How does the A320 Yellow hydraulic system work in a sim?
On the real Airbus A320, and in accurately modelled flight simulators, the Yellow hydraulic system supplies about 3,000 psi to selected flight controls, flaps, alternate and parking brakes, the No. 2 thrust reverser and cargo doors. Engine 2 normally powers it; an electric pump and the Green-system PTU provide other pressure sources.
Our Aviation & Real-World Flying answer uses the real aircraft as the baseline. Yellow is one of three independent hydraulic circuits, not a reserve tank for the whole aeroplane; our broader guide to the three A320 hydraulic circuits explains how Green, Blue and Yellow divide the workload.
What does the A320 Yellow hydraulic system power?
The Yellow system powers selected actuators and services so that losing one hydraulic circuit does not remove every means of controlling or stopping the aircraft.
| Aircraft function | Yellow-system contribution |
|---|---|
| Flight controls | One right-elevator actuator, one rudder actuator, spoilers 2 and 4, and one trimmable horizontal stabiliser motor |
| Flaps | One of the two hydraulic motors in the flap drive |
| Brakes | Alternate braking and the parking brake, supported by the brake accumulator |
| Engine | No. 2 engine thrust reverser |
| Ground services | Hydraulic operation of the cargo doors |
Yellow does not normally operate the landing gear, nose-wheel steering or normal brakes; those are principally Green-system services. The emergency gravity-extension arrangement is also not powered by Yellow. See our explanation of A320 landing-gear operation for that separate sequence.
How is Yellow hydraulic pressure produced?
Engine 2's hydraulic pump is the normal source, while an electric pump, the power transfer unit and limited stored or manual sources cover specific ground and failure cases.
- Engine-driven pump: Engine 2 mechanically drives the normal Yellow pump. With engine 2 running and
ENG 2 PUMPnot selected off, Yellow pressure should be close to its normal 3,000 psi. - Yellow electric pump: The
Y ELEC PUMPcan pressurise the circuit without engine 2. It is used as a backup and as a ground source; real A320 cargo-door logic can start it automatically. - Power transfer unit: The bidirectional PTU lets a healthy Green circuit mechanically power the Yellow pump, or Yellow support Green. It transfers hydraulic power, not fluid.
- Hand pump: A ground hand pump permits cargo-door operation when the normal electrical source is unavailable. Many simulators omit this.
- Brake accumulator: Stored Yellow pressure can operate the parking or alternate brakes for a limited period. It does not pressurise the entire Yellow circuit.
The PTU normally reacts when the Green-to-Yellow pressure difference reaches roughly 500 psi, subject to engine-start, parking-brake and cargo-door inhibition logic. A pump failure can therefore be masked by PTU assistance. A reservoir leak is different: the PTU cannot replace lost Yellow fluid. Our detailed explanation of PTU pressure transfer covers its automatic logic, limitations and familiar barking sound.
The ram air turbine is not another Yellow source. On the A320 it pressurises the Blue circuit, which supports the emergency electrical-generation arrangement.
What should the Yellow system do in a flight simulator?
A well-modelled A320 should connect Yellow-system pressure to the pumps, PTU, ECAM indications and individual aircraft services rather than showing a purely cosmetic pressure value.
- Open the hydraulic synoptic. Use the ECAM HYD page to monitor system pressure, reservoir condition, pump status and PTU state. Our guide to the A320 cockpit controls and displays helps identify the relevant overhead controls and ECAM indications.
- Observe the cold aircraft. With both engines and the Yellow electric pump off, the main Yellow circuit should not have normal pressure. The separate brake accumulator may still retain pressure.
- Supply pressure. Start engine 2 or select
Y ELEC PUMP. The ECAM indication should rise towards normal pressure; connecting external electrical power by itself should not pressurise Yellow. - Demonstrate PTU assistance. With Green pressurised and the PTU available, fail or switch off the engine 2 pump in a simulator training scenario. Yellow may remain pressurised through the PTU, depending on ground-inhibition logic.
- Isolate the failure. With both the normal Yellow source and PTU assistance unavailable, inspect the affected spoilers, flap speed, braking mode, No. 2 reverser and cargo doors.
This is a simulator systems demonstration, not a real-aircraft operating procedure. Add-ons differ sharply: a simpler aircraft may draw the right ECAM symbols while leaving brakes, flight controls and thrust reversers unaffected by a hydraulic failure.
Does a Yellow hydraulic failure disable the A320?
A single Yellow-system failure does not normally make the A320 uncontrollable because the remaining Green and Blue actuators retain the essential flight-control functions.
The right elevator and rudder retain other hydraulic actuators, while the stabiliser and flap drive retain another motor. Flap movement may be slower, and spoilers 2 and 4 are lost. Normal braking remains available if Green is healthy, but alternate and parking-brake capability may be reduced or limited to stored accumulator pressure. The No. 2 reverser and hydraulic cargo-door operation are also affected.
A Yellow pump failure and a complete Yellow system loss are therefore not equivalent. The PTU can compensate for a failed pump when Green is healthy, but it cannot cure an empty reservoir, major leak or failed Yellow-side plumbing.
Why does Yellow pressure look wrong in the simulator?
Most apparently incorrect Yellow-system behaviour comes from normal PTU inhibition, an unpowered pump, residual accumulator pressure or simplified add-on systems.
| Symptom | What to check |
|---|---|
| No Yellow pressure after engine 2 start | Confirm engine 2 is fully running, ENG 2 PUMP is not off, and no hydraulic failure remains active. |
| PTU does not restore pressure | Check that Green has pressure, the PTU is available and ground inhibition is not active. A Yellow fluid leak cannot be corrected by the PTU. |
| Parking brake still works after Yellow fails | The brake accumulator may retain stored pressure. Repeated applications should eventually reduce it in a detailed simulation. |
| Cargo doors move without pump activity | The add-on may treat the doors as visual animations and omit automatic Yellow electric-pump logic. |
| Failure causes no handling change | The aircraft may model ECAM warnings without modelling individual actuator or flap-drive consequences. |
A mistake we see constantly is treating the PTU sound as proof that fluid is moving between systems. It is only evidence that the PTU is operating; Green and Yellow retain separate reservoirs and hydraulic fluid throughout.