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Aircraft maneuvering, such as side slip and low speed flight at extreme CG position, can induce large flapping angles. The risk of excessive flapping and possible mast bumping increases when the aircraft approaches low g conditions.
A pilot encountering a ridgeline or tree line, executing a cyclic climb, and then nosing over using abrupt forward cyclic gives up G loading on the rotor disk. Higher speeds aggravate the situation. The combination of down collective and low g means that lift, and therefore thrust, has essentially disappeared. Absence of thrust means there is no lateral cyclic control, so cyclic movement cannot change fuselage position.
Tail rotor thrust, acting above the helicopter's center of gravity, starts the fuselage rolling. Alarmed by the excessive roll rate, the pilot abruptly applies left cyclic. The pilot still has rotor control, but he is controlling an unloaded rotor. Although he still has fore and aft control, he has lost roll control of the helicopter, and flapping increases drastically. The rotor hub strikes the mast violently, and the mast may separate.