Ram Air Turbine (RAT)

The RAT is the emergency power source for the Green hydraulic system. It is a drop-out turbine driven by ram air, coupled to a hydraulic pump, that pressurises Green to 2500 psi when both engines have failed or when reservoir low levels on the Green plus either Blue or Yellow system are detected. Beyond hydraulics, the same RAT also drives the emergency generator for the AC EMER bus — that electrical role belongs to ATA 24 and is referenced from there.

This article covers the engineering: a two-blade variable-pitch turbine governed mechanically to hold speed within a fixed band, a nine-piston variable-displacement hydraulic pump, the spring-driven release mechanism with dual redundant deployment paths, and the precise automatic-extension conditions including the 30-second post-gear-extension inhibit.

1. Where the RAT sits

The RAT and its supporting components are stowed inside the flap-track fairing #4 on the right wing. When stowed, the turbine and leg assembly are entirely enclosed; when deployed, the leg pivots out into the airstream below the wing.

                  Stowed inside flap-track fairing #4 (right wing)
                                       │
                          ┌────────────┴──────────────┐
                          │ Release command           │
                          │  • Manual: cockpit PB SW  │
                          │  • Auto: HSMU logic       │
                          └────────────┬──────────────┘
                                       │
                                       ▼
                       Retract lock releases
                                       │
                                       ▼
                       RAT actuator extends the leg
                       using spring force
                                       │
                                       ▼
                       Leg rotates into airstream
                                       │
                                       ▼
                       Extend lock engages
                                       │
                                       ▼
            ┌──────────────────────────────────────────────┐
            │  Two-blade variable-pitch turbine            │
            │  • 29.5" disc diameter                       │
            │  • Mechanical governor holds 4850–6370 rpm   │
            └─────────────────────┬────────────────────────┘
                                  │
                                  │ Angle gearbox at base of leg
                                  ▼
            ┌──────────────────────────────────────────────┐
            │  Drive shaft through the leg assembly        │
            └─────────────────────┬────────────────────────┘
                                  │
                                  │ Angle gearbox at top of leg
                                  ▼
            ┌──────────────────────────────────────────────┐
            │  Nine-piston variable-displacement pump      │
            │  • Anti-stall valve (low-airspeed protection)│
            │  • Inlet impeller (cavitation buffer)        │
            │  • Speed sensor (signal to FWC)              │
            └─────────────────────┬────────────────────────┘
                                  │
                                  ▼
                       RAT manifold → Green system at 2500 psi
                       
                       (Same unit also drives the emergency
                        generator — see ATA 24)

2. The turbine — two blades, mechanical governor

The turbine has two variable-pitch blades on a hub of machined aluminium alloy. Disc diameter is 29.5 inches (0.7493 m). There is no electrical control; the turbine is governed entirely by a mechanical flyweight-and-spring system.

Governor operation

The governor maintains turbine speed within 4850–6370 rpm across the full deployment envelope. The mechanism:

The closed-loop is purely mechanical. The RAT continues to govern even with all electrical systems failed — a deliberate design choice given that the RAT is the last-resort source in scenarios where the rest of the electrical architecture may also be in degraded state.

Two consequences worth noting:

• The turbine speed is stable. Across the operating envelope of aircraft speed and altitude, the rpm stays within the 4850–6370 band. What changes is the flow the hydraulic pump can produce — see §3.
• Time to operating speed. From release to stable governed speed takes seconds, not tens of seconds. The blades pitch fine to spin up the rotor, then move to operating coarse pitch as governor authority builds.

3. The hydraulic pump — nine pistons, 2500 psi

The RAT hydraulic pump is a nine-piston variable-displacement unit, similar in topology to the EDP but adapted for the lower input speed and the variable-airspeed environment.

Performance:

The output figure of 15% to 45% of EDP flow that appears in the FCOM corresponds to the 41.9–81.4 L/min range against the EDP's 175 L/min nominal. The variation with airspeed is significant — a slow approach delivers far less flow than cruise speed, even at the same regulated 2500 psi pressure.

The anti-stall valve

At very low airspeed, the turbine struggles to produce enough power to drive the pump at its nominal compensator demand. Without protection, the pump could stall the turbine — torque demand exceeds available power, rotor slows, blades cannot maintain governing authority, the turbine stops.

The anti-stall valve prevents this. When pump load threatens to exceed available power, the valve reduces output flow (effectively bypassing some demand internally) to a level the turbine can support at the current airspeed. Pressure regulation continues at 2500 psi at reduced flow.

The crew's interpretation: at low approach speeds with the RAT supplying Green, flow may be limited, which translates into slower surface response. This is documented as "aileron, elevator, and spoiler servo control operating speeds are reduced" in the FCOM, and the anti-stall valve is the architectural reason behind it.

4. Release mechanism — spring drives, hydraulic retracts

The RAT actuator has two motion modes:

• Extension: spring-driven. The actuator is preloaded with spring force that pushes the leg out when the retract lock releases.
• Retraction: hydraulically driven, using pressure from the main aircraft hydraulic system. The RAT can only be stowed on the ground.

Two mechanical locks hold position:

• Retract lock: holds the leg stowed during normal operation. Release of this lock allows the spring to push the leg out.
• Extend lock: engages once the leg reaches full extension, holding it in place against the airstream.

Two deployment solenoids release the retract lock — either one alone is sufficient:

The dual-solenoid architecture is intentional redundancy. Either path can deploy the RAT independently; failure of either solenoid does not prevent extension. The RAT is the last-resort source — its deployment must be reliable.

Extend-lock release on retraction

When retraction is commanded (on the ground), hydraulic pressure first releases the extend lock, then drives the actuator to pull the leg back into the stowed position. The retract lock then engages, holding the leg stowed for the next deployment cycle. Retraction requires the Green hydraulic system to be pressurised — which is why the RAT cannot self-retract during a failure scenario where Green is being supplied by the RAT itself.

5. Automatic extension conditions — the precise envelope

Per AMM 29-24, automatic extension fires only when all of the following apply for either trigger group:

Trigger group A — Dual engine failure

• Airspeed > 100 kt (in flight)
• Engine 1 AND Engine 2 N2 both < 50%

The 50% N2 threshold is the precise condition; "both engines failed" in the FCOM corresponds to N2 below half-idle on both engines simultaneously.

Trigger group B — Combined reservoir low level

• Airspeed > 100 kt (in flight)
• Either Green and Blue reservoirs both at low level, or Green and Yellow reservoirs both at low level
• Additional inhibit: automatic extension is suppressed for 30 seconds after the landing gear control lever is moved from UP to DOWN

The 30-second post-gear-extension inhibit deserves explanation. When the gear is selected DOWN, gear extension consumes Green flow heavily, transiently. If the Green pumps are stressed against this demand, the reservoir level may momentarily drop into the LO LEVEL band — triggering the automatic RAT extension on a transient that would clear on its own. The 30-second inhibit gives the gear cycle time to complete and the reservoir level to recover before the automatic logic is allowed to fire.

This is a subtle and important piece of architectural reasoning. The RAT cannot be stowed in flight; an inadvertent automatic deployment is operationally costly. The 30-second window prevents the most common false-positive case.

Manual extension — no envelope restriction

Manual extension via the RAT MAN ON pushbutton works at any airspeed, in any flight phase, with no inhibits. The crew may deploy precautionarily, but the consequences are non-reversible until landing.

6. The stowing constraint

Once extended in flight, the RAT cannot be stowed until the aircraft is on the ground. This applies whether the extension was manual or automatic.

The architectural reason: in-flight retraction would require the Green system to be pressurised (since hydraulic pressure drives the actuator retraction), but during dual-engine-out or dual-reservoir-low scenarios, Green is precisely the system being supplied by the RAT. Trying to retract the RAT in flight would either:

• Fail (because the Green source for retraction is the RAT itself, removed during retraction), or
• Succeed momentarily and then re-deploy when the source is lost.

The simplest safe behaviour is to lock the configuration once committed: RAT out for the remainder of the flight.

The RAT OUT memo behaviour reinforces this. The memo appears in green in cruise and approach — the RAT-out state is potentially the correct configuration for an ongoing abnormal. The memo appears in amber during takeoff phases 1 and 2 — the RAT should not be extended during takeoff or initial climb, and amber is a configuration warning. The colour change tracks whether the extension is appropriate for the flight phase.

7. Effects on the Green system when RAT-supplied

Per FCOM DSC-29-10-20:

Aileron, elevator, and spoiler servo control operating speeds are reduced when the RAT pressurises the Green system.

Reduced speeds, not reduced authority. The surfaces still reach their full deflection range; they reach it more slowly.

The cause is the reduced flow (15–45% of EDP, varying with airspeed). The compensator on the hydraulic pump regulates pressure at 2500 psi, but the flow available to drive surface actuators is lower. Under high simultaneous demand (rapid aileron input, large spoiler movements), surface motion lags.

In addition to the surface-speed effect, the priority valve sheds heavy consumers when system pressure indicates the RAT-supply configuration. Nosewheel steering and landing gear normal operation are disconnected; brake pressure is restricted to 1000 psi. The full set of restrictions is in Priority Function and Fire Shut-Off Valves.

8. The dual role — hydraulic and electrical

Within ATA 29, the RAT is the hydraulic source. The same physical assembly also drives the emergency generator that supplies the AC EMER bus — this is the electrical role, documented in ATA 24.

The architectural significance is that the same single deployment event restores both:

• Hydraulic Green at 2500 psi → flight controls retained, brakes limited, gear via gravity extension.
• AC EMER bus → essential avionics, navigation, a subset of cockpit displays, and the BAT supply maintenance.

The crew's mental model in a dual-engine-out scenario is therefore not "deploy the RAT for hydraulics, then think about electrics separately" — both come together from the same event. The cost is also bundled: if the RAT fails to deploy or fails after deployment, the aircraft loses both Green and AC EMER simultaneously.

9. Flow versus airspeed — the documented envelope

Per AMM 29-24, the RAT hydraulic pump flow varies with corrected airspeed. The published table for the in-service FSN groups:

Cross-check against FCOM's "15% to 45% of EDP flow":

• At Vc = 250 kt: 81.4 / 175 = ~46.5% of EDP flow → matches the FCOM upper bound (~45%).
• At Vc = 125 kt: 41.9 / 175 = ~23.9% of EDP flow → within the documented range.
• Below 125 kt: anti-stall valve becomes the limiting factor (see §10); FCOM's 15% lower bound corresponds to the band where the RAT can still produce some flow but at very limited rates.

The surface-speed columns capture the operational consequence: at 125 kt the elevator can deflect only 11 °/s (versus ~20 °/s under normal EDP supply); the rudder authority is reduced to 3.5 °/s, which significantly affects crosswind handling on landing. Approach planning under RAT supply factors these reduced rates into target speed margins.

10. Anti-stall valve — detailed working

The anti-stall protection in the RAT hydraulic pump prevents turbine stall at low airspeeds. The mechanism, per AMM 29-24:

RAT turbine speed → Gear pump flow (proportional to turbine speed)
                         │
                         ▼
                  [Restrictor in line]
                         │
              ┌──────────┴──────────┐
              ▼                     ▼
       Low ΔP across restrictor    High ΔP across restrictor
       (turbine slow / starting)    (turbine fast / normal)
              │                     │
              ▼                     ▼
       Anti-stall valve         Anti-stall valve
       reduces hanger angle     increases hanger angle
              │                     │
              ▼                     ▼
       Pump displacement        Pump displacement
       drops (unloaded)         rises (full load)
              │                     │
              ▼                     ▼
       Turbine not stalled     Pump delivers required flow
       by high torque

During startup, the anti-stall valve holds pump displacement at minimum so the RAT turbine spins up against essentially no load. Once the turbine reaches operating speed, the valve allows full pump displacement and normal flow delivery resumes.

125 kt is the documented anti-stall limit — below this airspeed the protection cannot guarantee against turbine stall. This is consistent with the flow table starting at 125 kt and with the operational airspeed envelope for RAT-supplied operations.

11. Blade index mechanism

A blade index mechanism on the RAT leg assembly locks the turbine blades in maximum coarse pitch when the RAT is stowed. Per AMM 29-24:

A blade index mechanism is installed on the leg assembly. It engages in a slot in the turbine hub and keeps the blades in the maximum coarse pitch position while the turbine is in the retracted position. The blade index mechanism disengages automatically on deployment when the RAT is approx. 10 degrees before the full extension.

The mechanism's purpose:

• In the stowed state: blades locked in maximum coarse pitch prevents in-fairing vibration and any blade movement that could damage the assembly.
• During deployment: the index pin disengages automatically as the RAT leg approaches 10° before full extension — leaving the blades free to rotate and allowing the governor to take over pitch control.
• For retraction (on the ground): the blades must return to a position where the index pin can re-engage. An interlock proximity switch prevents retraction if the pin is not correctly engaged. If the turbine is not in the correct position, the RAT cannot be retracted — maintenance must manually rotate the turbine to align the pin before stowing.

Per AMM 29-24:

The mechanism includes an interlock proximity switch which prevents the retraction of the RAT if the pin is not correctly engaged.

Operational consequence: a deployed RAT cannot simply be stowed by pressing the stow control on the ground. Maintenance may need to manually rotate the turbine to align the blade-index mechanism before retraction is possible.

12. RAT stow panel — on the Yellow ground service panel

The RAT stow panel (designator 12JR) is installed on the Yellow hydraulic-system ground service panel — not on the Green ground service panel, despite the RAT supplying the Green system.

Per AMM 29-24:

A RAT stow panel 12JR is installed in the Yellow hydraulic-system ground-service-panel.

This is a counter-intuitive placement worth flagging. For pilot awareness, this is mostly a maintenance-facing detail — the crew does not operate the stow panel — but knowing the panel's location prevents confusion when a maintenance technician is found at the Yellow ground service panel performing RAT-stow operations after a flight.

13. Output interface and the speed sensor

The RAT manifold is mounted on the RAT pump itself. It is the interface between the pump and:

• The Green main system (delivering 2500 psi to the system manifold).
• The RAT actuator (extend / retract logic).

A speed sensor in the pump sends turbine RPM data to the Flight Warning Computer (FWC). The FWC uses this to drive ECAM indications related to RAT operation status.

The RAT manifold and speed sensor are integrated into the RAT module. A single replacement module includes the turbine, leg assembly, hydraulic pump, actuator, and manifold — replaced as a unit if a major fault occurs.

14. Crew action chain on RAT automatic deployment

When automatic deployment triggers, the sequence from the crew's perspective:

[Trigger: N2 < 50% on both engines, or G+B/G+Y RSVR LO LVL, with Vc > 100 kt]
     │ HSMU energises the automatic deployment solenoid
     ▼
[Retract lock releases]
     │
     ▼
[Spring pushes RAT out of fairing into airstream]
     │ About 10° before full extension, blade index disengages
     ▼
[Turbine starts spinning → accelerates to 15–20% RPM]
     │ Blades shift from max coarse to fine pitch
     ▼
[Turbine reaches normal 4850–6370 RPM (governor regulates)]
     │ Anti-stall valve holds pump at minimum displacement during startup
     ▼
[Pump begins delivering fluid → Green system pressurised at 2500 psi]
     │ Speed sensor sends data to FWC → ECAM updates
     ▼
[Crew sees on SD HYD page: Green pressure recovers to 2500 psi]
     │ RAT OUT memo appears (green in cruise, amber during takeoff phases 1/2)
     ▼
[Crew operates within RAT-supplied envelope: reduced surface speeds, priority shed]

The crew has no direct action in this chain other than observing the indications and proceeding with the abnormal procedure for the underlying condition (dual engine failure, dual reservoir low).

15. Maintenance interfaces and BITE tests

RAT-related maintenance tasks documented in AMM 29-24:

The architectural choice to permit module-level replacement of the entire RAT (turbine + leg + pump + actuator + manifold as one part) reflects the rarity of in-service RAT faults. When a major fault occurs, the operational priority is to return the aircraft to service rather than perform component-level repair on the wing.

16. Indications

The RAT contribution is not shown as a separate pump on the SD HYD page. What the crew sees is Green at 2500 psi instead of 3000 psi, plus the RAT OUT memo confirming the source.

Self-test

[!note]- Q1. The crew sees the RAT OUT memo in amber during the initial climb after takeoff. Diagnosis?

Amber RAT OUT is the configuration warning specific to takeoff phases 1 and 2 — the RAT is out when it should be stowed. Two possibilities: an inadvertent manual extension (someone pressed RAT MAN ON), or an automatic extension triggered by an underlying condition (dual engine failure or dual reservoir low level in this phase). The latter is far more likely on a real climb. The crew identifies the underlying cause via the ECAM HYD page and engine indications, follows the appropriate abnormal procedure, and accepts that the RAT will remain extended until landing.

[!note]- Q2. Automatic RAT extension on dual-reservoir-low-level is inhibited for 30 seconds after the gear lever moves from UP to DOWN. Why?

Gear extension is a high-flow Green consumer. When the gear is selected DOWN, Green flow demand spikes briefly, and if pump capacity is already constrained, the reservoir level can transiently dip into the LO LEVEL band. Without the inhibit, this transient could trigger automatic RAT extension on a condition that would clear in a few seconds. Since the RAT cannot be stowed in flight, a false-trigger extension would commit the aircraft to the RAT-out configuration unnecessarily. The 30-second inhibit allows the gear cycle to complete and the reservoir level to stabilise before the automatic logic is allowed to fire.

[!note]- Q3. The RAT extends manually at FL250 in cruise as a precaution during an abnormal investigation. The investigation resolves; the original concern no longer exists. Can the RAT be stowed?

No. The RAT cannot be stowed in flight under any condition. The retraction mechanism uses Green hydraulic pressure to drive the actuator, and the entire scenario the RAT was deployed to address — possible Green degradation — is precisely the case where Green availability for retraction is uncertain. The architecture takes the simpler, safer route: deployment is one-way until landing. The crew has committed to the RAT-out configuration for the rest of the flight, including 2500 psi Green, reduced surface speeds, and the priority-valve restrictions.

[!note]- Q4. The mechanical governor on the RAT keeps turbine speed between 4850 and 6370 rpm. The hydraulic pump produces 2500 psi at regulated output. Why does flow vary with airspeed despite stable rpm and pressure?

Because the turbine's power input varies with airspeed. The governor holds rpm constant by adjusting blade pitch; if airspeed is high, the turbine can extract more power at the same rpm (blades at higher pitch); if airspeed is low, less power is available. The hydraulic pump downstream of the turbine has a compensator that regulates pressure at 2500 psi, but the flow it can sustain depends on the power available to drive the pistons. Higher airspeed → more turbine power → more flow at 2500 psi. Lower airspeed → anti-stall valve reduces flow to keep the turbine from stalling. The pressure stays at 2500 psi throughout; flow drops with airspeed.

[!note]- Q5. Two solenoids can deploy the RAT, one for manual control and one for automatic. Why two rather than one?

Redundancy. The RAT is the last-resort source in scenarios where the aircraft has already lost most of its normal power. A single deployment solenoid failure during such a scenario would prevent the RAT from extending — leaving the aircraft with no recovery path. The dual-solenoid design ensures that either path can fire the deployment independently. The manual path is also typically wired through a different power source than the automatic path, so a power failure on one path does not affect the other. The redundancy is symmetric to the role: RAT extension is too important to depend on a single signal chain.

References

Per FCOM DSC-29-10-20 (Ram Air Turbine description, automatic and manual extension, surface-speed effects, stowing constraint); AMM 29-24 (RAT physical components, turbine specifications, mechanical governor logic, deployment solenoids, actuator with retract and extend locks, automatic extension trigger conditions including 30-second gear-extension inhibit, anti-stall valve, flow envelope vs airspeed); FCOM DSC-29-20 (RAT OUT memo behaviour).

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, AMM, and QRH for operational use.