Gravity Gear Extension Interface

Gravity gear extension belongs to ATA 32 (Landing Gear) by chapter classification. From the ATA-29 hydraulic perspective, the critical interface is how the gear circuit is isolated from the Green system when gravity extension is selected. Understanding that interface is the foundation for the dual-system-loss decision rule — when Blue and Yellow have failed but Green remains healthy, why does the procedure still call for gravity gear extension?

This article integrates gravity gear from the hydraulic side: how Green normally couples to the landing gear; how the gear circuit is electrically cut from Green when gravity extension is selected; what physical valve performs the isolation (85GA L/G isolation safety valve); the power supply topology that lets gravity extension work without hydraulic or main-electrical support; and the conditions under which normal operation can be restored.

1. Normal landing-gear extension and retraction

Per AMM 32-31:

The Normal Extension and Retraction System... is electrically controlled and hydraulically operated. The hydraulic supply is from the Green Main Hydraulic Power System.

Landing gear is on the Green system's heavy-consumer list (see Power Distribution Map). Each gear cycle — extension or retraction — draws significant Green flow over a span of seconds. Normal operation:

Crew selects gear lever DOWN
        │
        ▼
LGCIU (Landing Gear Control and Interface Unit)
processes the selection
        │
        ▼
Green hydraulic supply pressurises the gear circuit:
   - Uplock releases
   - Doors open
   - Actuators drive the gear down
   - Doors close after gear locks down (normal sequence)
        │
        ▼
Gear extended and locked

The control is electrical (LGCIU); the actuation is hydraulic (Green). This coupling is what gravity extension specifically breaks.

2. Gravity extension — the complete behaviour

Per FCOM DSC-32-10-10:

LANDING GEAR GRAVITY EXTENSION The gravity extension system is an electro-mechanical system, controlled through two selectors located on the center instrument panel. It permits main and nose landing gear extension in case the normal system fails. When the related electrical selectors are set to down, the: - Landing gear hydraulic system is isolated from the green hydraulic system - Main and nose landing gear doors, and gears unlock - Main and nose landing gears extend by gravity - Locking springs assist the downlocking - Main and nose landing gear doors remain open. After a free fall extension, it is possible to restore normal operation, provided green hydraulic pressure is available. Note: In case of landing gear gravity extension, the nose wheel steering is lost.

The five immediate actions when the selectors are set to DOWN:

1. Landing gear hydraulic system is isolated from the Green hydraulic system. This is the ATA-29 critical interface — Green and the gear circuit are electrically cut apart.
2. Main and nose landing gear doors, and gears unlock. The mechanical locks holding the gear in the stowed position release.
3. Main and nose landing gears extend by gravity. No hydraulic actuator pushes them down; gravity does.
4. Locking springs assist the downlocking. Once the gear reaches the down position, springs help drive the lock pins home — preventing the gear from being pushed back by aerodynamic loads or friction.
5. Main and nose landing gear doors remain open. Unlike normal extension (where doors close after the gear is locked down), gravity extension leaves the doors open. Closing them would require hydraulic actuation, which has been isolated.

Five key facts:

3. The hydraulic isolation point — 85GA L/G isolation safety valve

The physical component that performs the Green-system isolation is the L/G Isolation Safety Valve, designator 85GA, located in the landing-gear bay area. The valve has two roles depending on the operational state:

The "dual function" interpretation — same valve serving safety in normal operation and isolation during gravity extension — is consistent with the FCOM text on isolation and with the AMM placement of the 85GA in the Green distribution to the gear bay. The detailed hydraulic actuator logic (how exactly the electrical signal drives the valve closed during gravity extension, versus the speed-based automatic close during normal operation) is documented in maintenance literature.

What matters for the pilot: when gravity extension is selected, the gear hydraulic circuit is cut off from Green by the 85GA, and Green's remaining flow can be directed to its other consumers.

4. Why Green flow conservation matters

The Green system, per Power Distribution Map, drives:

1. Landing gear (normal extension/retraction)
2. Normal brakes
3. Nose wheel steering
4. CSM/G (Constant-Speed Motor/Generator drive)
5. Primary and secondary flight controls (some channels)

When gravity extension closes the 85GA, the gear circuit is removed from Green's demand list. The remaining Green flow can supply:

• Flight controls (the most critical consumer)
• Normal brakes (needed for landing)
• Nose wheel steering (lost — see §5)
• CSM/G (still on Green)

Green can now devote 100% of its capacity to flight controls and braking. This is the foundation of the Blue+Yellow dual-loss procedure: with Blue and Yellow both lost and Green being the only remaining hydraulic source, the architectural choice is to deliberately isolate the gear circuit so that Green is not overloaded by gear extension demand at the moment of landing.

This is not a forced choice but a deliberate architectural design. FCTM "Remaining Systems" table for the Blue+Yellow loss column shows "Gravity (3)" for landing gear extension. Footnote (3) explicitly states:

For approach, landing gear will be extended by gravity to preserve green system integrity for flight controls.

The procedure mandates gravity extension specifically to preserve Green for flight controls — not because gear cannot be extended normally on Green, but because doing so would consume capacity that flight controls need more.

5. The loss of nose wheel steering

Per FCOM DSC-32-10-10:

In case of landing gear gravity extension, the nose wheel steering is lost.

The reason: nose wheel steering is part of the same Green hydraulic circuit as the landing gear (the gear hydraulic circuit downstream of the 85GA). When 85GA cuts the gear circuit, it simultaneously cuts the nose wheel steering circuit.

Operational implications:

• Cannot use nose wheel steering on landing rollout.
• Cannot use nose wheel steering for taxi to the stand.

Available alternatives:

• Differential braking — applying brakes to one main gear strut and not the other produces yaw moments for direction control. This is the primary tool for ground manoeuvring after gravity extension.
• Asymmetric reverse thrust — if reversers remain available (Engine 1's reverser is on Green, Engine 2's on Yellow), reverse thrust on one engine and not the other contributes to direction control on the landing roll.
• Tow-in to the stand — once decelerated, the aircraft is typically towed to the parking position by ground equipment rather than taxied under its own power.

The pilot's mental model: after gravity extension, the aircraft is committed to "landing without nose wheel steering" — the rollout uses differential braking, and arrival at the stand relies on ground crew rather than on-board steering.

6. Power supply — independent of main electrics

The gravity-extension electrical circuit is intentionally designed to operate without main electrical power, drawing instead from the battery-supported buses.

Per maintenance documentation (visible in AMM 32-33 free-fall electrical schematic):

The implications:

• No hydraulic dependency for the unlock command. The actuators are electric; they unlock the gear without any hydraulic action.
• No main-electrical dependency. HOT BUS is supported by the aircraft battery alone. Even in a scenario with both engines stopped, APU unavailable, and all main generators offline, the HOT BUS continues to provide 28 V DC long enough to execute gravity extension.
• Two independent channels. SYS 1 and SYS 2 route through different circuit breakers and supply different actuators. A single channel failure does not prevent the extension.
• Three actuators — one for each main gear leg (MLG L, MLG R) and one for the nose gear (NLG). Each actuator independently releases the uplock of its corresponding gear; gravity does the rest.

This is the "last-resort" design principle: an architecture where the safety-critical action (gear down) survives the loss of all upstream systems. The pilot's hand on the selector is the only requirement; the system handles the rest.

7. The selector design — locked toggle with link

Per FCOM DSC-32-10-30:

The landing gear gravity extension selectors are locked-toggle type selectors. The selectors are connected by a link, so that both are operated simultaneously. When the link is disconnected, each selector can be independently operated.

Selector design features:

• Locked-toggle type — the selector cannot be moved by accidental contact. The crew must consciously unlock and then move the selector. This prevents inadvertent activation by knee, sleeve, or stray hand contact.
• Link between two selectors — both selectors move simultaneously as a normal operation. The crew handles only one motion; both selectors arrive at the same position.
• Disconnectable link — in extremely rare circumstances (link mechanism failure, or specific maintenance situations), the link can be disconnected and each selector operated independently. This is not normal operation but is preserved as an architectural fallback.

The selector positions:

In the OFF position, the gear behaves as a standard Green-supplied actuator system. Moving the selector to DOWN triggers the five behaviours listed in §2 (isolation, unlock, gravity drop, locking-spring assist, doors remain open).

The locked-toggle + linked-selector design reflects the system's critical nature: easy to activate when needed, hard to activate by accident; both selectors aligned by design to prevent split-state issues; with a rare-use fallback (disconnect link) preserved for exotic cases.

8. Reversibility — restoring normal operation

Per FCOM DSC-32-10-10:

After a free fall extension, it is possible to restore normal operation, provided green hydraulic pressure is available.

Conditions for restoration:

• Green hydraulic pressure must be available (3000 psi normal, or 2500 psi via RAT).
• The selector is returned from DOWN to OFF.

The implications:

• Gravity extension is not single-direction-irreversible. Unlike the RAT (which can only be stowed on the ground), gravity gear can in principle be restored to normal operation in flight if Green pressure returns.
• There is a precondition: Green must be pressurised. With Green still lost, the extension stays as gravity-driven and the doors stay open.
• In practical operation, pilots almost never reverse a gravity extension during the same flight. Once the gear is gravity-extended for a landing, the approach typically proceeds to landing on the deployed gear. The reversibility provision exists more for scenarios where conditions change unexpectedly than for routine flexibility.

Compared with RAT deployment (Ram Air Turbine): the RAT is in-flight-irreversible (can only be stowed on the ground). Gravity gear is in-flight-reversible (with Green pressure). The architecture treats the gear circuit as more recoverable than the RAT.

9. Comparison — normal vs gravity extension

The comparison clarifies the trade-off: gravity extension gives up nose wheel steering, leaves doors open (small drag and aesthetic consequence), and consumes a non-standard operational profile, but it preserves the Green system for the most critical consumers (flight controls and braking).

10. When is gravity extension required

The procedural requirement comes from the FCTM "Remaining Systems" table, integrated with the hydraulic loss scenarios from Single-System Loss and Dual-System Loss:

The Blue+Yellow case is the architecturally interesting one — gravity extension is mandated even though Green is healthy, because using Green for gear extension while flight controls also depend on it would over-commit the system.

The other cases are simpler: with Green failed, normal extension is not an option; gravity is the only path.

11. Post-landing operational implications

After gravity extension, the rollout and arrival at the stand have specific considerations:

• Rollout direction control: differential braking and (if available) asymmetric reverse thrust. The pilot establishes a steady gear-track on rollout, using brakes asymmetrically to correct any drift.
• Arrival at the stand: typically by tow tractor rather than self-taxi, since nose wheel steering is unavailable. The crew coordinates with ground control to request towing as part of the post-landing arrangements.
• Maintenance involvement: maintenance confirms gear mechanical lock status (locking springs assist downlock but maintenance should verify), investigates why gravity was needed (whether the underlying hydraulic failure has been understood), and restores normal-operation availability before the next flight.

In rare cases where Green has recovered and the architecture would permit, the crew might attempt to restore normal operation and use nose wheel steering for taxi-in. In practice this is usually avoided as a procedural complication; the safer and standard practice is gravity-extended landing followed by towing.

12. Maintenance reset

After the flight, maintenance:

• Resets the gravity-extension selectors to OFF (manually returned to the normal position).
• Tests normal gear operation using a ground hydraulic source to confirm the gear circuit is fully functional and the 85GA isolation valve has restored to its normal-mode state.
• Investigates the root cause of the hydraulic event that triggered the gravity extension (Green leak, dual-system loss, etc.).
• Updates the technical log with the corrective actions and dispatch status.

The aircraft returns to dispatch only after the gravity-extension cause is understood and the normal hydraulic system is verified.

Self-test

[!note]- Q1. The selectors are set to DOWN for gravity extension. From the ATA-29 hydraulic perspective, what is the most important consequence?

The landing gear hydraulic system is isolated from the Green hydraulic system, via the 85GA L/G isolation safety valve. The 85GA is normally a safety valve preventing in-flight gear extension at high speeds; when gravity extension is selected, the valve is electrically driven closed, cutting the gear circuit from Green. The architectural significance: Green's flow capacity is now freed from the gear-circuit demand and can be entirely dedicated to flight controls, normal brakes, and other Green consumers. This is the foundation of the Blue+Yellow dual-loss procedure — gravity extension preserves Green for the most critical consumers rather than competing for capacity.

[!note]- Q2. The power supply for gravity extension is 28 V DC HOT BUS 1 and HOT BUS 2. Why this choice instead of main electrical buses?

HOT BUS is battery-supported and always energised, regardless of the state of the main electrical generators. Gravity extension may be required in scenarios where the main electrical system has already been lost — dual engine failure with APU unavailable would leave the main AC buses without power. Drawing gravity extension from HOT BUS ensures the function works under any electrical configuration, down to battery-only operation. The architecture treats gravity extension as a safety-critical last resort that must operate independently of all upstream systems. The use of two HOT BUSes (1 and 2) with independent channels (SYS 1 and SYS 2) provides redundancy on the electrical side as well.

[!note]- Q3. After gravity extension, nose wheel steering is lost. Why, and what alternative does the pilot use for ground manoeuvring?

Nose wheel steering is part of the same Green hydraulic circuit as the landing gear — the gear-circuit hydraulic supply that the 85GA isolation valve cuts off. When 85GA closes, both the gear circuit and the nose wheel steering circuit lose pressure simultaneously. The alternative for ground manoeuvring is differential braking — applying brakes to one main gear strut and not the other to produce yaw moments. If reversers are available, asymmetric reverse thrust on the landing roll adds additional directional control. For arrival at the parking stand, the aircraft is typically towed by ground equipment rather than taxied under its own power. The pilot's mental model is "land without nose wheel steering, rollout with differential braking, tow to stand."

[!note]- Q4. In the Blue+Yellow dual-loss case with Green healthy, why does the procedure call for gravity gear extension rather than normal Green-driven extension?

The architecture treats this as a deliberate design choice to preserve Green's flow capacity for flight controls. With Blue and Yellow both lost, Green is the only remaining hydraulic system; the flight controls depend on Green pressure for surface actuation. Gear extension on Green would draw significant flow at a critical moment (approach and landing), potentially competing with the flight controls. The FCTM table footnote makes this explicit: "For approach, landing gear will be extended by gravity to preserve green system integrity for flight controls." This is not a forced choice (Green could in principle extend the gear); it is a procedural commitment to dedicate Green to flight controls and braking, with gravity handling the gear extension instead. The cost is the loss of nose wheel steering and doors-open configuration; the gain is preserved flight-control authority.

[!note]- Q5. Gravity extension is described as "reversible" in flight if Green pressure is available. What does this mean in practice, and how does it compare with RAT deployment?

"Reversible" means that after a gravity extension, returning the cockpit selectors to OFF and confirming Green pressure is available allows the gear to operate again under normal Green-driven extension/retraction logic. The 85GA isolation reopens, the gear circuit reconnects to Green, and standard LGCIU control is restored. In practice, pilots rarely reverse a gravity extension during the same flight — once gear is extended for landing, the approach typically completes on the deployed gear. The reversibility is more about preserving architectural options than routine operational use. Compared with RAT deployment, gravity extension is the more reversible of the two: the RAT is in-flight-irreversible (can only be stowed on the ground), while gravity gear can in principle be returned to normal operation in flight if Green pressure returns. The architectural difference reflects the simpler isolation logic in gravity extension (a single electrical isolation valve) versus the RAT (which involves the turbine and its surrounding mechanical and electrical infrastructure).

References

Per FCOM DSC-32-10-10 (Landing Gear Gravity Extension — full procedure, isolation, nose wheel steering loss, reversibility); FCOM DSC-32-10-30 (gravity extension selector design — locked-toggle, link); AMM 32-31 (Normal Extension and Retraction System hydraulic supply); AMM 32-33 (gravity extension electrical schematic, HOT BUS sources, dual-channel architecture); AMM 32-31 schematic for the 85GA isolation valve placement; FCTM PR-AEP-HYD (Remaining Systems table footnote — Blue+Yellow loss case with gravity extension to preserve Green); cross-references to Power Distribution Map, Dual-System Loss, and Ram Air Turbine.

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