Eli5: How do the brakes on commercial aircraft work during landing
In: 14
Basically just likea car’s brakes. Just upscaled quite a lot. If you don’t know how those brakes work, you have a disc that spinns with the wheel and when you want to stop, a pair of non rotating discs/plates press against the spinning disc.
There is also usually some kind of airbrake, which is simply a large flat surface that you hold out in the wind.
They are actually not that different from the brakes on cars. They are composed of hydraulic cylinders mounted in a caliper which presses some brake pads onto the brake disk when pressure is applied. They even have a regular anti-lock brake system. However the pads and disks are quite massive in order to handle all the heat. They also usually have water cooled brakes, basically water spray onto the brake disks to cool them down. On the other hand an airplane does not use its brakes as often as a car does and the brakes do get a lot of time to cool themselves down each time they are used. For normal landings the brakes are not used that much, most of the braking action is done with the air resistance of the airplane in full flaps and spoilers as well as the engine thrust reversers. However during emergencies the wheel brakes can get red hot and it might damage them a bit, which is better then having to scrap the airplane.
There’s a brake for each wheel, except nose ones.
The brake is generally a multi disk carbon, with a fix plate on the wheel side, and pistons on the strut side, all aroud the axle. Each disc has a stator disc on each side, which is basically a massive circular brake pad the same size of the disc.
The whole thing left to right (and mirrored on the adjacent wheel) is fix plate-stator-disc-stator-disc-stator-pistons. When you brake, the pistons push onto the pack of stators and discs. Compared to a car system, that’s 10-30 times more surface area for a given wheel size.
Multiple hydraulic systems provide pressurized oil to the brake valves. Each brake is fed with a dedicated valve, and each valve is fed by at least 2 hydraulic systems, for redundancy.
The brake is commanded by pilot and copilot pedals.
The system is generally fitted with an anti skid computer. The computer prevents the lock of each wheel independently, by commanding the brake oil valves.
The controls are:
1 Pedals to give brake input. Left pedal activates left gears brakes, right pedal activates right gears brakes. Differential braking may be used by the pilot to control the plane on the runway or to help performing a turn, or as a substitute to the aircraft steering wheel (topic for another day) if it becomes unresponsive.
2 A knob to select the sensitivity of the autobrake.
Usual settings are something like 1-2-3-4, where 1 is a strong braking, and 4 is really strong brake.
Then there’s a RejectTakeOff setting which means “smashing your face in the seat in front of you”. It’s usually used during takeoff. You try take off, if something goes wrong, you hit the brakes and they will stop you with the maximum possible force.
3 a parking brake lever that locks all brakes to max force. That’s exactly the same purpose of a car’s handbrake.
Additional systems: a brake temperature indication system, a cooling fan for each brake.
How do they work during landing: you land and press the brake pedals. Ideally, you let the plane slow down (using airbrakes and thrust reversers) and then press the pedals, because you have only that much of a brake and if you brake full force at max landing speed, you will overheat the brakes, destroy your tires, and eventually set fire to the brake. But, in an emergency you can do that. Just call the tower in advance so firefighters are dispatched to deal with that. It’s something accouted for in the design, but it’s an undesired bonfire and it costs money.
Note, at landing you have four ways to brake: the brakes as described above, the air brakes on top of the wings (generally 6 sets, 3 on each wing, with 3 different hydraulic sources, again, redundancy), a thrust reverser for each engine (you can deploy each one independently, that’s for redundancy, if one fails the others may still work), and last, the airport grass. Cause if everything goes bad, that grass will stop you if you sink your gears into it.
And that’s why I’m boring at parties.
Here’s an article you can read. Pictures are pretty nice.
https://insights.globalspec.com/article/12903/how-do-aircraft-brakes-work
The brakes are basically brake pads, similar to the one in your cars, but with a key difference.
Most car brakes are disc brakes, where the wheel is also spinning a disc, which gets clamped on during brake application by the brake caliper.
Aircraft brakes are similar to drum brakes, but much much more powerful. Here, the inside of the wheel is the surface on which the brake pads, arranged in a 360 degree inside the wheel, presses against the wheel insides when applied.
Aircrafts also use spoilers and air brakes, in addition to reverse thrust.
Generally the big birds have 3 systems to stop.
1- friction brakes just like your car.
2. Reverse thrust- basically engine thrust if blasted forward to push backwards, and slow the plane.
3. Drag- Spoilers, airbrakes, flaps (flight surfaces on the wings)and such are extended to create as much drag as possible.
Most commercial planes will use up to three different types of brakes to stop them on landing.
The first are aerodynamic brakes – on landing flaps will open up on the wings to create aerodynamic drag to slow down the plane. Think of a slightly neater version of a drag car deploying a parachute behind it.
Second there are also engine/thrust brakes – by reversing the engines or using movable panels to redirect the thrust you can basically make the engine work backwards to slow the plane down.
Lastly they have friction brakes that work like the ones you have on a car or bicycle – there is a braking surface attached to the wheel, and when you want to brake you force a pad against it. The friction between the pad and disc causing the vehicle to slow.
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