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Why do aircraft have two brake pedals instead of one? If each pedal controls different sets of wheels e.g. the left wheels and the right wheels of the main gear, is that not a safety flaw given that at any point in time, it is impossible for the pilot to apply an equal amount of pressure to the brakes?

Isn't ABS enough to the point that only one pedal is enough?

Finally do both captain and pilot step on the brakes at landing? If not then why have two pedals?

Simon
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securitydude5
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    Related: What is differential braking? – Ron Beyer Jun 22 '17 at 15:00
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    The control yoke in front of the pilot is not a steering wheel, many aircraft the only way to steer it is to step on the right or left brake to make it turn... – Ron Beyer Jun 22 '17 at 15:01
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    There is indeed a learning "curve"! – Chris Stratton Jun 22 '17 at 16:35
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    Very few aircraft have ABS, most are flown by a single pilot. You also don't usually want to apply equal force to both sides: you apply whatever force is needed in order to get the plane to go where you want it to go. – jamesqf Jun 22 '17 at 17:53
  • Some airplanes don't have any brake pedals. A Yak52, for example, has a hand operated air brake with differential braking selected by the rudder pedals. – Riccati Jun 22 '17 at 23:39
  • tractors also have dual brakes, and for the exact same reason, tighter turns. – hildred Jun 23 '17 at 01:00
  • @hildred: Likewise tracked & skid-steer vehicles. That's the only way they can turn, either by differential braking or application of power. – jamesqf Jun 23 '17 at 06:01
  • There is a feedback loop, when pressing the pedals you observe what happens and modulate the pressure to each pedal accordingly, the same as braking in a car, just with two pedals instead of one. – Colin Jun 23 '17 at 06:36
  • I'd just like to comment on it is impossible for the pilot to apply an equal amount of pressure. Surely this mechanism has been in use since before many of us are born so it must work. A lot of tanks also work this way: two throttle/clutch levers instead of a steering wheel one for each track. And tanks can still travel in a straight line even though it's impossible to apply same amount of force to both levers. How does this work? The pilot/driver constantly make small unnoticeable corrections (same as when you drive a car or ride a bike). It turns out humans are excellent control computers – slebetman Jun 23 '17 at 11:53

4 Answers4

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In an aircraft the brake pedals control the respective side brakes. This allows for the pilot to turn the aircraft not only with the pivoting nose wheel (if it has one) but also with the brakes. This allows for a very tight turning radius.

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If each pedals controls different sets of wheels eg the left wheels and the right wheels of the main gear, is that not a safety flaw rather than measure given that at any point in time,

It is by design to allow for the tight turning. Some smaller aircraft also do not have a steerable nose wheel so differential braking is the only option.

it is impossible for the pilot to apply an equal amount of pressure to the brakes?

Pushing equally on each toe pedal will result in equal brake operation.

Isn't ABS enough to the point that only one pedal is enough.

The pedals allow for brake steering which a single pedal would prevent. Airplanes also do have ABS but that simply prevents the tires from skidding.

Finally do both captain and pilot step on the brakes at landing. IF not then why have two pedals?

Im not sure, when I fly with an instructor or co-pilot in smaller planes only one person operates the brakes.

Jasmine
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Dave
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    I find your spellings of "break" and "peddle" very funny (sorry couldn't resist). – kevin Jun 22 '17 at 16:39
  • Can't differential braking overload the nose wheel if the wheel is not steerable? – Ruslan Jun 22 '17 at 17:00
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    @Ruslan, when the nose wheel (or tail wheel) is not steerable, it is free castering (similar to wheels on an office chair or shopping cart). – Jan Hudec Jun 22 '17 at 17:06
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    @kevin - I make that mistake all the time, I'm glad someone else sees the humor in it. – Dave Jun 22 '17 at 17:07
  • @Ruslan generally no, the nose wheel is often castering as Jan Hudec mentions. Im not sure there are any big planes that have them but lots of small GA planes do – Dave Jun 22 '17 at 17:09
  • Also in many (maybe most) light planes that have a steerable nose or tail wheel, the steering is linked to the rudder/brake pedals. Also note that those pedal assemblies control the rudder as well. – jamesqf Jun 22 '17 at 17:50
  • @Dave The B-29 had a castering nosewheel. – Matthew Gauthier Jun 22 '17 at 18:04
  • @MatthewGauthier, I know some big military planes did I belive some even had castering mains to allow for cross wind landings without the longitudinal axis of the aircraft in line with the runway. – Dave Jun 22 '17 at 19:15
  • Tailwheel aircraft are pretty much impossible to steer on the ground without the ability to brake on one side and not the other. At speed on the ground the steerable tailwheel and propwash over the rudder allow small changes of direction, but to make sharp turns when taxi-ing, you need to be able to press one brake and not the other. – Flynn Jun 23 '17 at 18:09
  • @Flynn: Why is a steerable tailwheel on taildraggers so much less effective than a steerable nosewheel on tricycle-geared aircraft? – Vikki Feb 03 '19 at 04:45
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The left pedal is for brake(s) on the left side of the aircraft only, and likewise the right pedal for the right side.

We use two pedals because it is common where differential braking are applied, in other words different amount of braking on the two sides of the aircraft.

This is handy for maneuvering the aircraft around corners. For example if the plane needs to be turned to the left 90 degrees to enter a taxiway, and the length of the fuselage is long, applying left brake during the turn can reduce the turn radius, making it possible to turn without the outer set of wheels running onto the grass.

During a normal landing rollout, equal amount of pressure is applied to both pedals. The difference between both pedals caused by human application is minimal and does not have an observable effect.

Usually, only the pilot flying would be operating the controls, therefore only the pilot flying would apply the brake pedals. Except in an emergency where stopping distance is critical, the pilot monitoring may apply pressure to his own pedals to ensure maximum braking is achieved.

kevin
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7

Note also, the pedals are used to swing the rudder to the right and left, to yaw the aircraft, so two pedals are already there. It then makes perfect sense to split the brakes for all the above reasons.

JMR
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To add to the correct answer: two brake pedals allow for differential braking to steer the aircraft when it is on the ground

In light planes, differential braking eliminates the need for a steerable nose wheel. Instead, the nose gear simply casters, rotates as differential braking alters the path of the aircraft while in taxi. This makes for a much simpler and lighter nose wheel, plus no linkage to a wheel in the cockpit. With aircraft, simple is good. Simple doesn't go wrong as often as complex.

In heavy aircraft that usually have a steerable nose wheel, differential braking takes a lot of stress off of the nose wheel. Steering a heavy aircraft with the nose wheel alone puts a lot of angular stress on it. Heavy aircraft are, well, heavy. If all steering was done with the nose wheel, it would need to be beefed up to handle that, on top of being retractable. Differential braking on a heavy aircraft makes for a lighter nose gear arrangement.

tj1000
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