Hydraulics in the aircraft

Hydraulics uses an incompressible fluid (hydraulic oil) to transmit force. The basic principle is Pascal's law: pressure in an enclosed fluid acts equally in all directions — a small force on a small piston area can produce a large force on a larger piston area.

Formulas:

Pressure: p = F / A (force / area). Unit: bar, psi, Pa.
Force amplification: F₂ = p × A₂ (with p = F₁ / A₁).
Volume / stroke relation: V₁ = V₂ (small volume at small piston = large volume at large piston) → smaller stroke at the larger piston.

Example: small piston 1 cm² × 100 N force → pressure 10 bar (10⁶ Pa); large piston 10 cm² × 10 bar = 1 000 N. → 10× force amplification.

Properties of hydraulic fluid

Requirements:

Incompressible — compressible fluid would behave elastically and lose control precision.
High thermal stability — hydraulic systems heat up in operation.
Anti-wear additives — lubricate pumps, valves, cylinders.
Corrosion protection.

Common hydraulic fluids:

Mineral oil (MIL-PRF-5606) — red dye, older standard.
Synthetic phosphate ester (Skydrol, MIL-PRF-83282) — fire-resistant, yellow-purple dye, in modern airliners.
Water-glycol — rare, fire-safe.

Mixing forbidden: different hydraulic fluids must never be mixed — gel formation, seal damage.

Components of a hydraulic system

1. Reservoir — stores hydraulic oil, level shown through sight glass.

2. Pump — generates hydraulic pressure. Types:

Engine-driven pump — driven by the engine (airliners).
Electrical pump — for backup or small systems.
Hand pump — backup on pump failure.

3. Pressure relief valve — protects against overpressure; opens above limit.

4. Accumulator — pressure reservoir with gas pre-charge (usually nitrogen), absorbs pressure peaks and provides short-term reserve.

5. Selector valves — direct pressure to actuators per pilot input.

6. Actuators — hydraulic cylinders that produce mechanical motion (gear, flaps, brakes).

7. Filters — remove contaminants.

8. Lines — connect components; high-pressure tubing (typically 2 000–3 000 psi / 140–210 bar).

Hydraulics in PPL aircraft

In light aircraft (C172, PA-28, DA40) hydraulics is rare and limited to simple applications:

Brakes — almost always hydraulic:

The brake pedal pushes a small master cylinder → hydraulic fluid transmits force to the brake caliper at the wheel.

Variable-pitch propellers — constant-speed propellers use oil pressure for blade pitch (engine oil, not a separate hydraulic system).

Retractable gear — on retractable types (C172RG, C182RG, Mooney, Bonanza) hydraulic or electric.

Flaps — rarely hydraulic in PPL aircraft; usually electric or mechanical (PA-28 flaps by hand lever).

Pneumatics in the aircraft

Pneumatics uses compressed air instead of fluid. Differences from hydraulics:

Compressible — springy behaviour, less precise control.
Cleaner — no oil leaks in the cockpit.
Lighter — no heavy fluid.
Higher pressures difficult — air is harder to compress.

Applications:

Pneumatic de-icing boots: inflatable rubber tubes on leading edges crack ice off.
Gyro drive by suction pump (vacuum system): vacuum pump creates suction (~5 inHg), air flows through gyro nozzles and spins the gyro.
Pressurised cabin (airliners): bleed air from the engine maintains cabin pressure.
Starters (turbines): pneumatic starters on some types.

Vacuum system in PPL aircraft

Many classic PPL aircraft have a vacuum system for gyro instruments:

Vacuum pump (engine-driven) creates suction.
Vacuum gauge in the cockpit (4.5–5.5 inHg normal).
Powers attitude indicator (AI) and directional gyro (DG) with airflow.
Turn coordinator is electrically driven — redundancy on vacuum failure.

Vacuum failure: AI and DG slowly fail (gyros spin down). The pilot must use electrically driven instruments (turn coordinator) and the magnetic compass.

Safety aspects

High-pressure hydraulics: injury hazard from oil jet on leaks; hydraulics maintenance only in approved shops.
Contamination: foreign matter clogs valves; check filters regularly.
Water ingress: causes corrosion and foaming — hydraulic fluid must remain dry.