Quote from bsieker:
“...both main engines and the APU each have two starter/generators. The APU's SGs are designated "left" and "right".
A single dedicated starter would probably suffice, but since they are starter/generators, and two generators are installed for redundancy, one ends up with two starters as well.”
Thanks for the clarification, Bernd, and for reminding me that they are starter/generators. I’m disappointed with Boeing’s left/right terminology in this instance.
The twin starter/generator configuration is shown in the only document that I have seen, entitled “B787 Systems and Performance”, which dates from 2005. In addition to being at least six years ahead of actual entry into service, it has the look more of a publicity document than a technical overview, so I’ve been taking its contents with a large pinch of salt. The fact that it schedules certification and delivery in 2008 does little to inspire confidence in the currency of its information.
The document also shows both APU gennies, together with the 4 ED gennies, supplying a common, nebulous “230Vac Distribution”. I presume you must have a more detailed, authoritative source?
Somewhere in the multitude of PPRuNe threads (is it 5?) started since the APU battery failure in Boston earlier this month, I’ve picked up the idea that the APU battery can only be charged by energy from its dedicated APU charge circuit; not by the main electrical system. So, bearing in mind what you have said, I tentatively conclude that the R/H starter/generator feeds the APU battery charge circuit, while the L/H starter/generator feeds the main 230Vac system. Is that correct?
The APU battery and the Main battery seem to be identical in themselves, each rated at only 65Ah. Unlike the B777, it seems there is only one Main battery unit. There has been speculation on whether the separate APU and Main battery failures were similar or not, and whether they were under charge or under load. It seems unlikely that the ANA aircraft’s Main battery would have been subject to a high load, although the same cannot be said for the APU battery in the JAL case. If the failures were from a common cause, it would probably be the chargingsystem?
Chris