Remember the autobrakes are looking for a rate of deceleration. This deceleration is achieved by a combination of three different components. Drag due to flaps and spoilers, reverse thrust and wheelbrakes.
Here are two scenarios. Same aeroplane and conditions using two different techniques.
In the first case we set autobrakes one (low level of deceleration). On touchdown the spoilers deploy and by magic we go straight into a high level of reverse thrust. THE RATE OF DECELERATION MAY BE HIGH ENOUGH SO THAT THE WHEELBRAKES ARE NOT REQUIRED TO PROVIDE THE SELECTED DECELERATION. The brakes will hardly be applied. At about 80 knots you will come out of reverse, the aircraft will sense deceleration has stopped and now the brakes will automatically apply until the aircraft stops. The brakes will hardly be warm.
Scenario two. You select a high autobrake setting, 3+. On touchdown the spoilers deploy as before but now you are late selecting reverse or you use very little reverse. IN ORDER TO PROVIDE THE SELECTED RATE OF DECELERATION THE AUTOBRAKES WILL IMMEDIATELY APPLY TO A HIGH LEVEL. The brakes do most of the work and absorb most of the landing energy. They will be very warm.
The point is that autobrakes are only part of the stopping equation. Proper reverser use is equally important. I notice a strong tendency on the line for pilots to use too little reverse too late. This is argued to be better for the passengers as they are not frightened by the noise and forces. So the brakes get very hot.
With carbon brakes you do not want them to cycle on and off frequently during the landing run as this increases wear. With a low autobrake setting this may happen. Therefore, with carbon brakes you select a setting which puts the brakes on and tries to keep them on without cycling. This is why level two is recommended in some cases. THE REVERSERS ARE STILL THE KEY TO REDUCING BRAKE ENERGY ABSORBTION.
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