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How Auto Brakes Work On Jets

Flying Photog

What are auto brakes, and how do they work? We'll look at how the auto brakes work on an MD-80, but for the most part, the concept is the same for all airline jets.

Grant Wickes

Auto Brakes: Doing The Work So You Don't Have To

The Automatic Brake System (ABS), automatically applies brake pressure during landing. Basically, it does the braking work for you immediately after touchdown, so you don't have to press the brakes down yourself.

So why would you want the brakes to be automatic? Auto brakes help ensure that the brakes are engaged at the right time during landing: not too early, and not too late. That's especially important when you're landing on a wet or snowy runway.

When Auto Brakes Engage

Auto brakes engage after the spoilers deploy on landing (the spoilers automatically deploy when they sense the main gear spinning up during touchdown). But there is a delay, based on the auto brake setting that's used.

There are three auto brake settings that can be used for landing in the MD-80: MIN, MED, and MAX. When the MIN and MED settings are used, auto brakes engage approximately 3 seconds after the spoilers deploy, which helps you touch the nose wheel down normally. But when the brakes are set to MAX, they engage approximately 1 second after the spoilers deploy, ensuring that you have the most braking, for the longest period of time on the runway.

The different settings are used based on runway length and condition. But when the runway is contaminated with snow, like what happened on the Delta flight, the MAX setting is typically used.

Max Brakes and Anti-Skid

Max brakes (or any of the brake settings, for that matter) need anti-skid in order for them to work. That's because when MAX auto brakes engage, they apply the maximum braking effort they can.

That's where anti-skid comes into play. The anti-skid system releases pressure on the brakes when it senses the wheels are starting to skid. When they spin up again, it re-applies brake pressure. This release and re-application of brake pressure happens very quickly, much like what happens with your car's anti-lock brake system.

What all of this means is that combined, MAX auto brakes and anti-skid give your aircraft the ability to apply the most brake pressure possible, for the longest time on the runway, without allowing any of the wheels to lock up and skid.

When Auto Brakes Disarm

Obviously, in most cases, you don't want auto brakes to bring you to a complete stop on the runway. You want to keep rolling so you can taxi off and let the aircraft behind you land.

So how to you disarm auto brakes? There are a few different ways.

The first method is accomplished by pressing down on the brake pedals, which, like most aircraft, are on the top portion of the rudder pedals. When you press down to about 25% of the travel of the pedals, the auto brakes disengage. At that point, you are in full control of the brake pressure applied to the wheels. Remember, though, when this happens, the anti-skid system is still working, preventing you from skidding the tires.

The second method to disarm the auto brakes is by pushing the thrust levers forward. If you needed to go-around, you definitely wouldn't want your brakes at max, so when the thrust levers start to come forward to about 22 degrees, the auto brakes automatically disengage.

Fly For Fun

Third, if the flaps are retracted to less the 26 degrees and the jet's speed is above 70 knots, the auto brakes disarm. Again, this is useful in a go-around scenario. And finally, if the spoilers are stowed, the auto brake pressure is released.

What Happens If The Spoilers Don't Deploy On Landing?

Auto brakes engage after the spoilers deploy during a landing. But what happens when something goes wrong and the spoilers don't automatically deploy?

In the MD-80, you can manually deploy the spoilers by using the lever, if for some reason they don't pop up automatically when you touch down. And when they are manually deployed, the auto brakes are activated.

Putting It All Together

Auto brakes help you stop by automatically adding brake pressure after touchdown and spoiler deployment. And while you're in the most demanding phase of flight (landing), it's nice to know your jet can take care of something like that for you.


Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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