Under what condition can the rotor brake be effectively used?

• A. Above 80% Nr
• B. 40% Nr
• C. Below 40% Nr
• D. When stationary

Answer: (C)

The rotor brake can be effectively used when the rotor system is operating below 40% rotor speed (Nr). At this lower speed, the rotor blades have decreased centrifugal forces acting on them, allowing the brake to be applied without risking damage to the rotor system. Engaging the rotor brake at this condition helps to bring the rotor to a stop efficiently and safely, thus facilitating ground operations or shutdown procedures.

Using the rotor brake above this threshold, such as at 40% Nr or higher, could pose significant risks. Higher rotor speeds mean that the blades are under considerable stress, and applying the brake could lead to potential damage, excessive wear, or safety hazards for both the aircraft and personnel involved. Thus, the rotor brake is specifically designed to function optimally when the rotor is stationary or operating below the critical speed threshold.