Hydraulic System (None)

What Is an Aircraft Hydraulic System?

An aircraft hydraulic system is a high-pressure fluid power network that converts mechanical or electrical input into precise, powerful actuation force. Hydraulic systems are the primary means of moving flight control surfaces, extending and retracting landing gear, operating wheel brakes, and deploying flaps and slats on the vast majority of commercial and military aircraft.

How It Works

Hydraulic fluid — typically Skydrol or phosphate ester-based compounds — is pressurized by engine-driven pumps (EDPs) to approximately 207 bar (3,000 psi) on most commercial aircraft. Some modern aircraft, including the Airbus A380 and A350, use 350 bar (5,000 psi) systems to reduce component size and weight. Electric motor pumps (EMPs) and air-driven pumps (ADPs) provide backup pressure when engines are not running.

Pressurized fluid is routed through redundant circuits to hydraulic actuators — linear cylinders or rotary motors — that translate fluid pressure into mechanical movement. Return fluid drains back to reservoirs via low-pressure return lines. Accumulators store pre-pressurized fluid for instantaneous response and emergency backup, such as deploying landing gear with a single brake application.

Redundancy is paramount: commercial airliners operate three or more independent hydraulic systems (typically labeled Green, Blue, and Yellow on Airbus; System 1, 2, and 3 on Boeing). Each system independently powers critical functions, ensuring no single failure removes all hydraulic control. Fly-by-wire aircraft use electrohydraulic actuators that receive electrical command signals and respond with hydraulic force.

Key Components

• Engine-Driven Pumps (EDPs): Primary pressure source; one per engine, producing up to 100 L/min (26 gal/min).
• Actuators: Linear cylinders for gear, brakes, and spoilers; rotary actuators for rudder trim.
• Accumulators: Pre-charged nitrogen/fluid vessels maintaining pressure during peak demand.
• Reservoirs: Fluid storage, typically pressurized to 4–5 bar (58–73 psi) to prevent cavitation.
• Priority Valves: Ensure flight-critical functions (e.g., flight controls) receive pressure before secondary systems (e.g., cargo doors).

Aircraft Examples

• Boeing 747-400: Four independent systems (P1, P2, P3, P4) at 207 bar (3,000 psi); total fluid volume approximately 760 L (200 gal).
• Airbus A380: Green, Yellow, and a backup Blue system; first airliner to use 350 bar (5,000 psi) saving 500 kg (1,100 lb) in system weight.
• Boeing 787-9: Two 207 bar systems; more electric architecture reduces hydraulic dependency versus predecessors.
• Airbus A320: Three independent systems (Blue, Green, Yellow); Blue powered exclusively by an EMP and ram air turbine.