With reference to previous posts:
My understanding is that the yellow pump is not designed for continuous operation, (unlike the blue pump), and it will overheat after a few minutes. It's purpose is to assist retraction of the flaps in the event of low yellow hydraulic pressure. It's other uses are for on ground operations: to operate the cargo doors, recharge the parking brake accumulator (on some variants), and pressurise the hydraulics for engineering tests. But it is not designed as a full-time system.
The emergency electrical generator powered via the RAT and Blue hydraulics is of limited size, and therefore does not have enough spare capacity to keep the batteries charged as well.
Why are both these things limited? To save weight and cost. And also because a risk analysis has been applied.
rudderrat: You need hydraulics
and an ELAC to control either aileron, (320/321). The right aileron is normally operated by ELAC1 controlling green pressure. If green hydraulics or ELAC1 fail, ELAC2 takes over, controlling Blue pressure.
Chris - your APU start explanation is spot on. A normal APU start is battery
assisted, as you imply.
