The Classic and the 400s are slightly different.
Hyd.
The extra pumps on the Classic were termed ADPs and were air driven. They cut in automatically when the system pressure dropped below 2400psi (I think), or could be manually selected. There was also the DC Aux powered pump for pushback swithched on the FE's panel
The 400 extra pumps are known as Aux pumps. 1 and 4 are air driven and run when a servcie such as gear or flaps are selected. 2 and 3 are AC powered and are only really there to give pressure to the flying controls in emergency situations when main system pressure drops. They won't run otherwise.
No 4 pump in all 400s also had an Aux position which operates a separate DC pump to power the brakes. It's an option to fit it to the number 1 system as well. These tend to be found on GE powered aircraft where you can start two engines on one side at once on pushback. But if you do, you don't want to start the outside of the turn engines, otherwise the tug loses the battle!
The ADPs/Demand pumps are there to increase flow or back the the EDP in the event of it's failure. In the event of an engine failure and no ADP/Demand pump you lose the system as well as there is no way of cross feeding the systems. But if the HP core is still windmilling, the EPP will still produce workable pressure down to about 120 knots I gather.
Electrics:
The batteries on the Classic were on the flight deck (bloody silly place to put them - if one has a thermal runaway it fills the flight deck with noxious fumes. It happened on one of my company's aircaft and the guys had to land on Oxygen. How that got certifiied I don't know!). On the 400, the main batteries were in the radio electrics bay. They are normally charged by the TRUs from the IDGs, but they could be charged from a GPU or the APU on the gound. But if they were very flat they had to be removed as charging very flat NiCads with big currents can make them go bang.
Flight on batteries only is measured in minutes. Not a likely scenario or one you would want to be faced with. The batteries only power the hot batt busses, the batt busses and the AC standby busses.
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.
The SSB joined sync bus 1 and 2 to sync bus 3 and 4. The only time you would do this is if you had lost either sync bus to a double gen failure as you wouldn't connect one bad sync bus to another in case you lost the lot. But 90KVA is a massive supply, so one gen per sync bus was more than adequate.
The APU gens are the same as the ones on the IDGs and can be intercahnged. But as the APU gen frequency is controlled by the APU speed and there is no way of independantly adjusting them, they cannot be parralleled. The Classic ahd no safety interlinks and closing the SSB with both APU gens on line was not a good idea. The APU gens can't be used in flight or parrallled to the IDGs on the ground.
The elec system is very good on the 747 - it was almost a direct copy of the VC10's. The VC10 designers all got jobs at Seattle when Vickers made them redundant.
Pneaumatics.
The de-ice blows air around the inside leading edges and is a total loss system. It doesn't work itf the LE devices are deployed. It's very hot and an anti-ice overheat is a biggy as far as emergencies go as it can start melting metal!
The cat is an option on 400s only (I think). It's between the packs and the plenum chamber. You have no control over it from the flight deck.
All of the info on this thread is general as Boeing build aircraft the way the customer wants. so there are a lot of user specific options. there are also a lot of differences between aircraft in the same fleets in some airlines.
And I haven't flown one for over 4 years, so my memory is getting dim!
Last edited by Dan Winterland; 16th Nov 2009 at 01:51.