Brakes - ATPL 021 — Airframe, Systems and Powerplant

A landing aeroplane arrives with kinetic energy that would boil hundreds of kettles, and most of it ends up in a handful of modest disc stacks inside the main wheels: four on a narrow-body, a dozen on the largest types. This lesson is about those stacks: how a disc brake works and what it is made of, the energy accounting that limits it, the systems that apply, store and monitor its pressure, and the two computers, anti-skid and autobrake, that apply the brakes better than feet can.

The disc brake

A disc brake works by friction: stationary pads or discs are squeezed against discs rotating with the wheel, converting the aircraft's kinetic energy into heat. On a transport aeroplane the single disc of a car grows into a multi-disc stack filling the wheel: rotors keyed to rotate with the wheel, stators keyed to the axle between them, and a ring of hydraulic pistons at one end that clamps the whole sandwich when the pedal is pressed. More discs mean more friction surfaces and more mass to soak up heat, which is the entire game: a brake is an energy sink first and a friction machine second.

Two disc materials are used: steel and carbon, and the comparison is a standing exam item:

Weight — carbon is far lighter for the same energy capacity: on a large aeroplane the saving runs to hundreds of kilograms, which is why carbon owns the transport fleet.
Temperature limits — carbon works to much higher temperatures, and takes its enormous heat loads without warping.
Internal friction coefficient — carbon's friction holds steady as the stack heats up, where steel's falls away when very hot: hot carbon keeps braking, hot steel fades.
Wear — steel wears with the energy each stop puts through it; carbon wears mainly with the number of brake applications, almost regardless of how hard each one is. Hence the carbon-brake taxi technique: a few firm, deliberate applications wear the brakes less than continuous light riding, which for carbon is the most expensive way to brake.

The energy limit

A brake absorbs energy, and its certified limit is exactly that: a maximum brake energy it can absorb in one stop, set by the mass of the stack and the temperature it may reach. The sizing case is the rejected take-off at maximum mass from close to the decision speed V1 (the last speed at which a take-off can still be rejected): the heaviest aircraft at the highest speed, all of it into the brakes. The operational consequences follow: brake energy and cooling tables limit how soon the next departure may follow a landing or a rejected take-off, an overheated brake may fade just when it is needed, hot brakes can ignite leaked fluid or grease, and hot enough wheels will melt their thermal plugs, also called fuse plugs, and deflate the tyres deliberately (the next lesson's subject). The brake temperature indicator exists for exactly these decisions: it confirms the brakes are within limits before take-off (a rejected take-off must arrive on brakes that can absorb it), reveals a dragging brake by its odd-one-out reading, and tells the crew when cooling, by time or by fans, is actually complete.

Applying, storing and watching the pressure

Brakes are actuated hydraulically on most aeroplanes: the pedals command metering valves that send hydraulic pressure to the brake pistons in proportion to foot force, with the full system pressure of the hydraulics module available for a maximum-effort stop. Electrically actuated brakes replace the hydraulic pistons with electromechanical actuators pressing the same stack: no fluid to leak or catch fire at the hot wheel, finer computer control, and one less demand on the hydraulic systems; some of the newest types use them.

Brake accumulator — the hydraulic accumulator dedicated to the brakes: charged with system pressure and holding it behind check valves, it guarantees braking after the pumps have failed, for a limited number of applications. It is the reason a hydraulically dead aeroplane can still stop.
Parking brake — a valve arrangement that applies the brakes and traps the pressure on them, held by the accumulator for hours. It also, on most types, deactivates the anti-skid, which has no business releasing a parked aircraft's brakes.
In-flight wheel braking — a brief automatic brake application after lift-off, stopping the wheels before the gear enters the bay: a heavy wheel spinning at take-off speed is a gyroscope that resists the leg's folding motion, a source of vibration, and a sling for whatever water or slush the tyres picked up.
Brake wear indicators — a pin that protrudes from the brake housing and retreats as the stack wears: with the brakes applied, a pin still standing proud means life remaining, a pin flush means the brake is due. It is the first lesson's on-condition maintenance in its purest form: measured condition, read in seconds on the walkaround.

Anti-skid

The friction between tyre and runway has a maximum, and it is not at a locked wheel. Anti-skid works by releasing brake pressure automatically whenever the applied pressure exceeds what the tyre-runway friction can use, then reapplying it: braking rides just below the skid, at the friction peak. The intelligence is a comparison: the anti-skid computer compares each wheel's speed with the aircraft's reference speed, and a wheel running notably slower than the aircraft is a wheel starting to skid. From that one comparison the computer provides four protections:

Slip-ratio control — holding each wheel near the small amount of slip where friction peaks, for maximum braking performance;
Locked-wheel protection — releasing a wheel that has stopped rotating entirely while the aircraft still moves: a deep skid on one wheel means a burst tyre seconds later;
Touchdown protection — refusing brake pressure until the wheels have spun up after touchdown, so a landing can never be made onto already-applied brakes;
Hydroplane (aquaplaning) protection — on a flooded runway a hydroplaning wheel (aquaplaning is the same phenomenon by its other name) slows or stops without any brake being applied; the computer reads it like a skid and keeps pressure off rather than locking the wheel completely.

The performance impact is why anti-skid is more than a convenience: certified landing and rejected-take-off distances assume it working, because it brakes each wheel at the friction peak continuously, which no pilot modulating one pedal pair can do. With anti-skid failed, the crew brakes manually and carefully (firm, progressive, released at the first hint of a skid), accepts substantially longer distances, and accepts the real risk that a locked wheel bursts its tyre; the performance tables and minimum equipment provisions carry the penalties.

Autobrake

An autobrake system applies the brakes automatically after touchdown or a rejected take-off, at a setting armed in advance, so braking starts symmetrically and immediately while the pilot flies the touchdown and the reversers. It requires the anti-skid to be operative: autobrake commands braking blind, with no idea what the runway friction offers (whether it is chasing maximum effort or a deceleration target), and the anti-skid beneath it is what turns that blind command into peak braking rather than locked wheels. No anti-skid, no autobrake.

Its modes of operation are a three-state cycle:

OFF — the system is off, or has been reset after use;
Armed — a setting is selected and the system waits for its conditions (ground sensed, wheels spun up, thrust at idle or reject);
Activated / Deactivated — the system is applying brake pressure, and it deactivates when the pilot takes over on the pedals, disarms it, or advances the thrust levers.

The settings mean two different things:

RTO (or MAX) — commands maximum braking: every rejected take-off gets everything the brakes and the friction can give;
The numbered landing settings — command a rate of deceleration, not a brake pressure, and that changes how the brakes behave: with reverse thrust working, the reversers supply part of the target deceleration, so the autobrake applies less brake (cooler brakes from the same setting); on a slippery runway the friction may not support the selected rate at all, so the anti-skid caps the braking at what the surface offers, and the aircraft decelerates below the target however the autobrake asks.

The setting is a request; the runway has the veto.

What the exam asks

The steel-carbon comparison, the four anti-skid protections and the autobrake settings logic are the reliable marks here. The overview, following the learning objectives:

Describe the disc-brake principle, the two disc materials, and the steel-carbon comparison for weight, temperature, friction and wear

Explain the brake energy limitation and its operational consequences, and the reason for the brake temperature indicator

Explain hydraulic and electric actuation, the brake accumulator, the parking brake, in-flight wheel braking and brake wear indicators

Describe the anti-skid principle and the computer's four protections; give the performance impact and the implications of anti-skid failure

Describe the autobrake principle, why anti-skid must be available, the OFF/Armed/Activated modes, and how RTO/MAX and rate settings respond to reverse thrust and slippery runways