I don't know if this is working at intended, but if start up the APU, then open the APU bleed, I get duct pressure around 10 PSI, booth needles as I have the iso valve on auto, so one would expect it to be open with only APU bleed and no engine bleed, and this seems to be the case. Then I turn on the left pack to AUTO, and both duct pressure needles jump to about 38 PSI, turn the right pack to AUTO and both needles fall to about 18 PSI. Now my though was the duct pressure should be very high when the APU bleed is open and no packs are on, and then as I turn on packs they should then go down, yet with one pack on the pressure jumps. Also I get silly readings in the supply duct temps, like 60+ degrees, even with the temp gauges set fairly far to the left (the cold) side. These were set from the previous flight. In order to get the duct temp to go down, I need to cycle the trim air switch on and off and then "jiggle", for lack of a better word, the appropriate CAB TEMP knob. If this is all how the real 737NG is, then ok thats fine, but the duct press right now seems counter-intuitive to me. Why would duct pressure go up, and up dramatically, after I turn on a pack?

I believe I read somewhere it's supply and demand, the pack demands more, so it ups the pressure, when you turn on the 2nd pack, you have the same pressure, but going to twice as much "space" so to speak, so pressure goes back down. (The last part is just what I am figuring/assuming, so I am probably very wrong)

I don't know if this is working at intended, but if start up the APU, then open the APU bleed, I get duct pressure around 10 PSI, booth needles as I have the iso valve on auto, so one would expect it to be open with only APU bleed and no engine bleed, and this seems to be the case. Then I turn on the left pack to AUTO, and both duct pressure needles jump to about 38 PSI, turn the right pack to AUTO and both needles fall to about 18 PSI. Now my though was the duct pressure should be very high when the APU bleed is open and no packs are on, and then as I turn on packs they should then go down, yet with one pack on the pressure jumps. Also I get silly readings in the supply duct temps, like 60+ degrees, even with the temp gauges set fairly far to the left (the cold) side. These were set from the previous flight. In order to get the duct temp to go down, I need to cycle the trim air switch on and off and then "jiggle", for lack of a better word, the appropriate CAB TEMP knob. If this is all how the real 737NG is, then ok thats fine, but the duct press right now seems counter-intuitive to me. Why would duct pressure go up, and up dramatically, after I turn on a pack?

The packs contain a recycling machine which would make the pressure higher in the duct.Try it with the isolation valve set to Open.The Apu bleed air can then enter the right hand side of the system and get to the right pack also. Fred.

I believe I read somewhere it's supply and demand, the pack demands more, so it ups the pressure, when you turn on the 2nd pack, you have the same pressure, but going to twice as much "space" so to speak, so pressure goes back down. (The last part is just what I am figuring/assuming, so I am probably very wrong)

This is one situation I considered, and it makes sense that way, that turning on a pack triggers the APU to up its bleed air output. And Fred, when trying this out I see no difference between ISO valve set to Auto or set to Open, which I expected should things function the way they are supposed to and I have not suffered a service based failiure.

I found that psi stays at 10 today's flight.Sometimes I fail to starting engine no.1 with EXT PWR and AIR. After a few second after Engine 2 start switch turn back to Auto from GND, duct pressure suddenly drops around 10. When I try starting no.1, psi rises only up to 20.At this point even I turn on the APU, psi doesn't go up, But rises beyond 30 AFTER I turned switch to GND. Is this the same "demand" theory? All other switches are at correct position. Packs off, ISO Valve open.

If the packs are off then there is not a demand for the APU bleed air. If you use a source that needed higher bleed air like a engine start you will see it go up to 30+ psi if everything is right. But it is a demand system.

This diagram is good for understanding the different systems that use bleed airand can cause the duct pressure to rise or fall. Fred.

What is then the min psi required for an engine start?

If the packs are off then there is not a demand for the APU bleed air. If you use a source that needed higher bleed air like a engine start you will see it go up to 30+ psi if everything is right. But it is a demand system.

Oh There's so many things to learn with NGX!I've never heard 737NG series had changed to demand-affected Duct Pressure system anywhere even NGX FCOM. As you guys know, Many sims we used always shows high duct pressure whenever air is connected even no demand (yes, classic 737s). But I'm still curious, what is the reason I failed to start second engine? (started 2, stabilized, starting 1. psi don't go up beyond 20psi, N2 stays barely at 15%)APU off, ALL ext sources connected, Packs off, APU/ENG bleeds off. opening of closing ISO Valve doesn't make any difference.(I learned should start Eng 1 first but started Eng 2 for test. And this order doesn't make change for me)Is this intended behavior to prevent injuries starting both engines starting at the gate?

If engine No. 1 was started using a ground air source, to minimize the hazard to ground personnel, the external air should be disconnected and engine No. 2 started using the Engine Crossbleed Start procedure.

For ground starter and external air, try to start No1 then give a little trhotle until psi rises to 30. Then try to start No2.

Hi guys, and I have little problem with start engines... Let's suppose following case: At the gate, I have connect GRD POWER, ASU and AIR COND. units, and APU is OFF. My AIR COND. PANEL settings are:L/R RECIRC FAN - ONL/R PACKS - HIGHISOLATION VALVE - OPENAPU BLEED - OFF (because APU is OFF and Ground Air is connect)ENG BLEED - ON Ok, we get clearance to pushback, so:I disconnect GRD POWER (earlier, I turn on APU of course)I disconnect ASU and AIR COND. carts - unfortunately, DUCT PRESS falls dawn to about 10psi.APU BLEED - ON (duct press increase to ab. 20 psi) ...then start pushback. While pushback, I try start the engines ... and the problems begin:both PACK - OFF (as FCOM "says")ENGINES/APU BLEED - still ONISOLATION VALVE - still OPENduct press - ab. 20 psi - so, too low... ...but... when I turn L PACK - OFF and R PACK - AUTO, duct pressure increase to nearly 40 psi.So, now I can start engine no.1 and no.2 (without crossbled procedure) but I don't know if my proceedings are correct.I guess - not. If not - what is the reason of rapidly decreasing duct press after disconnect AIR carts - and why duct press increase to only about 20 psi after APU BLEED turning on?

What is then the min psi required for an engine start?

I've had the same question. But thinking about it, if it's really by design that pressure is low UNLESS there is demand, there probably won't be a minimum pressure AS the pressure would go up as soon as the starter demands more pressure. This seems to be an obsolete thought as in the classics you would in fact check start pressure to be at around 30 psi at sea level, decreasing with altitude. Apparently this is no longer a concern on the NG. However, as has been mentioned before, a cross bleed start will definately benefit from adding a wee bit of thrust on the running one. In fact it seems if you fail to remember this you might not get the other engine started at all, with it's N₂ fluctuating around some low value. You'll immediately see it increase once you add bleed pressure by advancing the thrust lever on the running engine.

The APU automatically adjusts to air demand. That is why you see an increase in duct pressure when you turn the pack on. If you watch the duct pressure after turning the engine start switch to GRD you will again see duct pressure going up after a couple of seconds, just like it does when you turn a pack on. The cross-bleed engine start procedure is described in FCOMv1.

APU with no bleed demands will give low pressure output to the bleed, as soon one of the users (packs, starter) require pressure, the APU bleed valve adjust the demand.With both packs selected (and engine bleeds selected on) the isolation valve will close (in auto position) it can be forced by opening it manually or by turning off either engine bleed or a pack.Both packs on High will drain a lot of air from the APU, more than it can feed at the correct pressure, so the pressure will drop and its EGT rises (in NGX I seen only 20-40°C of EGT increase).It is normal that if air is used the pressure drops, it is also normal that if no air is used, APU bleed is controlled to mantain less pressure to save APU and bleed ducts (and to prevent overheating of the ducts eventually) When groun cart is avaiable pressure is present on the right side so it will feed only the right side if all packs/bleed switches are set to ON, ISOL to AUTO. Just switch one engine bleed or one pack to OFF to restore pressure on the left side.On ground one pack is sufficient for air conditioning.

Thanks for replies, guys.