If the aircraft is equipped with auto brakes, you can use them. It is up to the pilot to decide whether to use them or not.
Auto brakes function by applying a deceleration rate to the brakes. For example, in the A320, we have three levels of auto braking. LOW, MED (medium) and MAX (Maximum). LOW and MED braking can be used for landing and MAX is only reserved for takeoff which becomes active if the pilots elect to reject the takeoff for some reason.
If we decide to use auto brakes, we arm them during the preparation for descent and landing. LOW is used if the runway is long and dry. If the runway is wet, contaminated, and/or short we use MED braking. The biggest difference between LOW and MED is the deceleration rate. When LOW braking is active, the computers send a signal to brake so that the aircraft deceleration is about 1.7 m/s^2 and when MED is used the deceleration rate is increased to 3 m/s^2. The auto brakes will function until the aircraft comes to a stop. If required, it can be overridden by the pilots by simply braking manually with a simple tap the top of the rudder pedals. Normally, we take over the brakes once the aircraft is significantly slowed down (below 100 knots). However, this is not always the case.
The auto brake control panel of an A320. To arm them, you simply press the desired push button
When we land an aircraft, we decide which taxiway to get off the runway. Some really busy airports mentions in the charts where they expect you to vacate the runway and many times, the controllers will let you know where they want you to get off the runway. In the case, where we make a long landing, there is a chance that the deceleration rate of the auto brakes might not be enough to make the turn-off. In this case, we disengage them earlier and use manual braking to increase the brake pressure to vacate from the designated taxiway. In some situations, we may be asked to vacate from a taxiway a bit further away. This may not require the auto brakes as if we use them the aircraft may come to a stop earlier because the deceleration rate may be too much. So, in such situations, it is recommended that you do not use the auto brake function or disengage it as soon as the aircraft touches the runway and brake manually as required.
More modern aircraft, like the Airbus A380 and A350 have a much more sophisticated auto brake system. It is called Brake to Vacate (BTV). With BTV, the pilots can simply tell the aircraft the taxiway they want to vacate from. The BTV is a complex function of the basic auto brakes of the aircraft which can modulate brake pressure to make the pilot commanded taxiway to vacate.
Author – Anas Maas (Pilot)
