According to the wiring schematics for the Packs, if the overhead Pack switches are in AUTO, the Packs will go from Low flow (when the airplane is on the ground) to HI flow when both engine bleed switches are selected to OFF (engines running or not).I can't see the ...er... "logic" behind the Pack logic.... Why would it do this?Thanks.Cheers.Ian.P.S. On the ground, the engine bleed switches seem to be left on at all times (at least that's the impression I get from my limited exposure to NG's).

Ian,My best guess says this was the asiest way to force a pack to high flow on the loss of a bleed system, I am kind of suprised that they didn't put an Air/Grd logic in it but this does seem to be a fairly simplified Boeing compared to its older siblings. RegardsPaul Gollnick :-coolTechnical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.com

One Engine bleed switch I could understand, Paul, but both? In the air with both Eng bleeds inoperative/off either leaves you with no bleed air to drive the packs... or APU air to drive the packs(if below a certain altitude). I wouldn't think you would want to put all that strain on the APU at this point in time.I may have to look at the more detailed schematics to make any sense of this ;-)Cheers.Ian.

Ian,Since you have the schematic, I am just guessing but my thought is that they simply tied it to either switch so either one or the other (and also both) would cause the High Flow circuit to activate. Kinda of a simplified wayof doing things. Of course if you do turn off both bleeds in flight then you get the "Rubber Jungle" effect in the back. :-lolPaul Gollnick :-coolTechnical Operations/Customer Operational SupportPrecision Manuals Development Groupwww.precisionmanuals.com

"Since you have the schematic,..."Er... In this case, ownership doesn't necessarily mean understanding ;-)Check your Inbox and see what I mean (although you may need more than the one diagram to fully understand what's going on here).Cheers.Ian.P.S. Suitcases packed yet? :-)P.P.S. Good point about the rubber jungle... Don't try it on your next trip ;-)

Ian,In a 737 Airplane Flight Manual it is stated that

"

Hi ian,the engine bleed air switches are always in the on position, because when a bleed switch is in the OFF position a relay in the BAR (bleed air regulator) is energized. so to extend the life time of the relay the switch is always left in the on position. so the are only set to the off position when there is a problem.In cruise, if a pack shuts down from an overheat or if a (L or R) pack switch is OFF, a solenoid de-energizes. This sets the operating pack to high flow mode.APU high flow mode when:- the pack switch is in the HIGH position - the APU bleed switch is in ON - the APU operates above 95% - the airplane is on the groundFrom the 737NG training manual: "fcsov electrical funtional descr"When solenoid B is energized, the FSCOV operates in the low flow mode.Solenoid B deenergizes when K18 energizes. K18 energizes when both engine bleed switches are in the OFF position.Also, solenoid B deenergizes when these things occur: - airplane in the air - flaps upWhen solenoid B deenergizes, the FSCOV operates in the HIGH flow mode.alex

The manual I refer to is for the 3-5-7-8-9 series, but of course it does not include any Wiring Schematics or any explanation why you should need

>the engine bleed air switches are always in the on position,>because when a bleed switch is in the OFF position a relay in>the BAR (bleed air regulator) is energized. so to extend the>life time of the relay the switch is always left in the on>position. so the are only set to the off position when there>is a problem.This explains a comment from another source that state about the start procedure:The engine bleed switches are normally kept in ON position. When the engine is running, the bleed air valve opens and engine bleed air enters the bleed air system.Cheers,Bert

"Well, the

>Don't forget bleed air duct pressures on the overhead panel,>Bert. Especially on the ground with the APU ramping up the>pressure to meet demand ;-) (See previous thread with APU>Input Monitoring photo shown).>>Also, I heard a mighty rush of air the other day when I>switched one pack on an NG to "HIGH" with the APU providing>bleed pressure. It's very noticeable when you go from Auto LOW>to APU HIGH mode.>>Cheers.>Ian.OK, so what you mean is that when I switch the pack switch from AUTO to HIGH, the reduced duct pressure is an indication of high airflow. However, I have never heard any sound from the fan. So I decided to search this forum, and found a thread that said that I had to switch the STANDBY POWER off and then on again to get the fan-sound. Is this a known bug?Also this sound (PMDG_fans.wav) has a constant volume independent of any pack switches, and the sound is on even with engine cutoff, battery off and ground power off. Cheers,Bert

"OK, so what you mean is that when I switch the pack switch from AUTO to HIGH, the reduced duct pressure is an indication of high airflow."How an APU Bleed system handles different demands varies from aircraft to aircraft, Bill. On a Classic 737, the APU puts out a nominal max pressure, say, 40psi, and every time a bleed user system is switched on, as a rule, the duct pressure goes down. On the real NG, the APU puts out, on some days, 20psi with no load/no user systems switched on, but higher psi if packs are switched on, or even higher if packs are switched to HI... or even higher still if an engine is started (I haven't witnessed a engine start yet on an NG, but I suspect the duct pressure would rise to 40psi or thereabouts... The same pressure a Classic might put out normally). The pressure will eventually start to go down however if you try to run everything at once. The APU does have its limits, especially so, if your airport is hot and high. The PMDG bleed system seems to be modelled on a Classic. I don't know if this will be corrected in SU-2.... It's not the sort of thing you can reprogramme in a hurry.The reason more modern aircraft put out lower pressures when not being asked to run bleed user systems, I believe is for reduced wear and tear... with, perhaps, minor savings in fuel."However, I have never heard any sound from the fan. So I decided to search this forum, and found a thread that said that I had to switch the STANDBY POWER off and then on again to get the fan-sound. Is this a known bug?"I'm not sure that this is the sound of aircon fans running. It sounds more to me like the general background noises you would hear if you switched on Standby Power (e.g. gyros running up.... not that there are many old fashioned mechanical gyros on the NG... and all the clicking and whirring that cockpits make. I haven't had much experience with the NG, so I don't know if the NG cockpit is quite as noisy as the Classic during Stby Power ops). You would have to ask the PMDG team if the sound recordings for this were taken from a Classic or an NG.The fact that you don't get the Stby Inverter supplying power to the aircraft when you switch on the battery has been recognized by the PMDG team. With the Stby Power Switch in Auto, I believe that the Standby Inverter should start running on the ground and in the air whenever the Main battery switch is selected ON (or if the battery switch is not on, you can force the Standby Inverter to use the battery with it's third position, BAT... but I'm not sure this is modelled in PMDG). Hope this makes sense?Cheers.Ian.

>>I'm not sure that this is the sound of aircon fans running. It>sounds more to me like the general background noises you would>hear if you switched on Standby Power (e.g. gyros running>up.... not that there are many old fashioned mechanical gyros>on the NG... and all the clicking and whirring that cockpits>make. I haven't had much experience with the NG, so I don't>know if the NG cockpit is quite as noisy as the Classic during>Stby Power ops). You would have to ask the PMDG team if the>sound recordings for this were taken from a Classic or an NG.Well, I think it is supposed to be aircon fans if you consider the name of the sound-file. Perhaps someone from the PMDG team can jump in to confirm? >>Hope this makes sense?>Yes, thanks Ian, as you said in another thread, it helps more than it hurts.:-hah Bert

Paul, or Ian, or anyoneJust as a matter of interest, and please excuse my ignorance, but what is the 'Rubber Jungle' effect? Has it something to do with the passengers getting upset because the air-conditioning isn't working anymore?