Originally Posted by
dubfan
.... is there not a case for aircraft brakes to be redesigned so that hydraulic pressure keeps the brakes in the off position so that in the event that total hydraulic pressure is lost, the brakes would automatically engage?.
What you have to look for now are the new failure cases that you have introduced.
You're replaced the case where total loos of hydraulics (at the brakes) means total loss of braking.
You've also replaced the case where loss of hydraulics in one system (at the brakes) means loss of the corresponding braking capability.
But you've now added the case that loss of hydraulic pressure (at the brakes) means inadvertent brake application - either on a system-by-system basis or as a total system.
Cases that would have to be addressed include hyd failure on TO causing full brake application of the corresponding brakes, with the performance impact (and associated hazards), the handling impact of possibly asymmetric brakes (depending on where the hyd failure occurs) and the risk to the brake of overheating and of tyre burst.
Since the existing cert rules more than adequately address the case of single hyd loss leading to associated braking loss, all that this proposal mitigates is the total braking failure case, which is a very improbable case (perhaps even "extremely improbable" in which case the certification is trivial). But now there's a whole bunch of single failures resulting in inadvertent brake application. Loss of one hyd system is typically a 10e-4 to 10e-5 per FH event, so that inadvertent braking is going to be pretty frequent - certainly I'd be concerned that the new risks are more dangerous than the case being mitigated.