Airframe, Systems and Powerplant
Airframe and rotor head
Understand the load paths: mast to transmission to airframe, and how the anti-torque system reacts torque. For articulated heads, know flapping hinges (relieve bending at the hub), drag hinges (relieve in-plane chordwise loads), and the feathering bearing. Semi-rigid (teetering) heads use a single teetering hinge and undersling to control Coriolis effects. Rigid heads carry bending moments into the hub and rely on composite flexures.
Transmission and drive
Trap: questions about freewheel units and autorotation. The freewheel (sprag or overrunning clutch) disconnects the engine from the rotor when engine rpm falls below rotor rpm, allowing the rotor to drive the transmission. Know that the tail rotor drive shaft has a flexibly coupled segment and that a loss of tail rotor drive is not the same as loss of tail rotor thrust.
Powerplant
For turboshaft engines, learn the compressor, combustion chamber, turbine stages, and the distinction between N1 (gas producer) and N2 (power turbine). Governors, fuel control units, and the overspeed protection system each have a question type. Piston helicopters require understanding of carburettor icing and mixture control.
Worked example
If rotor rpm decays in a descent with the engine at idle, the freewheel unit allows the rotor to overrun the engine. Rule of thumb: the freewheel only ever transmits power one way - engine to rotor. Any option implying the engine can be back-driven by the rotor in normal flight is wrong.