It will try and maintain a fixed derate however if you are on a surface that is slippery it will be harder for it to do so since the wheels will lock up more easily and the anti skid will be working overtime. However I havnt come across anything so far that causes a problem. We are given braking coefficients if required which we can relate to good or medium or poor braking. We are not allowed to operate if the conditions arein the poor category.