Airbus vs. Boeing Hydraulic Systems: Why Airbus Uses Colors and Boeing Uses Letters

Step into an Airbus cockpit and a technician will talk about the Green system or the Blue system. Step into a Boeing and the same conversation turns into System A, System B, and Standby. Same job, wildly different vocabulary, and it says something real about how each manufacturer thinks about its aircraft. The clearest way to see it is by comparing the two most common narrow-body jets in the sky, the Airbus A320 and the Boeing 737.

The Job Both Systems Do

Hydraulics on any airliner move things too heavy to move by cable and pulley alone, flight controls (primary and secondary), landing gear, brakes, flaps, spoilers, steering. Both the A320 and the 737 run at roughly 3,000 psi, and both build in more than one independent circuit so losing one does not mean losing the aircraft entirely. Controls and operations are split between the three systems.

Airbus: Green, Yellow, Blue

On the Airbus A320, each system powers a distinct set of functions:

1. Green — engine-driven pump (EDP) on Engine 1 powers primary flight controls (ailerons, elevators, rudder), normal brakes, the landing gear, slats/flaps, yaw damper 1, etc.
2. Yellow — engine-driven pump (EDP) on Engine 2 plus its own electric pump powers cargo doors, alternate braking, flaps, rudder, yaw damper 2, etc. 
3. Blue — electric pump in normal operation, backed up by the Ram Air Turbine (RAT) powers essential flight controls, including spoilers, rudder, elevators, ailerons, etc. and keeps the aircraft controllable if both engines are lost

The three systems never share fluid. When pressure between Green and Yellow drifts more than about 500 psi apart, a Power Transfer Unit (PTU), which has a distinct dog barking sound, kicks in automatically and passes mechanical power between them, not fluid, so a leak in one system can never contaminate another.

Boeing: System A, System B, Standby

The Boeing 737 incorporates the same idea around letters instead of colors:

1. System A — engine-driven pump (EDP) on Engine 1 powers primary flight controls (ailerons, elevators, rudder), the ground and flight spoilers, landing gear, and Autopilot A
2. System B — engine-driven pump (EDP) on Engine 2 plus its own electric pump powers trailing edge flaps, normal brakes, the flight spoilers, the yaw damper, and Autopilot B
3. Standby — the safety net often called, powers the rudder, the standby yaw damper, etc. and in certain failures the leading-edge devices

System A leans toward landing and slowing down and the system B towards configuring for lift. The rudder alone draws power from all three sources at once, about as much redundancy as Boeing builds into any single control surface on the aircraft.

What the Cockpit Actually Watches

Both aircraft give pilots a running readout of their hydraulics rather than leaving anything to guesswork. The A320's ECAM HYD page displays pressure, temperature, and fluid quantity for Green, Yellow, and Blue in real time, so a crew can see a system trending toward trouble before it fully fails.

The 737 leans on cockpit hydraulic panels that let pilots monitor pressure across System A, System B, and Standby. Crews are trained to read two distinct failure signatures, a downstream leak shows falling fluid quantity, while a pump problem shows steady quantity but dropping pressure.

What the Naming Actually Reflects

Airbus color-codes because its systems are parallel, largely interchangeable systems distinguished mainly by which physical source powers them. Boeing's lettering leans functional, A and B split responsibilities by flight phase, and Standby exists purely as a last-resort layer.

There is a deeper split behind the labels too. The A320 was built from the outset as a fly-by-wire aircraft, so its hydraulics work hand in hand with computers that interpret pilot input. The 737 is not fly-by-wire, and if it somehow lost System A, System B, and Standby all at once, the crew could still fly it through manual reversion, moving control surfaces by muscle force through the same cables Boeing has used since the earliest days.

None of this naming is trivia for pilots and maintenance crews. Green, Yellow, and Blue feel almost like a traffic light system, easy to picture and hard to mix up. A, B, and Standby read more like a hierarchy; primary, secondary, last resort. Both approaches get a loaded jet safely on the ground they just start from two different ideas about how to describe the same kind of power.