Final answer:
Turboprop engines have a propeller control unit that prevents the propellers from entering the reverse range during cruise by employing safety mechanisms such as a mechanical lock and an overspeed governor.
Step-by-step explanation:
The question What stops the propellers on a turboprop from entering the reverse range during cruise flight? is related to the mechanical and control systems of aircraft, particularly turboprop engines. In normal operation, a propeller control unit (PCU) will prevent the propellers from entering the reverse thrust range while in flight. The PCU ensures the propellers maintain the correct pitch, varying this pitch for different flight phases such as takeoff, climbing, cruising, and landing. To prevent the reverse range during cruise flight, several safety mechanisms are in place including a mechanical lock, a hydraulic or electronic governing system to monitor and control pitch, and critical operational protocols that pilots must follow. For example, there is an overspeed governor that prevents the propeller from spinning too fast, and a beta range (reverse thrust) lockout mechanism that activates only when the aircraft is on the ground and at a slow speed, thereby preventing engagement of reverse thrust during flight.