Hydraulic Power
Gearbox-driven pumps, dual independent systems for the irreversible control servos, accumulators and filtration.
Hydraulic power — summary notes
Free- 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.
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.
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