Eli5: How do the brakes on commercial aircraft work during landing
In: 14
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
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