’t there when the pilot landed. Take equal care during landings; it might seem impossible for dynamic rollover to occur during landing – but landing gear can collapse, or apparently solid ground can crumble. Know what to do before it happens, as once really started, and with a high angle of bank, dynamic rollover cannot be stopped.

If at any point the take-off (or landing) looks like being anything other than textbook perfect, lower the collective smoothly but firmly, and get back on the ground and start again. There is no shame in being extra careful. This is one reason why you will rarely see helicopters lift off quickly…except in the movies of course!

Author: FTN

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