Helicopter Structure, Powerplant and Systems
Airframe and rotor system
A helicopter's structure carries flight, ground and vibratory loads. The main rotor system may be articulated (fully articulated with flapping and lead-lag hinges), semi-rigid (teetering), or rigid. The tail rotor provides anti-torque and directional control; some designs use fenestron or NOTAR. Know the swashplate and pitch links that convert cyclic and collective inputs into blade pitch. The transmission reduces engine RPM to rotor RPM and includes freewheeling units so the rotor can autorotate if the engine fails.
Powerplant and systems
Turboshaft engines drive the main transmission through a combining gearbox in twin-engine types. Know the engine control unit, fuel control, and the meaning of N1, N2 and TOT/ITT limits. The lubrication and fuel systems, hydraulic boost for controls, and electrical generation are all systems whose failures appear in questions. The spray system is an aircraft system: pump, tank, boom, nozzles, and dump valve, with its own controls and pressure gauge.
Traps
Questions confuse N1 (gas generator) with N2 (power turbine/rotor) indications, or ask what happens to rotor RPM after an engine failure (it decays unless the pilot lowers collective). Another trap: assuming the freewheeling unit is a brake, or that hydraulic failure means total loss of control (it means higher control forces). Skid versus wheeled gear and ground resonance are also frequent.
Rule of thumb
For any system-failure question, ask "what does the pilot do first?" The answer is almost always to maintain rotor RPM and control, then manage the system. Lowering collective converts rotor energy into time.
Worked example: A question describes an engine failure in cruise. The correct sequence is to lower collective to maintain rotor RPM, enter autorotation, select a landing area, and only then attempt a restart. Choosing "increase collective to stretch the glide" is always wrong.