Mast bumping is definitely not something you want happening to your precious billion dollar blender. This slightly dirty-sounding term refers to an event that at best can cause severe damage to a helicopter's rotor mast, at worst tear the main rotor off completely. Either way, it clearly isn't a desirable thing to have happen. It can happen to helos with two blades on the main rotor, like the Bell Jetranger or the Huey UH-1. It was the cause of several puzzling losses of the UH-1 during the Vietnam War that did not involve combat or other helicopters.

Background

Why only helicopters with two rotor blades? Because helicopters with two rotor blades almost universally use a teetering hinge to join the rotor hub to the rotor mast (the driveshaft for the rotor). This hinge is used so that the blades have some freedom of up-down movement (called "blade flapping"), so that differences in lift generated by the two rotor blades compensate for one another. Sometimes the blades are individually hinged where they join the hub, but by and large they are attached rigidly to the hub, which is hinged to the rotor mast. This is explained further here and here. Anyhow, mast bumping can happen to any helicopter that uses such a teetering hinge. The diagram below shows roughly how it all fits together on a twin-bladed rotor system:


         _______Rotor Blades________
        /                           \
       /         Rotor Hub           \
      |              |                |
      V              V                V
 _______          ________          ________
_\      |--------/   __   \--------|       _\
\_______|--------\__|__|__/--------|_______\
                    /||
                   / || <--Rotor mast
                  /
         Hinge joint

Mast Bumping

Obviously the flapping of the main rotors must have a physical limit; the term mast bumping is fairly explicative. It means that for whatever reason, the rotor blades have flapped too far in one direction, causing the rotor hub/yoke to tilt far enough that one side of it makes contact with the rotor mast as indicated below:


 _______          ________          ________
_\      |--------/   __   \--------|       _\
\_______|--------\__|__|__/--------|_______\
     Points of  ---->||<----
     Mast bumpage    ||

The main rotor would tilt so far in one direction that the rotor hub would hit the rotor mast (the first source for this node has an illustration).

What causes this to occur is a sudden reduction in g-loading on the main rotor: that is, the helicopter falling faster than gravity is pulling it, meaning the rotor blades are not loaded by the weight of the helicopter as they are meant to be. This could happen if the pilot:

  • Transitioned suddenly from horizontal flight to a dive by pushing the cyclic stick forward
  • Transitioned suddenly from a climb to forward flight or a dive without levelling out gradually
  • Transitioned from a vertical climb to a freefall
  • Attempted any strong, rapid cyclic movements in low-G conditions or otherwise
  • Tried to land the helicopter on a steep slope.
However that's not all that has to happen to cause mast bumping. Some subsequent, misguided action by the pilot is usually required to aggravate it.

What can happen

Taking one of the cited examples, if an inexperienced pilot were to perform a push over: to apply strong forward cyclic when climbing as if to dive (when cresting a ridge during contour flying, for example), the following sequence of events may occur:

  1. As the helicopter approaches the apex of the climb and begins to fall, the g-loading on the rotor blades is reduced, as is their angle of attack, as the helicopter becomes temporarily weightless. The rotor blades become able to flap up and down freely and any cyclic control by the pilot will now have virtually no effect. *

  2. Because the main rotor is now not generating any thrust, the tail rotor's torque is now unopposed. Since the tail rotor is above the helicopter's centre of gravity the helicopter rolls sharply in the direction of the tail rotor's thrust. Most modern helicopters' main rotor turns counter-clockwise, so would roll to the pilot's right.

  3. The pilot, attempting to correct the roll, pushes the cyclic stick in the opposite direction. Because the cyclic is not having any effect the pilot will apply more and more. This causes the main rotor, still not g-loaded, to flap to its limit. The rotor flaps so far in one direction that the rotor hub hits the rotor mast, very fast and with considerable force. At this point the pilot will probably hear one or more loud bangs from the main rotor.

  4. The main rotor hub very likely separates from the rotor mast - the flapping of the rotor blades basically wrenches the hub from its hinge - or the rotor mast snaps off completely.

  5. The helicopter and main rotor both fall out of the sky. Separately.

  6. No, that's it.

In-flight mast bumping doesn't always end as in the example above - sometimes the helicopter survives with damage to the rotor hub and mast - but the outcome can be disastrous. A major structural component of the helicopter (the rotor mast supports the whole weight of the helicopter fuselage, quite apart from the huge torque it must withstand) has suffered damage and becomes more susceptible to failure.

Blade flapping can damage more than just the rotor mast though; in extreme situations the main rotor may even strike part of the helicopter fuselage, most likely the tail boom. There have been recorded incidents of helicopters slicing off their own tail booms as a result of excessive blade flapping.

With slope landing, mast bumping can occur as the helicopter is settling to the ground (one side will touch the ground first). As the helicopter fuselage begins to tilt in the direction of the slope, the angle of the main rotor relative to the rotor mast will gradually increase: the main rotor is still providing vertical thrust, but the rotor mast is tilting in the direction of the slope! If the slope is too steep mast bumping will occur, though in this case it may not be fatal. For the pilot, anyway.

How to experience mast bumping and not die

Mast bumping is of course, best avoided. The best way of doing so is to avoid conditions like turbulence that are likely to encourage it. Violent manoeuvres under low-G conditions or in turbulence are possible triggers, as are landings on steep slopes. If forced to fly in turbulence, avoid making sudden moves with the cyclic control. Also, know when to quit.

However, maybe it's too late for that. Obviously if mast bumping happens on the ground your chances are much better than if you're in the air. Your main worry is the metal blades that are spinning at horrendous speed a couple of feet above your head, but any discussion of how to avoid death by rotor blade is pretty much academic if one decides to head your way. However, if you are in the air when mast bumping occurs, the following corrective action is essential if the helicopter has not already decapitated itself:

Regardless of any manoeuvre the helicopter is trying to make, immediately but gently and gradually apply aft cyclic. This will re-load the main rotor, causing it to produce thrust again. Once this is done you should be able to correct the roll as normal.

Almost the only variations on this are used if mast bumping occurs while flying sideways, flying backwards or while landing on a slope (in which case you should move the cyclic control to the centre to stop the bumping).

Finally, land immediately. This applies to any situation. You've survived mast bumping, so it'd be an awful shame to die now because you decided to keep flying and the main rotor sheared off.


Note: Mast bumping can also occur with helicopters that have more than two blades with flapping hinges on the main rotor, but the results are rarely as serious as described.

* - If you want an analogue to this, consider a string puppet. On the moon. Stay with me. If you're holding the puppet by its hanger, then quickly move the hanger downwards, all the strings become limp and no tilting of the hangar will make the puppet's limbs move until you stop moving the hanger downwards and the strings become taut again. Of course this would work on any planet with gravity, but it's easier to see the effects in lower gravity. I suppose you could always go up in the vomit comet. I'm not greedy.

See also:
Sources:
  • helicoptersonly;
    • "Helicopter Mast Bumping"; <http://www.helicoptersonly.com/sayagain_MastBumping.html>
    • "Mast Bumping part 2"; <http://www.helicoptersonly.com/sayagain_MastBumping2.html>
  • (Author unknown) webmaster@helicfi.com; "SFAR 3.61; Mast Bumping"; <http://www.helicfi.com/sfar.htm>
  • (Author unknown); "Accidents Waiting to Happen; Mast Bumping"; <http://www.mygyroplane.com/MAST_BUMPING.pdf>
  • navygouge.com; (title unknown); <http://www.navygouge.com/helos/hillgouge/fams/fam08.html>