Some helicopters, like the Robinson R66, have tall masts putting the rotor system high above the tail boom; others, like the MD500, have a more compact design. Engineers take into account flight characteristics of a design when considering the distance between the tail boom and the rotor disk. Even so, tail-boom strikes can and do happen.
One of the more common scenarios is when a helicopter makes a hard landing following an autorotation. Touching down too nose-high on the aft part of the skids can cause a nose-down pitch that instinctively causes the pilot to pull back on the cyclic to counteract it. This action in combination with the fact that the blades are still moving downward can result in the blades contacting the tail boom. An in-flight entry to an autorotation will also cause a nose-down pitch because of the advancing blade seeing a greater reduction in lift than the retreating blade. A pilot who overreacts with sudden aft cyclic will cause the rotor system to flap back while the tail boom is still rising, which can lead to the blades to come in contact with the tail boom. In-flight blade-to-tail boom strikes are normally fatal.
Strong wind gusts can also create a problem. A tail-boom strike can happen as rotor rpm gets lower and the centrifugal force holding the rotor stiff drops. A helicopter that is starting up or shutting down in high winds or near another hovering helicopter is particularly vulnerable. Manufacturers have used droop stops in teetering rotor systems to support the blades at slow speeds. One design uses spring-loaded droop stops with weights that pull them out of the way when the rotor speed–and related centrifugal force–gets high enough. Even so, many manufacturers and operators have maximum wind speeds for start up and shut down.