Will touchdown protection function if the parking brake is set ?
Interesting question! Touchdown is a function ANTISKID. IAW AMM 32-42-00
"The normal antiskid valves release unwanted brake pressure through the parking brake valve. Therefore, the antiskid system monitors the parking brake system for a fault. When the parking brake valve is not in its commanded position, the parking brake relay does not send a ground to the AACU. This causes a fault in the outboard and inboard antiskid, and the amber ANTISKID INOP light comes on" (AACU: Antiskid/Autobrake Control Unit). Interestingly enough,
AACU monitors not parking brake position but parking brake valve disagreement. If the positions of the parking brake lever and the parking brake valve disagree ANTISKID INOP light will go on. In other words I cannot see a reason for ANTISKID to send an ANTISKID INOP signal with the parking brake set in flight. The subsequent question is "ok, ANTISKID can arm but can it work?". I think it can't because the released pressure from
any kind of ANTISKID protective functions (touchdown protection included) is
routed through the parking brake valve so even if the system passes BITE with parking brake on it will never work and probably it will fail not in it's arming logic but in it's application logic internal test as it will not be able to release the pressure. The result will be blown up tires and ANTISKID INOP light on. Contrary to ANTISKID, AUTOBRAKE will not arm at all because it will sense the pressure sent to the brakes (>750 psi which is the AUTOBRAKE disarm pressure) as brake application through the
NORMAL BRAKE METERING VALVES. AUTOBRAKE doesn't know if it's the crew or the parking brake that has applied the brakes because the brake pedals are
mechanically held in a BRAKES ON position. That's what I can make out of the AMM. Real life examples are welcome.