Not going against what rodders says about big jets. . .. .But you can also get self exciting alts which have an exciter on the shaft.. .This takes the form of a DC generator using perm magnets, so as long as the shaft is rotating there is a DC supply.. .. .But as rodders says its way to deep for PPL and you will get a 2" thick manual on this stuff if you do your ATPL exams. Both oxford and bristol ground school notes are pretty good on it.. .. .If you have a look at this web page. .. .<a href="http://www.perr.com/gen.html" target="_blank">http://www.perr.com/gen.html</a>. .. .under generator basics there is a good description how the thing gets its self going.