Bleed air for leading edge heating is indeed blown into a cavity behind the leading edge devices.
The leading edge is forward of the leading edge devices (at least when the leading edge flaps are stowed). The A/I air is only circulated in the very forward part/tip of the wing leading edge... an area only a few inches deep. There is no heating effect on the LE devices. The air is then expelled overboard through slots as previously said. Optionally, the anti-ice system is inhibited when the leading edges are deployed.
Then in the next statement it comes to a bit contradictious statement with previous one (each engine has air driven pump) as manual says that engine 1 and 4 have bleed air driven AUX hydraulic pump and engine 2 and 3 have AC electric driven AUX hydraulic pump.
This statement from the PMDG manual has been mentioned previously on the PMDG forum. I'm surprised you didn't ask there
A minor point to be sure, but I believe you can isolate the DC busses on the overhead panel: by pressing the Bus Tie Breaker switch, you also open the respective DC Isolation Relay...
Correct. Also, DC isolation also occurs during a normal Autoland. There are Circuit Breakers to isolate the DC systems, but none in the cockpit.
You can start a 747 on batteries only. I did it once in a 400, but you have to watch the charge rates carefully afterwards. the APU battery takes a massive hit and you only really have one start attempt. All three batteries are the same and interchangebale. But you have to lug batteries the whole lenght of the aircraft to do that.
3 batteries? An option?
I'm not sure what you mean by starting a 747-400 on batteries. Bleed air is required, so you need more than batteries. Also, batteries will not open the AC-powered Wing Bleed Isolation Valves if they have been previously closed, so you cannot get External Air into the wing bleed ducts for engine start. Just curious, but why would there be a large drain on the batteries? Other than the ignitors and EEC power(for less than a minute) for a single engine and the usual Standby Power stuff, I don't see any additional requirements. A single 747-400 battery can pump 50 amps for an hour on the test bench.
[quote]The APU gens can't be used in flight or paralleled to the IDGs on the ground.[quote]
Later/modified Elec Bus Control Units allow APU power if all engines generators are out (however, the APU cannot be started inflight ... at least not without a lot of CB pulling).
Rgds.
NSEU