The wheel speed signal is the PRIMARY signal and the IRS signal is the secondary. For hydroplane and autobraking protection there is, to some degree, cross referencing of speed signals from both the wheel transducers and the IRS by the BSCU.
This is actually where I was heading. Will hydroplane/touchdown protection be lost as a result of no IRU groundspeed? If so, will the antiskid deem itself unserviceable, and by doing so, inop the Autobrakes (which is dependent on the Antiskid system being serviceable).
My books say
Hydroplane / Touchdown Protection:
Hydroplane / Touchdown protection is provided directly to each of
the aft wheels by comparing wheel speed to airplane ground speed
from the Intertial Reference System (IRS).
• Touchdown protection ensures the rear brakes do not have any
hydraulic pressure applied upon touchdown.
• Hydroplane protection occurs during ground roll to ensure
hydraulic pressure is released at the rear brakes.Protection of the
forward wheels is thus by the locked wheel function.
You can't compare IRU groundspeed with wheelspeed if you don't have groundspeed. There is no mention of primary/secondary references (in relation to this). If there is no touchdown protection, all the tyres may blow on landing (after that, antiskid and autobrake is of not much use). It may be similar to setting the parking brake in flight and then landing (messy).
It may be completely unlike body gear steering system where IRU groundspeed is factored into the logic (yet I know that it will work with with IRUs switched off).
Any comments from PinHead? I'm not even sure why he asked the question
