Niall
While PS1.3 is not TOTALLY accurate in every respect, it certainly is re. the APU. No in-flight start, but you can start on the ground and run for take-off and initial climb providing AIR ONLY, for one pack, up to 15,000'. Must be shut down by 20,000' Why???
It is able to provide air for one pack during take-off and initial climb to offload the engines for hot-and-high or max weight departures. On the R-R powered 44s this gave (from memory) about 18 degs C advantage on the EGTs.
Why no in-flight continuous running or inflight start? Simply not necessary. The (automatic) electrical redundancy available with four totally separate 'gennies', any one of which could take care of the normal, basic, load requirements makes a FIFTH source a statistical non-starter. That fifth source, were it to be available, could not be connected into the system because neither APU 'genny' can be parallelled with an IDG.
Note, by the way, the other B7 variants ALL suffer from their own problem - ONLY TWO ENGINES !!
They need all that extra stuff.
As to Boeing philosophy - (to paraphrase) "Mr Boeing, why do you always fly on four-engined airplanes?" "'Cos they ain't yet made a FIVE-engined airplane, son!"
Finally, as regards the four engine failure case, read Capt Eric Moody's account of how to deal with that (and more besides).
