A rigid (or sometimes called hinge-less) rotor system is capable of transmitting high bending forces to the main rotor shaft. When a pilot makes a cyclic movement causing the main rotor disc to tilt, the fuselage wants to follow. In flight, with a rigid rotor the mast bending moment is low. However, when the fuselage is in contact with the ground and cannot follow the main rotor disc the bending moment can be very high.
This type of rotor system is used on the helicopters designed and built by the German manufacturer MBB (now Airbus Helicopters). Because large cyclic displacements on the ground have the potential to damage the mast assembly, a mast moment indicator (MMI) is installed. The gauge is a single dimension indicator that shows the total moment being applied to the mast. When the gauge reads high, the pilot has to figure out what direction to move the cyclic to reduce the mast moment. Over time, experience makes knowing how to keep the mast moment low a natural reaction, however, pilots new to these types of helicopters would have to be very careful not to exceed the limit. Recently, to help reduce any possible confusion a new style gauge has been developed. It is two dimensional (using a circle instead of a straight line) which makes knowing the correct direction to move the cyclic control easier.
Normal pick-ups and set-downs require care as to not exceed the limits on the MMI. Generally, this is not difficult. However, slope landings and running landings can be more challenging. In these situations, the pilot needs to be comfortable with the MMI being close to limits and making very small cyclic adjustments. If a limit is exceeded, the amount (in percentage) and duration dictate how extensive an inspection or repair will be.
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