Mast Bumping

The fixed wing pilot may push the stick forward to lower the nose and descend to avoid the obstacle. A low rotor RPM horn can be mistaken for a stall warning, apparently requiring the nose to be lowered.

Never do this in a Helicopter! To understand why that action would decrease RRPM even further, read 'Coriolis Effect', also 'Danger of Low or Negative G Conditions'.

The only action to take in either case is to maintain the attitude. With the avoidance of obstacles, lower the collective. Enter autorotation if you have to. With a low RPM warning increase throttle and lower the lever. In a severe case flare the Helicopter as well, never 'dive'.

Low or Negative G

The aircraft could be in a climb or in level flight. It could be that an inexperienced or unwary pilot pushes the stick forward, attempting to avoid cloud, birds or another obstacle. Disc tilted forward, attitude nose down. This is the dangerous part, a 'pushover'. The above effects are now present in the main rotor. As coning angles decrease, Coriolis Effect reduces RRPM, again reducing total rotor thrust.

The disc is now unloaded. The weight of the aircraft and its occupants are no longer suspended by the rotor disc. A feeling of weightlessness may occur.

The fuselage is not now directed by the disc. It is free to move in any direction. The most significant force acting upon it is tail rotor thrust to the right.

Looking from above it is seen that tail rotor thrust pulls to the right, resulting in the nose moving to the left.

Keeping in mind that the tail is higher than the nose (it was said that the dangerous part was the nose being lowered) it is now apparent that there is a rolling couple set up between the nose movement and tail rotor thrust. The rolling couple always try to straighten out (Like pulling on two ends of a rope.)

The fuselage will roll to the right irrespective of the disc attitude. The cyclic stick (and therefore the disc) has now lost most of its authority over the fuselage.

The fuselage roll develops - There is now little or no clearance between the rotor hub and the mast. A pilot who is unaware of the danger may use left cyclic (ineffective anyway) in an attempt to correct the right roll. That is the worst possible thing to do. Mast bumping will occur. The rotor can be separated from the mast.

The only action to take is to reload the disc by applying gentle aft cyclic. The roll to the right can be corrected later. Take care not to apply aft cyclic too aggressively. As the disc regains its thrust it may flap back whilst the tail is still high.

To recover from a Low G situation

• 1. Recognise the symptoms – A feeling of weightlessness, you might lift off the seat much like going over the top of a hill.
• 2. Reload the disc by using gentle touch when applying aft cyclic only.
• 3. After you have reloaded the disc you can then deal with the right roll as you would do in a normal recovery from a turn or roll to the right.

Keep Away From Low G

No abrupt pull ups and push overs. Let the disc catch up with stick inputs.
Never ‘Dive’ for any reason.
Lower the collective instead. Only use the cyclic for attitude and the collective for height.
PAT (Power Attitude Trim) for any entry to a descent, whatever the reason.
Avoid gusty, variable and windshear wind conditions.

Backwards Flight

The temptation to fly backwards faster when the 'knack' is gained can be quite strong. However, the backward flight speed limit is set for good reasons. The horizontal stabiliser is presented to the relative airflow first when flying backward and cannot function as an aid in keeping the mast at roughly 90 degrees to the disc. It will work in reverse, bring down the tail and the disc (which is flapping forward anyway) can be so far away from 90 degrees to the mast that mast bumping could occur. If you need to go further or faster, turn and go forwards, it's much safer.

Sideways Flight

The sideways speed limit is also made for good reasons. Some of them are; Risk of mast bumping similar to the rearward limit (as the relative airflow will tend to roll the fuselage and the disc will flap in the opposite direction), effects of relative airflow causing an increase in tail rotor induced flow, relative airflow resisting tail rotor induced flow to result in vortex conditions. (See 'Vortex Ring and the Tail Rotor) If you need to go further or faster, turn and go forwards, it's much safer.

The disc (which is flapping forward anyway) can be so far away from 90 degrees to the mast that mast bumping could occur. Also beware of flying back to meet your own downwash, a potentially dangerous condition.