Hydraulic Power, Module 12, Certify66

Notes

Main ideas

Hydraulic power operates the flight-control servos (main-rotor and tail-rotor) and, on some types, other services, using a fire-resistant fluid at high pressure; the pump is usually driven by the MAIN GEARBOX so it keeps working in autorotation.
Because the flight-control servos are irreversible and essential, helicopters commonly have DUAL independent hydraulic systems (or a hydraulic system plus a backup) so a single failure does not lose control.
Components: reservoir, pump, filters, accumulators, pressure/return lines, servo actuators and pressure/quantity indication; an accumulator stores energy and damps pulses.
Loss of hydraulic boost makes the controls heavy (high feedback forces from the rotor); procedures and, on some types, a limited manual-reversion capability address this.
Contamination control (filtration) is vital, particles jam the servo valves that fly the aircraft.
⚠ Exam trap: the hydraulic pump is typically driven by the MAIN GEARBOX, so control boost remains available during autorotation; the flight-control servos are irreversible and essential, so dual/independent systems are provided.

Solved examples

Why is the helicopter's hydraulic pump usually driven from the main gearbox rather than directly from the engine?
The flight-control servos must keep working even after an engine failure, during autorotation. By driving the hydraulic pump from the main gearbox, which is kept turning by the autorotating rotor via the freewheel unit, the controls stay powered whether or not the engine is running. An engine-only-driven pump would lose boost the moment the engine failed, exactly when control is most needed.

Hydraulic Power