autobrake onset
Having done some work with the braking system on the Airbus I can confirm that the A340-600 will have 5 levels of autobrake with rotary switch, exactly for the reasons given by javelin and Leslie.
On the ‘bus each autobrake setting corresponds to a deceleration rate using feedback from the ADIRUs. I think that the system that controls brake pressure described by devils advocate is found on the Boeing classics.
During flight testing of the A340 a lot of effort was put into trying to obtain smooth onset characteristics for autobrake. When this was achieved and the aircraft was subsequently used by a crew who had not been involved in the autobrake testing it was thought that this would be a good test of the improvements. On landing they used LO autobrake and found the onset so smooth that they thought the system had failed and snagged it in the logbook. I think that Airbus really got the message that LO was TOO LO. I’m not so sure what it is like now.
As for brake wear there are so many factors as has been pointed out that it is difficult to draw more than general conclusions. Basically carbon brakes do like being hot and don’t like repeated applications. One brake application down to taxi speed would appear to offer the best brake wear as indicated by mutt and as far as the carbon is concerned it does not matter if it is controlled by the pilot or autobrake.
I hope that there are some brake system designers looking at this thread since they can learn a lot about what you get up to with the system when it gets into the real world.
Keep on stopping !