14 CFR § 27.675

Each control system must have stops that positively limit the range of motion of the pilot's controls.

Each stop must be located in the system so that the range of travel of its control is not appreciably affected by—

Wear;

Slackness; or

Takeup adjustments.

Each stop must be able to withstand the loads corresponding to the design conditions for the system.

For each main rotor blade—

Stops that are appropriate to the blade design must be provided to limit travel of the blade about its hinge points; and

There must be means to keep the blade from hitting the droop stops during any operation other than starting and stopping the rotor.