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Autobrakes are an automated wheel-based hydraulic brake system standard on most modern commercial aircraft, particularly larger jet planes. The purpose of an autobrake system is to automatically apply even, adequate braking force to the wheels of the aircraft at stages of the landing or takeoff where the pilot's workload is intense. This also applies the brakes more evenly and reacts to deceleration dynamics more accurately. Autobrake systems are typically manually engaged by the pilot as part of the before-takeoff and landing checks. These systems typically have several settings designed to suit most environmental conditions and aircraft configurations.
The pilot flying is under the most pressure during takeoff and landing. There are more than enough things to do and concentrate on even under ideal conditions. When the cross wind, low visibility, and wind shear "gremlins" are present, things can get very interesting indeed. Autobrake systems are one of the automated features present on modern airliners designed to lighten the pilot's workload and ensure that high stress situations can be easier to control successfully.
The autobrake system usually consists of a manual control on the flight deck which interfaces with the aircraft's systems to apply main gear braking automatically under set conditions. These controls are typically located on the central instrument panel in the vicinity of the landing gear lever and have several different settings. Although different aircraft manufacturers have alternative configurations, settings typically include an off, low, medium, high, and max setting in addition to a rejected take off (RTO) setting. The RTO setting engages by default prior to takeoff; the other settings are generally engaged at the pilot's discretion when preparing to land.
The RTO function of an autobrake system applies maximum main gear braking should the pilot decide to abort the takeoff on the runway. This is a typically dangerous and stressful procedure; the autobraking function takes a load off the mind of the pilot when this happens. This function is generally ground speed and throttle position activated; it automatically kicks in should the throttles be brought back to idle while the ground speed is above a certain point, generally 80 knots. While optional, the use of autobrakes during landing is generally employed, depending on a number of factors.
When preparing to land, the crew will assess a number of environmental and aircraft status factors when deciding whether to engage the autobrakes and which level of braking to use. Depending on these variables, the autobrakes will engage during the landing preparations at an appropriate setting. When the aircraft touches down, the autobrakes will automatically engage when the throttles switch to idle. Simultaneously the spoilers will automatically deploy and the pilot will engage the reverse thrusters; this combination of braking inputs can safely and smoothly decelerate the aircraft.