Autobrake

The most-misunderstood thing about autobrake is the belief that "pressing LO/MED/MAX brakes the aircraft". It does not. Pressing the pushbutton only arms the system; the actual braking is triggered later by ground-spoiler extension. Autobrake holds a constant deceleration rate, on landing and on a rejected take-off (RTO). Per FCOM DSC-32-30-10:

The A/BRK is a function that automatically decelerates the aircraft during landing, or in the case of a rejected takeoff (RTO). The A/BRK: Avoids any delay for brake application in the case of an RTO / Reduces flight crew workload during landing... / Reduces the number of brake applications, and therefore, brake wear.

Three programmes — LO and MED for landing, MAX for take-off. Per AMM 32-42-00:

Automatic braking is available during a landing (LO or MED) or a take-off (MAX).

1. Arming — pressing the pushbutton checks a set of conditions

Per FCOM DSC-32-30-10:

The crew may arm the system by pressing the LO, MED, or MAX pushbutton provided all the following arming conditions are met: Green pressure available / Antiskid electrically-powered / No failure in the braking system / At least two PRIMs are available and able to command the ground spoilers extension / At least one ADIRU available. Note: Autobrake may be armed with the parking brake on.

(The AMM calls the flight-control primary computer the FCPC; the FCOM calls it the PRIM — the same computer.) Note the arming condition "at least two PRIMs able to command ground-spoiler extension" — it foreshadows that activation depends on the spoilers. Armed, the AUTO BRK mode shows blue on the FMA; the BSCU lights the pushbutton's ON (blue) light and shows AUTO BRK with the programme (LO/MED/MAX) in green on the WHEEL page. If the conditions are not met, AUTO BRK shows amber on the WHEEL page.

   Press LO/MED/MAX pushbutton
        │ all arming conditions met? (green pressure / antiskid powered /
        │  no brake fault / ≥2 PRIMs able to command spoilers / ≥1 ADIRU)
        │  (may be armed with parking brake ON)
        ▼
   ┌─ ARMED ───────────────────┐  ON (blue) light; FMA blue AUTO BRK; WHEEL page AUTO BRK + LO/MED/MAX green
   │  (conditions not met → amber) │  ◄── no braking yet!
   └────────────┬─────────────────┘
                │ activation: BSCU receives ≥2 ground-spoiler extend signals (from the FCPCs)
                │  · LO/MED: energise selector valve after a set delay
                │  · MAX:    spoilers + wheel speed >40 kt + nose gear compressed → immediately
                ▼
   ┌─ ACTIVE ──────────────────┐  FMA green AUTO BRK; holds a constant deceleration rate
   │  rate reaches 80% of programmed │  ──► DECEL light on (MAX: at 2.65 m/s² = 8.7 ft/s²)
   └────────────┬─────────────────┘
                │ disarm: press pushbutton again / lose an arming condition /
                │         enough pedal deflection / after take-off or touch-and-go
                ▼
        manual takeover (pedal past the threshold) or full release

2. Activation — it only brakes once the spoilers extend

Arm is not braking. Activation needs the BSCU to receive a ground-spoiler extend signal. Per FCOM DSC-32-30-10:

Automatic braking is activated: At the command for ground spoilers extension, for LO and MED modes, or At the command for ground spoilers extension, when the wheel speed is above 40 kt and when the nose landing gear is compressed, for MAX mode.

The AMM adds the "delay vs immediate" detail and the in-flight MAX-arm case. Per AMM 32-42-00:

When the BSCU gets not less than two ground spoiler extend signals (from the FCPCs) it energizes the brake selector valve: after a set delay if LO or MED braking is set / immediately if MAX braking is set.

And: If MAX AUTO/BRK is armed in flight it will not be active until: ...not less than 2 ground spoiler extend signals and the nose landing gear shock absorber is compressed / ...not less than 2 ground spoiler extend signals plus eight seconds. When active, the FMA AUTO BRK turns from blue to green.

3. The 72 kt RTO trap (must understand)

Per FCOM DSC-32-30-10:

Consequently in the event of an acceleration stop, if the deceleration is initiated with the wheel speed below 72 kt, the automatic braking will not activate because the ground spoilers will not be extended.

This is the key A330 RTO chain:

Ground spoilers auto-extend only above a speed threshold (72 kt) — below it, bringing the thrust back does not auto-extend them.
Autobrake (even armed MAX) is triggered by spoiler extension.
So in a low-speed reject (below 72 kt) autobrake will not act — you must brake manually.

This is why the procedure stresses confirming manual braking in a low-speed RTO rather than waiting for autobrake. (Note the MAX activation also requires "wheel speed above 40 kt" — that is the autobrake-side threshold; 72 kt is the spoiler-side threshold. Two thresholds in series, and the spoiler one is higher and trips first.)

4. The DECEL light and the ground reference speed

Per AMM 32-42-00:

When the aircraft deceleration rate is 80% of the programmed deceleration rate, the related DECEL light comes on. If MAX deceleration was set the related DECEL light comes on when the aircraft deceleration rate is 2.65 m/sec/sec (8.7 ft/sec/sec).

A lit DECEL light means autobrake is decelerating as set and has reached 80% (or the MAX absolute threshold) — the crew's confirmation that autobrake is working. DECEL on = autobrake is doing its job; if it does not come on, be alert (the set deceleration may not be reached, and manual intervention may be needed).

Autobrake needs to know "how fast to decelerate", which uses the reference speed, computed slightly differently from pure antiskid on the ground. Per AMM 32-42-00:

When the aircraft moves on the ground the BSCU calculates a reference speed... from: an average of the wheel speed signals... and data from the ADIRS (true ground speed). The BSCU makes a new reference speed 25 times each second. If the brakes are put on, the reference speed is changed only from the ADIRS.

The subtle last sentence: once the brakes are applied the wheel speeds are no longer trustworthy (the wheels are being braked and may skid), so the reference switches to the ADIRS (inertial ground speed) only — the same "inertia does not lie" idea as in Antiskid.

5. Disarming and the manual-takeover thresholds

Per FCOM DSC-32-30-10:

The system is disarmed by: Pressing the pushbutton of the corresponding armed autobrake mode / Losing one or more arming conditions / Applying sufficient deflection to at least one brake pedal when autobrake is active / After takeoff, or touch and go.

The "sufficient deflection" has precise thresholds — the critical point for taking over manually from autobrake. Per AMM 32-42-00:

Action	LO	MED	MAX
One pedal	50% travel (≈7.3°)	50% travel (≈7.3°)	80% travel (≈11.7°)
Both pedals	25% travel (≈3.7°)	25% travel (≈3.7°)	50% travel (≈7.3°)

Reading it: MAX is the "stickiest" — it takes 80% (one pedal) to take over, because MAX is for the RTO and the system does not want a light touch to release maximum braking. LO/MED are for landing and a light press (50%, one pedal) takes over. Both pedals together lower the threshold (two feet = clear manual intent). After disarming, the selector valve de-energises, and if manual braking is not continued the automatic selector valve shifts to blue (as in Normal Braking). The AMM also lists a disarm the FCOM does not stress: both MLG bogies in the fully-trailed position beyond a set time (an abnormal bogie tilt) — a ground-configuration protection.

[!warning]- Six misconceptions this article corrects (1) Pressing LO/MED/MAX does not start braking — it only arms; activation needs spoiler extension. (2) A low-speed reject does not get automatic braking — below 72 kt the spoilers do not extend, so autobrake does not activate and you must brake manually. (3) MAX activation is not the same as LO/MED — MAX also needs wheel speed above 40 kt and the nose gear compressed, and acts immediately (LO/MED after a delay). (4) Autobrake can be armed with the parking brake ON. (5) A light pedal touch does not always disarm — there are precise thresholds, and MAX takes 80% (one pedal) to take over. (6) The DECEL light is not a fault light — it confirms the deceleration target is being met.

Self-test

[!note]- Q1. After arming, what actually activates autobrake, and how do LO/MED and MAX activation differ?

Ground-spoiler extension. For LO/MED, the BSCU energises the selector valve after a set delay on receiving ≥2 spoiler-extend signals. For MAX, it also requires wheel speed above 40 kt and the nose gear compressed, and it acts immediately (no delay).

[!note]- Q2. Why does autobrake not brake in a low-speed RTO below 72 kt?

Ground spoilers auto-extend only above 72 kt, and autobrake is triggered by spoiler extension. So below 72 kt the spoilers do not extend, autobrake does not activate, and the pilot must brake manually. (The 40 kt MAX threshold is the autobrake side; 72 kt is the spoiler side, in series, and trips first.)

[!note]- Q3. List the five arming conditions, and can it be armed with the parking brake on?

Green pressure available, antiskid electrically powered, no brake-system failure, at least two PRIMs able to command spoiler extension, and at least one ADIRU available. Yes — it may be armed with the parking brake on.

[!note]- Q4. What lights the DECEL light?

The aircraft deceleration reaching 80% of the programmed rate; for MAX, the DECEL light comes on at a deceleration of 2.65 m/s² (8.7 ft/s²). A lit DECEL light confirms autobrake is meeting its deceleration target.

[!note]- Q5. What are the manual-takeover thresholds, and why is MAX the highest?

One pedal: LO/MED 50% travel, MAX 80%. Both pedals halve it (LO/MED 25%, MAX 50%). MAX is highest because it is for the RTO — the system resists releasing maximum braking on a light touch; both feet signal clear manual intent, so the threshold is lower.

Key takeaways

Theme	The one thing to remember
Arm ≠ active	Pressing LO/MED/MAX only arms; spoiler extension activates
72 kt RTO trap	Below 72 kt the spoilers do not extend → autobrake does not act → brake manually
MAX vs LO/MED	MAX also needs >40 kt + nose compressed, and acts immediately (LO/MED delayed)
FMA colour	Blue = armed, green = active
DECEL light	Confirms 80% of programmed (MAX: 2.65 m/s²) — not a fault light
Takeover	MAX is stickiest (80% one pedal); both pedals lower the threshold

References

A330 specifics per FCOM DSC-32-30-10 (Autobrake — function, arming conditions including arm-with-parking-brake, FMA blue/green, activation on spoiler extension with the LO/MED vs MAX 40 kt + nose-compressed distinction, the 72 kt RTO trap, the four disarm methods) and AMM 32-42-00 (Automatic Braking — ON light and WHEEL-page display, delay-vs-immediate, in-flight MAX-arm 8-second / nose-compressed cases, DECEL light 80% / 2.65 m/s², ground reference-speed algorithm 25 Hz / ADIRS-only after braking, pedal-deflection takeover thresholds, fully-trailed-bogie disarm). The state-machine diagram is an integrative synthesis of the FCOM and AMM text. The full ground-spoiler auto-extension logic (the spoiler side of the 72 kt threshold) belongs to ATA-27. Autobrake is inoperative under alternate braking — the fallback is in Alternate Braking.

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.