Helicopter rotor blades are potentially lethal. In certain situations, such as high winds or uneven terrain, the clearance between the main rotor blades and the ground can reduce to strike any person or object within the rotor blade arc (the rotor disc).
The clearance height of the main motor differs between helicopters. With certain helicopters the rotor blades can oscillate vertically several feet when starting up and shutting down, creating a potential hazard to personnel entering the rotor blade disc at these times.
It is important that you are aware of the entry/exit points for the type of helicopter that is operating.