APU management and ETOPS
Some time ago I argued that, if special restrictions were put on the discharge and charge of the APU battery in advance of a complete fix, ETOPS flights requiring the APU would still be possible if the APU was started - perhaps in flight - prior to the beginning of the ETOPS leg, using the main electrical system, as a pre-emptive measure. I pointed out, however, that starting it in flight would be a hostage to fortune, in that cold-soak MIGHT (though should not) inhibit APU start. A more reliable tactic would be to start it before departure, as an idling APU uses little fuel, particularly at cruise altitude.
Quote from cockney steve:
"WRT charge/discharge.....APU should be started off bus fed by 4 main gen's before shut-down....APU feeds all demands in conjunction with main Gen's until engines are shut down.
IF Apu is shut down for ANY reason, the aircraft is then dependent on an outside GPU for the next half-hour ,before the APU can be started again. As soon as the main engines are restarted (GPU?) the power is restored, but temperature/time limitation means that the APU will be inop, under these conditions, for takeoff."
What are these temperature/time limitations? Not something I've ever found a problem on other types, and a typical long-haul turnround time is about 90 minutes or more.