Slick,
To the best of my knowledge Boeing test useing half worn cold brakes
Totally depends on when the aircraft was certified, the present FAR's (25.109) states A flight test demonstration of the maximum brake kinetic energyaccelerate-stop distance must be conducted with not more than 10 percent of the allowable brake wear range remaining on each of the airplane wheel
brakes. However, that only applies to aircraft certified post 1998.
Mutt.