A helicopter does not want to fly. It is maintained in the air by a variety of forces and controls working in opposition to each other, and if there is any disturbance to this delicate balance the helicopter stops flying immediately and disastrously. There is no such thing as a gliding helicopter.

- Harry Reasoner, circa 1970

Dynamic Rollover is one of the very many Bad Things That Can Happen to helicopters because of their inherent behaviour. It can occur within seconds in common flight conditions and result in the destruction of the helicopter and possibly the occupants.

Rollover is a fairly rare cause of helicopter mishaps (using very rough-hewn NTSB statistics) which might seem somewhat strange given how easy it is to induce. But then, flying into a building is easy too.

Rollover is just what it sounds like: for some reason, the helicopter rolls onto its side. It occurs when the helicopter is on the ground with its rotors turning or in the process of takeoff or landing. There are actually two flavours of rollover: static and dynamic.

Static Rollover

This is such a rare occurrence it mainly serves to help in explicating the dynamic variety. By definition it is the helicopter on the ground, being at such a sideways tilt that gravity causes it to fall over onto its side.

The angle at which this occurs (the 'static rollover angle') is different for every model of helicopter, but is always the balance point of a helicopter. If you were to get a helicopter and tilt it one way or the other until it were balancing on one skid or wheel (i.e. the centre of gravity is directly over that skid or wheel), that's the static rollover angle. If it is exceeded, the helicopter will tip over.

Dynamic Rollover

Dynamic rollover is essentially the same but the angle above which it will occur (the dynamic rollover angle) is described by the effect of such elements as wind, airspeed and helicopter weight on the static rollover angle. Dynamic rollover occurs when the helicopter flight controls are not sufficient to stop it tipping over. The rotor system may actually accelerate the tipping over.

It may occur during:
  • slope takeoffs
  • slope landings
  • sideslipping (the helicopter flying sideways)
  • Earthquakes or other comparable events (!)

If we look at sideslipping, dynamic rollover occurs if the helicopter skid/wheel facing the direction of travel touches the ground with any significant force. This action alone induces the helicopter to roll, much as a person tripping over something will fall in the direction they were moving when they tripped. That is part of it but some more physics are involved, as are the aircraft's particular circumstances and the reaction of the pilot to the situation. Before explaining further why Dynamic Rollover occurs, I will first explain what happens in the example we're using.

What can happen

The helicopter in our example is a modern type, with a counter clockwise-rotating main rotor. It is facing north in an easterly wind, hovering a foot or so above the ground. Here's what happens:

  1. The pilot, pushing the cyclic control to the right, makes the helicopter fly sideways, to his right.
  2. For some reason - perhaps the pilot uses too much cyclic without increasing collective or perhaps a rise in the ground is encountered - the helicopter's right landing skid hits the ground.
  3. The friction of the skid on the ground combined with the lateral motion of the helicopter induces it to roll to the right, pivoting about the skid that hit the ground. The wind helps it along, as does the thrust from the tail rotor.
  4. After a second or more of thinking time the pilot realises what's going on and pushes the cyclic stick all the way to the left in an attempt to correct the roll. By this time the helicopter has already tilted past a critical point. At best the pilot's action slows the roll slightly but does not stop it. At worst it may speed up the roll.
  5. The helicopter continues rolling. Rotor blades hit the ground at hundreds of miles per hour. Bits of rotor and helicopter fly off in multiple directions and the remainder rolls over completely, possibly disintegrating. See Black Hawk Down (it's not dynamic rollover but the result has to be similar).
  6. Seriously, that's it.

What causes Dynamic Rollover

The insidious aspect of dynamic rollover is that the roll rates which precipitate it are within the range the pilot would normally allow in flight.

- U.S. Navy Naval Air Training and Operating Procedures Standardization manual

To explain why this happens, a bit of background on moments of inertia should help. The moment of inertia for a rotating object describes its resistance to being rotated, much as inertia describes the "unshoveability" of an object (to quote a physics teacher of mine). The key to applying moments of inertia to this context is this general rule:

For any two objects of equal mass, the moment of inertia will be greater for the object that has the greatest concentration of mass the furthest away from the axis of rotation.
The simplest example of this is a solid circle versus a ring. If you had a solid circle of material and a ring of material, both with equal diameter and mass, the ring would require more effort to rotate about its centre.

When flying a helicopter will pitch and roll about its lateral centre of gravity, which is a point somewhere in its fuselage. This point is slightly different for every helicopter but usually somewhere between the vertical midpoint of the helicopter fuselage and the top; it is the point about which the helicopter will rotate when it rolls. When the pilot applies some left/right cyclic control, the speed of the subsequent roll depends mainly on two things:

  1. The amount or strength of control the pilot is applying
  2. The moment of inertia of the fuselage along the axis of rotation (aside: the moment of inertia is different along all three axes of rotation).

Fair enough, but in a rollover situation one of the helicopter skids or wheels is touching the ground. I'm not even going to attempt ascii-ising that because the first source has a perfectly good illustration. When one of the skids touches the ground that skid (or more precisely, the point where the skid meets the ground) instantly becomes the new rotation point for the fuselage. The helicopter will now rotate about the skid, not about its lateral centre of gravity.

The effect of this on control effectiveness is tremendous: depending again on the specific helicopter the moment of inertia of the fuselage may increase up to five times its original value. Put another way, the helicopter will require five times more cyclic effort to produce a given rolling motion. Put still another way, the cyclic control will be five times less effective at controlling the lateral rolling motion of the fuselage than it was when the helicopter was aloft.

In order to explain why the moment of inertia increases so, consider a pair of circles of equal size and mass, Circle A and Circle B. Circle A pivots around its centre. Circle B pivots around a point on its edge. Now, rotate both of these circles by applying force opposite the pivot point and mark out the circle that each one traces. You would of course find that Circle B traces a circle twice the size of Circle A (which traces a circle equal to its own size). Not only that, but Circle B requires a lot more force to rotate than Circle A does.

Now, imagine these circles weighed several tons each: the difference in force required to move each of them would be substantial. This is a rough analog to the helicopter's situation. When it places a skid on the ground its pivot point is displaced from its centre of gravity and as a result it not only becomes more difficult to roll, but becomes more difficult to stop rolling once it has started.

Hopefully this is now clearer: if a helicopter is sideslipping and the skid/landing wheel on the side of travel contacts the ground, not only does this cause a roll in that direction but it displaces the helicopter's pivot from its centre of gravity to the skid. Further, cyclic control now has much less effect so control is difficult at best, impossible at worst. The result is that the critical tilt angle - the dynamic rollover angle, past which a tilt is unrecoverable - is reduced. This angle can be as little as 7°, depending on the helicopter.

However, there's a little more to it: as we've said the dynamic rollover angle is dependent upon several factors. Crosswinds (in such a direction as to encourage a roll), the direction the main rotor rotates (because this dictates which direction the tail rotor thrusts) and the direction the helicopter is moving in will all affect the dynamic rollover angle. It helps if the tail rotor is providing thrust in the opposite direction to the roll, as does the wind blowing in the opposite direction to the roll.

Quick reactions by the pilot can also save the helicopter, though if the roll is fast enough the dynamic rollover angle will be exceeded inside the pilot's thinking time, and they won't even have time to make their peace with God.

Other causes

To mention the other situations, dynamic rollover can occur during slope landings or takeoffs, particularly the latter. Obviously during slope operations there will be a point at which only one wheel or skid of the helicopter is touching the ground, since helicopters cannot climb or descend diagonally. They must land or take off vertically: one skid or wheel at a time will leave the ground or touch down.

The danger when taking off from a slope is applying full collective too early, before the helicopter has completely left the ground. If a wheel or skid is still touching the ground it is still the helicopter's pivot point and means that roughly five times more cyclic strength will be needed to keep the helicopter level than if it were hovering.

The moment the helicopter leaves the ground altogether its rotation point will revert back to being the helicopter's lateral centre of gravity. If the cyclic control has not been reduced accordingly it will be far too strong, making a very real possibility that the helicopter will roll towards the upslope and crash. Not to mention the risk of mast bumping from excessive cyclic input combined with reduction of collective that may result from any corrective action by the pilot (see mast bumping).

The risk of dynamic rollover when landing on a slope is more to do with bounce than with helicopter behaviour. If you're landing on a slope, have one wheel or skid in contact with the ground and reduce the collective too much to settle to the ground, the helicopter may fall too quickly causing the upslope skid or wheel to bounce, possibly a downslope roll. The general rule in slope operations is to be as gentle as possible with the controls until fully landed or aloft.

As for the earthquake scenario, this regards oscillations of the surface the helicopter is resting on, causing a rollover. If the supporting surface tilts suddenly and stops suddenly, the angular velocity the helicopter gains will cause it to continue rolling in the direction of the original tilt, possibly rolling over completely.

There is a recorded instance of a helicopter rollover occurring on an aircraft carrier when the ship was hit by a large wave. The helicopter was on the flight deck with its rotors running and the action of the deck tilting caused the helicopter to roll in the same direction; it rolled onto its side and its rotors were driven into the deck. The helicopter was destroyed but the crew were unhurt.

One other thing that can cause rollover is getting one of the skids or wheels caught on something. When taking off the helicopter would roll in the direction of the stuck side. There have been recorded mishaps where a helicopter has been operating in icy conditions and one skid has frozen to the surface while it was grounded, causing the helicopter to roll over on takeoff. Variations on this are not hard to imagine.

What to do about Dynamic Rollover

This is one of those things you should avoid if you can. The best way to do so is to keep your control inputs smooth and gentle under conditions that are likely to encourage rollover. When taking off from a slope any rolling momentums (caused by the tail rotor, for example) should be kept trimmed with the cyclic; if you find your bank angle cannot be arrested with full cyclic, immediately reduce collective to reduce the rolling effect.

If you're caught in a dynamic rollover situation, the sooner it is comprehended and corrected the better. You - the pilot - literally have two or three seconds to do both of these, depending on such factors as wind, lateral speed and helicopter load/weight.

To be honest, there is little you can do here. Getting out of this one will be as much good fortune as any skill on your part. If you are in the process of taking off the temptation (if inexperienced) may be to bump the collective all the way up to speed the takeoff before the helicopter rolls over completely.

Don't do that.

It will almost certainly speed up the rollover, making things even worse. The first and most significant thing you can do is reduce the collective control to zero over about two seconds. The main rotor will then not be providing any thrust and if you've done it soon enough, the helicopter will not have exceeded the static rollover angle and gravity may pull it back level. Do not reduce the collective too quickly though, otherwise you may get a bounce as the helicopter drops to the ground and end up rolling in the opposite direction (quite apart from the risk of mast bumping)!

If none of the above works I can only suggest consulting how to survive a helicopter mishap, but if you're reading this you've probably already done that.


See also:
Sources:
  • Brennan, William T; Helicopter Dynamic Rollover;
    <http://www.avcommusa.com/download/ac90-87.pdf>
  • Dynamic Flight Inc.; "Dynamic Rollover";
    <http://www.dynamicflight.com/aerodynamics/dynamic_roll/>
  • (Author unknown); "Accidents waiting to happen - Ground Resonance and Dynamic Rollover";
    <http://safecopter.arc.nasa.gov/Pages/Columns/RayProuty/pdf_files/Ground_Resonance.pdf>

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