Someone please correct me if any of this data is wrong.
Both engine IDGs and the APU Gen can supply 120kva each. Any one of these three power sources alone, is capable of supplying all of the power needs of the entire aircraft. The RAT Gen can supply only 7.5kva and only supplies power to the standby systems.
I don't have very complete info on the backup Gens, and this to me seems an important area for this discussion thread. I don't know what their power output is, but they do feed DC power into 400Hz AC converters, so they can supply AC power. I just don't know HOW MUCH AC power they can provide. This to me seems an important question to answer, as to whether you would dispatch or not by considering whether to regard the back Gens as primary AC power sources (surely the manuals indicate this). My guess would be that the backup Gens can supply AC power with some load shedding, but may not have enough AC power available to supply the entire aircraft. If this were the case, you probably could not consider these as primary AC power sources in making the MEL and ETOPS dispatch decisions.
I do know that the backup Gens are considered the primary DC power sources for the flight controls, but I could do with more data on the backup Gens.
Assuming then that the Backup Gens are NOT considered primary AC power sources, and you dispatch (ETOPS=120), here are your scenarios.
Suppose you dispatch with the APU Gen INOP, and you lose one of the IDGs. By the manual your flight is over (in or out of the ETOPS segment), and you land at the nearest suitable airport.
Suppose you dispatch with one IDG INOP (with the APU shutdown in flight), and you lose the other IDG. You have more drama here as I assume the Backup Gens will supply some load shedded AC, while you get the APU fired up and its Gen online. Again your flight is over (in or out of the ETOPS segment), and you land at the nearest suitable airport.
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Safe flying to you...
[This message has been edited by Flight Safety (edited 20 March 2001).]