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An autobrake is a type of automatic wheel-based hydraulic brake system for advanced airplanes. The autobrake is normally enabled during takeoff and landing procedures, when the aircraft's longitudinal deceleration system can be handled by the automated systems of the aircraft itself in order to keep the pilot free to perform other tasks - Wikipedia

  • How does the aircraft "know" when is time to activate the autobrake systems on a rejected takeoff and landing?

  • Does it apply full brake to all the aircraft's wheels?

  • Is it really used by commercial jets?

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Not all commercial jets have autobrakes, and the ones that do usually have a company policy dictating when it will be used.

The criteria for activating the brakes are different depending on the manufacturer and model of aircraft, but are typically some combination of:

  • System must be armed
  • A certain takeoff power setting must be set by the throttles
  • A certain airspeed must be obtained
  • Thrust levers must be pulled back to idle
  • Aircraft must still be on the ground

Usually for a rejected takeoff it does use max braking, however the autobrake usually has different settings for landing along the lines of "light/medium/max" braking. Each setting will use a different amount of runway for landing, much like lightly pressing on the brake pedal in your car versus jamming it to the floor.

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    $\begingroup$ Modern Boeings (I know 737, 747 and 777) have an "RTO" auto brake setting. E.g. for 777, with all of the criteria Lnafziger mentions, max braking is applied if the throttles are retarded above 85kts. In other words, standing on the pedals will not stop you more quickly. $\endgroup$
    – Simon
    Commented Mar 23, 2014 at 11:34

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