Someone Somewhere

In normal flight, the A350 hydraulics are powered by engine-driven pumps, not electric pumps. Starting the APU does not restore these. In-flight with APU/RAT only, the APU will only power the side 1 buses (1A and 1B). 2A/2B are unpowered and EMER 1 / EMER 2 are powered by the RAT. The electric pumps are 400Hz only and are inhibited if any engine is running. It's not clear from the FCOM what engine signal that is - generator online, above idle, switch in on position? The Green electric pump is on 1B so could be powered by the APU - you need to start it manually with the maintenance switch, or press ACCU REINFLATE. The Yellow electric pump is on 2A so cannot be run in APU/RAT-only flight. What should happen is the braking accumulators (on both yellow and green systems) would give you enough fluid/pressure for about 6 full brake applications, enough to stop the aircraft and apply the parking brake but not taxi to the gate. (you would also likely get some degree of residual hydraulic pressure from windmilling engines, as long as no high-flow items like gear or flaps were selected)