Which component allows either or both engines to disengage from the transmission?

• A. Main gearbox/Main Transmission
• B. Free-wheeling unit
• C. Tail gearbox
• D. Accessory Gearboxes/Drives

Answer: (B)

The concept being tested is how the system decouples an engine from the transmission. This is accomplished by a free-wheeling unit, a one-way clutch between the engine and the main gearbox. When an engine is producing power, the free-wheeling unit locks and transmits torque to the gearbox. If the rotor begins to drive the engine (such as during autorotation or if an engine is shut down), the unit allows the engine side to freewheel and not be driven by the rotor, effectively disengaging that engine from the transmission. In a twin-engine helicopter, this setup lets either or both engines be decoupled as needed, while the other engine can still drive the main gearbox. The other components—main gearbox, tail gearbox, and accessory gearboxes/drives—are parts of the power path or support systems, not mechanisms that selectively disengage engines from the transmission.