kijangnim, I agree with the principle of using brakes, but the logic in detecting brake faults might be flawed.
One of the weaknesses of auto-brake is that the lack of “feet-to-seat” (brake application to deceleration) feedback; this degrades the ability to assess a system’s effectiveness in comparison to manual brake application.
With auto-brake, judgement of satisfactory performance is made on deceleration alone, but in many circumstances the brake system is not providing the majority, or any deceleration at all. This is particularly important on slippery runways where for other reasons the brakes may not very effective; this has been discussed in the thread
http://www.pprune.org/forums/rumours...-honduras.html.
During the initial landing roll at high speed with low auto-brake setting and reverse thrust, and providing runway conditions permit, the auto-brake desired deceleration can be fulfilled by reverse and aerodynamic drag alone, i.e. no brakes applied. As speed decreases so do the effects of reverse and aerodynamic braking, then the auto-brake applies brakes to maintain the desired level of deceleration. This is illustrated in
‘Braking Devices’ - Fig 3. The brakes are only applied as the reverse decreases below 80 kts, which might be too late to take remedial action.
I don’t believe that this is a big issue for brake faults, particularly as brake system’s reliability (and back up) are far superior when compared to the problems of human performance in detecting less than adequate deceleration (from any means) and the judgement of runway conditions leading to the choice of the auto-brake setting or manual brakes in adverse conditions prior to the landing.
The above suggests that if there are doubts about the level deceleration which can be achieved on the runway – possible low friction but you decide to land, then manual braking (auto-brake off) might be a better option. Good judgement might choose to use ‘half power’ manual brakes, assess the situation and then apply maximum brakes as required. However, if you have already suffered poor judgement in runway assessment, then all this might do is minimise the speed during the overrun.