Hi everybody, I was wondering if anybody could explain to me what the bleeding into the duct means. I set the switches during the starting of the apu and engines, but I realise that i don't know the principles behind these actions. What is this bleed, and how do the airconditioning packs function?Greetings,Arjen Nederstigt

A very short and simplified (since I don

Hi Arjen,I will try to explain this as accurate as possible.First of all you have 3 bleed switches Bleed1, bleed2 and APU bleed.These are your 3 sources of pressurised air (you have the possibility of ground connection as well, but is only used if the APU is malfunctioning).The APU is started with a generator/starter (electricity), but the engines bacause of the size an other factors would be hard to start this way, hence we are using pressuriezed air supplied by the APU when the engines are starte, this is to crank the engines to a point where it will be able to spinn itself up (N2 around 25-30%) when fuel is supplied.The other use for the pressurized air is for pressurisation of the cabin and air conditioning. At this point the engines are usually the only sources.Pressurisation is accomplished by supplying air and by using controllable valves locks the air in the cabin, hence pressure build-up. Opening and closing of the valve regulates the cabin pressure.The air is supplied throigh the air conditioning system which consists of the packs.And now to your packs. In the cocpit you have 2 switches Pack1 and Pack2. By switching on the packs you start the packs (its just a word for the arrangement of components that make the air conditioning system). You open for valves which supplies hot pressure regulated air from the engines to two possible routes.1. Through one heat echanger > compressor > second heat exchanger > and a turbine. The result of this arrangement is very cold air, close to freezing point.2. The other way for the air is around this arrangement.And the temp. of the air is regulated by a mix valve which lets either the varm air or the cold air come trough and of cource a mix of these two. This air is mixed in a mixing chamber and is directed into the cocpit and the cabin. This is a simplified explanation of the system, but should give you an idea of how it works. Hope it helped :)Please ask if it still is in the blue.

Guten TagA little add:http://dict.leo.org/?p=2Ib6..&search=packThis English-German dictionary translates'pack' = [aviat.] K

PACK = Pressurization and Air Conditioning Kit.Tero

>PACK = Pressurization and Air Conditioning Kit.>>Tero>Cool,Been trying to figure that out for weeks :)

Great, thanks everyone, that cleared up a lot. However I have another question. Why is it a problem to have dual bleed, as it is called, in my understanding this is when both the engines AND the APU are bleeding air into the duct system. Why is this a bad thing, do you risk some kind of overload or something?

Hi,During dual bleed you have all SOV (Shut Off Valves) open from all bleed sources i,e ENG 1, 2 and the APU. There is no problem with having a dual bleed warning as long as you have the engines running at idle. But when you are throtteling up above idle N1 the engine bleed air pressure is much higher than the APU bleed air pressure.To prevent back flow there are check valves on the supply bleed ducts. But check valves can get stuck in open oposition and are generally more vulnerable for malfunctions than the SOV. A malfunctioning check valve could cause the bleed air from the engines to go backwards through the APU bleed air ducts into the APU. This is not desired. It can damage the APU. At least this is what I remember from the top of my head. ;-)Cheers,

Guten TagMany Groundpower Units are capable to provise Bleed Air as well.I could'nt manage an engine start this way.Is this not modelled in the PM 737 ??G

Gunter, isn't this what you do when you flip the engine starter switch to "ground" to fire up the engines?Thanks everybody, I think I understand the PMDG 737 a little better now. That's just waht makes the PMDG series great, all the little things to find out and discover (as long as you are not a real airline pilot, that is)Greetings,Arjen Nederstigt

>PACK = Pressurization and Air Conditioning Kit.>>Tero>From where the Aussie expression "Packed up" came from, no doubt!!!FYI "Packed Up" is an Australian (and possibly from other parts) which means Unserviceable, Not Working, Broken, Buggerred etc.:-wink2