Airbus A320 RAT: when does the ram air turbine deploy, and what does it power?

Yes. The Airbus A320 has a RAT (ram air turbine), an airflow-driven emergency source that normally deploys after loss of both main AC buses when airspeed is above its threshold. It pressurises the Blue hydraulic system, allowing the emergency generator to supply the essential electrical network; it is not a normal generator or APU replacement.

For our Aviation & Real-World Flying readers, two distinctions prevent most confusion. Does the A320 have a RAT? Yes. What does the Airbus A320 RAT power? Blue hydraulics first, then the hydraulic emergency generator and a reduced essential electrical network. It does not deploy in response to every hydraulic fault.

The RAT is one part of a layered backup arrangement. Our explanation of A320 electrical sources, buses and load shedding shows where the emergency generator fits among the engine generators, APU generator, batteries and essential buses.

When does the A320 RAT deploy?

The A320 RAT normally deploys automatically when both AC BUS 1 and AC BUS 2 are unpowered and aircraft speed is above the automatic-deployment threshold, generally about 100 knots.

1. Both main AC buses lose power. The trigger monitors the buses, not simply the number of failed engines or generators.
2. Airspeed is above the deployment threshold. Adequate airflow is needed to turn the turbine.
3. The RAT extends into the airstream. Once extended, it begins driving its hydraulic pump.
4. Blue hydraulic pressure becomes available. This depends on the Blue circuit containing fluid and being able to retain pressure.
5. The emergency generator supports the essential network. Blue hydraulic pressure drives the generator, essential AC and DC supplies are maintained through the relevant distribution equipment, and non-essential loads are shed.

Loss of one engine generator does not normally cause RAT deployment because the remaining generator can supply both main AC buses. Even loss of both engine generators does not necessarily deploy it: if the APU generator or another valid AC source continues to power AC BUS 1 and AC BUS 2, the automatic trigger has not occurred.

Batteries support essential equipment during the interval between bus loss and emergency-generator availability. RAT output also declines with airspeed, so batteries become increasingly significant as the aircraft slows.

Can the crew deploy the RAT manually?

The crew can command deployment manually when an approved procedure requires it. The hydraulic RAT MAN ON control commands physical RAT extension and Blue-system pressurisation, while the emergency-electrical manual control initiates the electrical sequence subject to the applicable system conditions.

A deployed RAT cannot be retracted from the cockpit. Restoring an engine or APU generator can recover the normal electrical network and disconnect the emergency supply, but the turbine remains extended until ground maintenance resets it.

What does the Airbus A320 RAT power?

The A320 RAT supplies mechanical power to a hydraulic pump; it does not generate electricity directly.

• Blue hydraulic system: RAT-driven pressure supports the Blue circuit and selected flight-control actuators connected to it.
• Emergency generator: Blue hydraulic pressure drives the generator that supplies a restricted essential electrical network.
• Essential equipment only: The system prioritises aircraft control, essential instruments and communications. Cabin, galley and other non-essential loads are shed.

The RAT does not pressurise the Green or Yellow hydraulic systems. It is also separate from the Green–Yellow power transfer unit and its hydraulic logic; the PTU transfers mechanical power between those two systems without transferring fluid, while the RAT acts on the Blue system.

Is the A320 RAT the same as the APU or batteries?

No. These sources overlap during an emergency, but they obtain energy differently and do different jobs.

The RAT is therefore not a substitute for the fuel-powered APU and its electrical role. The APU can restore a broader electrical supply; the airflow-driven RAT is a last-resort hydraulic and electrical source in flight.

Does an A320 RAT deploy after a three-hydraulic-system failure?

No. Failure of all three hydraulic systems, by itself, is not the normal automatic RAT trigger on the A320; the normal trigger is loss of both AC BUS 1 and AC BUS 2 with sufficient airspeed.

If Green, Blue and Yellow pressure are lost while the main AC buses remain powered, the RAT should not automatically extend merely because three hydraulic systems show low pressure. If the hydraulic event also causes both main AC buses to become unpowered, the RAT may deploy because the electrical trigger has been met.

What happens after deployment depends on why the Blue system failed. If normal pumps were unavailable but the Blue circuit still contains fluid and can hold pressure, the RAT-driven pump may restore useful Blue pressure. A leak, depleted fluid supply or physical failure affecting the Blue circuit may prevent the RAT from building or retaining pressure.

Even successful RAT operation restores only its contribution to the Blue system. It does not repair damaged lines, recover the Green and Yellow systems or guarantee normal flight-control capability after a genuine three-system hydraulic failure.

How can you test the A320 RAT in a simulator?

In a sufficiently detailed A320 add-on, create loss of both main AC buses while safely airborne, then verify the hydraulic and electrical results rather than relying solely on an external RAT animation.

1. Prepare a safe simulator test. Save the flight, use ample altitude and keep airspeed comfortably above the model’s RAT deployment threshold. This is a simulator exercise only.
2. Check the starting electrical state. Confirm both main AC buses are normally powered and ensure the APU generator is not connected.
3. Remove both normal AC sources. Use the add-on’s failure system or switch off both engine generators. Do not introduce engine or hydraulic failures unless they are part of the specific test.
4. Watch the system indications. On the ECAM ELEC and HYD pages, look for RAT extension, rising Blue pressure, emergency-generator availability, essential-bus supply and load shedding. Our guide to reading A320 ECAM pages and messages explains how to interpret the display hierarchy.
5. Restore a normal source. Reconnect an engine generator or the APU generator. Normal bus power should return, although the RAT should remain physically extended.

Simulated three-hydraulic-failure caution

Do not assume that an “all hydraulics failed” selection will automatically extend the RAT or restore the controls. If the add-on leaves both main AC buses powered, the automatic deployment condition has not been met. If it models the Blue failure as a leak or loss of fluid, an extended RAT may be unable to recover useful pressure.

Failure definitions vary between add-ons. One may model a failed pump, another a leaking circuit, and a simpler aircraft may display only a generic warning. Use the individual add-on’s failure documentation and test electrical bus loss separately from hydraulic-system loss.

Why did the A320 RAT not deploy in my simulator?

The most common causes are a remaining AC source, insufficient airspeed or incomplete systems modelling.

Across our simulation community, the mistake we see most often is switching off both engine generators while leaving the APU generator online. That removes the engine-driven sources, but it does not remove power from both main AC buses, so automatic RAT deployment is neither required nor expected.