Parking and Ultimate Emergency Braking

The parking brake is not just "hold the aircraft still" — it is also the ultimate emergency brake when every normal and alternate mode has failed. The AMM puts both functions in one section because they are one set of hardware. Per AMM 32-45-00:

The parking braking system is an electrohydraulic system. Its primary function is to prevent movement of the aircraft when it is parked. It can also be used to stop the aircraft if control in all the other braking modes is not available.

Its hydraulic source is shared with alternate braking — blue main hydraulics or the park/alternate accumulator. Per AMM 32-45-00:

The parking braking system gets its hydraulic power supply from the accumulators of the Alternate Braking system or the Blue Main Hydraulic Power system. The accumulators have sufficient capacity to hold the brakes on for a minimum time of twelve hours.

This article reuses many components from Alternate Braking — the park/alternate accumulator, dual valve, dual shuttle valve, second piston set, and 175 bar limit — so read that first.

1. Borrowing the lower half of alternate braking

The parking brake does not build a new circuit; it borrows the lower half of alternate braking (dual shuttle valve → servovalve → second piston set, as in Alternate Braking), simply swapping the upstream source from the pedals/dual valve to a control valve (4GZ) direct feed.

 PARK BRK selector 3GZ ──ON──► control valve 4GZ (two circuits, two motors)
                                 │ (1) electrical contacts → BSCU: isolate normal braking
                                 │     + minimum current to alternate servovalves → max braking (faster on)
                                 │ (2) connect to NWS de-activation box 5GC → PARKING BRAKE light on
                                 │ (3) connect accumulator (or blue) to two service ports:
                                 ▼
              ┌──── port C (reduced, held at 175 bar) ────► dual shuttle valve 5404GG
              │                                              │ closes the dual-valve 5403 supply
              │                                              │ connects parking pressure → second piston set
              └──── port D (full pressure) ──┬─► automatic selector valve 5202GG (closes alternate-servovalve return)
                                             └─► park brake operated valve 5801GG (isolates dual-valve 5403 supply)
 pedals ──✗── (have no effect in this configuration)
              triple indicator 2GK: accumulator pressure + per-MLG brake pressure when ON

Three dedicated parts: the selector switch 3GZ (OFF/ON), the control valve 4GZ (the core — connects hydraulics, reduces to 175 bar, two redundant circuits), and the park-brake operated valve 5801GG (isolates the dual valve so it cannot leak pressure when parked).

2. The control valve — reduce to 175 bar with dual-circuit redundancy

Per AMM 32-45-00:

The control valve is an electrohydraulic valve that connects the hydraulic power supply to the brakes. It also decreases the pressure supplied to the servo valves to 175 bar (2538 psi). The system has two independently connected circuits each with a different power supply. Usually the circuits are energized at the same time but each circuit operates the control valve if the other is not available.

175 bar matches the alternate dual-valve ceiling (Alternate Braking) — the parking brake delivers the same pressure level as alternate braking, not an excessive accumulator-direct pressure. Two independent circuits on different power supplies make the control valve single-fault tolerant.

3. What happens at PARK BRK ON (the core cascade)

Per AMM 32-45-00:

When the park-brake selector-switch 3GZ is set to ON... electrical contacts in the control valve send a signal to the BSCU to isolate the Normal braking system. The BSCU sends a minimum current to the alternate brake servo valves to give maximum braking. This helps the brakes come on quicker.

This reuses the alternate servovalve's "minimum current = applies pressure" property (Alternate Braking). The two service ports then divide the work. Per AMM 32-45-00:

Full pressure is supplied from port D to: the automatic selector valve 5202GG which closes the return line from the alternate brake servo valves / the park brake operated valve 5801GG which isolates the supply pressure to the dual valve 5403GG. The pressure from port C of the control valve causes the dual shuttle valve 5404GG to operate. It closes the supply line from the dual valve 5403GG and connects the pressurized fluid to the brakes.

Port D full pressure "blocks the roads" first (closes the alternate-servovalve return, isolates the dual valve), and port C's 175 bar feeds the brakes through the dual shuttle valve. The result, per AMM 32-45-00:

Movement of the brake pedals does not affect the system when it is in this configuration.

Why the pedals are defeated: the pedal path runs through the dual valve → dual shuttle valve, but the dual valve is now isolated by 5801GG and the dual shuttle valve 5404 has switched to the control-valve side — the pedal path is physically cut. So with the parking brake ON, the brake pressure is locked by the handle/control valve, and pressing the pedals does nothing.

4. The 4-second logic — ON but no pressure → normal braking returns

Per AMM 32-45-00:

When the parking brake is set to ON with correct alternate system pressure, the normal braking system is isolated. When the parking brake is set to ON with no alternate system pressure for 4 seconds or more, the normal braking system becomes available. When correct alternate system pressure becomes available again, the normal braking system becomes isolated again.

This solves a dilemma: if the crew has left the parking brake ON before taxi or landing while the parking brake itself has no pressure (the accumulator is exhausted), the system will not leave you with neither parking nor normal braking — it automatically gives normal braking back. Once the accumulator recharges, parking-brake priority returns. It is a safety backstop against falling between two stools.

5. As the ultimate emergency brake

ULTIMATE EMERGENCY BRAKING in the title is exactly this: after normal, alternate-with-antiskid, and alternate-without-antiskid have all failed, this hardware (especially the accumulator) is the last brake. The FCOM points to the scenario in alternate-without-antiskid. Per FCOM DSC-32-30-10:

The antiskid system is either deactivated... Hydraulically (B + G system low pressure, the brakes are supplied by the brake accumulator only). ...The pilot must modulate brake pressure at, or below, 1 000 PSI in order to avoid wheel locking.

The ultimate emergency brake has no antiskid — it works on the accumulator's limited applications (at least 7 full applications — Alternate Braking), modulated gently and progressively on the PARK BRK handle. The step-by-step procedure is in Brake Failure and Degradation.

6. Indications and warnings

Per AMM 32-45-00:

The warning message PARK BRK ON shows on the E/WD if: the parking brake is set to ON and the engines are at takeoff power. When the warning message shows, a continuous repetitive chime operates and the MASTER WARN lights come on.

And: If the park brake pressure decreases to less than 40 bars at the alternate brake pressure transducers (when the park brake is selected on), the warning message PARK BRK LO PR shows after 4 seconds.

The two warnings:

PARK BRK ON (at take-off power) prevents a take-off with the parking brake set — a potentially fatal error, so it carries the highest level: MASTER WARN plus a continuous repetitive chime.
PARK BRK LO PR (below 40 bar) warns that parking-brake pressure is insufficient and the aircraft may not be held — prompting a recharge (cycle the PARK BRK to refill the accumulator) or chocks.

[!warning]- Seven misconceptions this article corrects (1) The parking brake is not only for parking — it is also the ultimate emergency brake. (2) With the parking brake ON you can not add pedal braking — the pedals are defeated (dual valve isolated, dual shuttle valve switched). (3) The parking brake does not use the first piston set — it uses the second (like alternate braking). (4) It is not a single-circuit system — two independent circuits on different power supplies. (5) PARK BRK ON does not always suppress normal braking — ON with no pressure for 4 seconds returns normal braking (anti-"two stools"). (6) The accumulator does not hold for only a short while — at least 12 hours. (7) The ultimate emergency brake has no antiskid — modulate at ≤ 1000 PSI, gently and progressively, to avoid a lock.

Self-test

[!note]- Q1. What does setting PARK BRK ON rearrange internally, and why are the pedals defeated?

The control valve's electrical contacts tell the BSCU to isolate normal braking and send minimum current to the alternate servovalves for maximum braking (faster on). Port D full pressure closes the alternate-servovalve return and isolates the dual valve (via 5801GG); port C's 175 bar drives the dual shuttle valve to feed the brakes. The pedals are defeated because their path through the dual valve and dual shuttle valve is physically cut (5801GG isolates the dual valve, 5404 has switched to the control-valve side).

[!note]- Q2. Which piston set does the parking brake use, and which lower-half components does it share?

The second piston set. It shares alternate braking's lower half — the dual shuttle valve 5404GG → the alternate servovalves → the second piston set — and the park/alternate accumulator.

[!note]- Q3. What does the "ON but no pressure for 4 seconds → normal braking returns" logic solve?

It prevents the crew being left with neither parking nor normal braking. If the parking brake is ON but has no pressure (the accumulator is exhausted) for 4 seconds or more, normal braking is made available; when accumulator pressure returns, parking-brake priority resumes.

[!note]- Q4. What triggers PARK BRK ON versus PARK BRK LO PR?

PARK BRK ON shows (with MASTER WARN and a continuous repetitive chime) when the parking brake is ON and the engines are at take-off power — preventing a take-off with the parking brake set. PARK BRK LO PR shows after 4 seconds if, with the parking brake selected ON, the pressure falls below 40 bar at the alternate-brake transducers.

[!note]- Q5. How is the ultimate emergency brake used?

After all other braking modes have failed, on the park/alternate accumulator, with no antiskid — modulated at or below 1000 PSI, gently and progressively on the PARK BRK handle, within the accumulator's limited applications. The step-by-step procedure is in the brake-failure article.

Key takeaways

Theme	The one thing to remember
Two functions, one hardware	Park the aircraft, and the ultimate emergency brake
Source and pistons	Blue or accumulator (≥ 12 h hold); second piston set, shared with alternate braking
PARK BRK ON	Control valve isolates normal braking, locks the brakes via the shuttle valve; pedals defeated
4-second logic	ON with no pressure for 4 s returns normal braking — never both lost at once
Warnings	PARK BRK ON at take-off power = MASTER WARN + chime; PARK BRK LO PR below 40 bar
Ultimate emergency brake	No antiskid — modulate at ≤ 1000 PSI, gently and progressively

References

A330 specifics per AMM 32-45-00 (Parking/Ultimate Emergency Braking — the two functions, accumulator 12-hour hold, control valve 4GZ at 175 bar with dual circuits, the PARK BRK ON cascade with the port C/D division and pedal defeat, the 4-second logic, and the PARK BRK ON / PARK BRK LO PR warnings at 40 bar) and FCOM DSC-32-30-10 (the ultimate-emergency scenario on the accumulator only, ≤ 1000 PSI). The PARK BRK ON hydraulic-rearrangement diagram is an integrative synthesis of the AMM operation text. The control-valve internal reducing-servo mechanism is maintenance-layer; the pilot procedure for the ultimate emergency brake is in Brake Failure and Degradation. 40 bar (PARK BRK LO PR), 175 bar (control-valve limit), and 1000 PSI (the manual-modulation target) are three distinct values with distinct meanings and must not be conflated.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.