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Guest hangar744

744 cabin pressurization on ground

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Guest hangar744

Hi there,I can't pressurize the cabin on the ground. Is there a trick to do it somehow, or is this issue just another fs limitation?APU, 3 PACKS (one would be enough as well), OUTFLOW VALVES MAN L & R ON. Valves are slightly closed causing -500 ft/min cabin rate of climb.Cabin differential pressure mysteriously stays zero after hours of blowing air into the cabin.Next time I'll try this with engine bleed air...perhaps delta P is coded only into the engines (just like the T/E flaps).One little thing. The outflow valve OPEN/CLOSE toggle switch animation in VC has only two positions. Regardless of LMB/RMB, the switch goes only to CLOSE position. The animation of this switch in the 2D cockpit is correct.Sure I had a lot of fun.regards,delcom http://forums.avsim.net/user_files/190949.jpg

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Not sure about this bird, but pressurized aircraft normally "dump" cabin pressure on the squat switch (when the gear is compressed by the weight of the aircraft).Just wondering, why would you want a pressurized cabin on the ground?


Dan Downs KCRP

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Guest hangar744

Hi Dan,thanks for your interest. I believe what you mean is simply the air/ground logic. The nose squat sensor input for ground mode is only needed if HYD SYS 1 and/or HYD SYS 4 pressure is below 1200 PSI for more than 2 seconds. Previously I wrote a very brief introduction about air/ground logic. For details check that post: http://forums.avsim.net/dcboard.php?az=sho...id=145193&page=The cabin pressure control system in MANUAL control mode bypasses the cabin pressure controllers. In MANUAL mode the outflow valves are "straight" connected to the open/close toggle switch. Changing back to automatic mode will cause the valves to move to a position computed by the cabin pressure controller software. In my case that was the fully open position, because the plane was in ground mode and the groundspeed was less than 65 knots.One might (probably Qavion) suggest me to command the forward overboard valve to close as well. Can be done in ground mode, too. I'm not sure yet if PMDG included that in the pressure model. I think I'll find that out soon. Still with this valve open I should be able to get delta P higher than 0.0 PSI.Anyone taking off right now? Check if the outflow valves move the moment you pass 65 knots. I don't know...I never really paid attention to it.Dan, you also asked why would I want a pressurized cabin on the ground. Well, I wanted to know how airtight the PMDG bird really is. Pretty normal maintenance procedure. We do it really often. Personally I love it, despite the poppin' ears (or that's the reason).Uh...did someone just pull the smoke evac?Have fun guys.regards,delocm

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"One might (probably Qavion) suggest me to command the forward overboard valve to close as well. Can be done in ground mode, too."I think the Forward Overboard Valve was discussed during beta testing, but I don't know if the logic was modelled. Where would you get data for the outflow rate?Yes, cabin pressure should rise (albeit more slowly) with the Forward Overboard Valve open and the outflow valves closed, but are you saying the FOV can be closed on the ground? I thought this could only be done by maintenance, not from the cockpit??? Last time I did it, I had to pull the canon plug off the valve (in the left hand crawlway of the MEC) and manually close it with the lever. Did you try closing the outflow valves completely?Cheers.Q> P.S. And did you close the cabin, MEC, CEC, top hatch and cargo doors? :(

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Guest hangar744

Hi Q,lol...yeah all doors were closed :-) and also tried it with outflow valves fully closed. Removing the connector and then closing the valve manually is the normal procedure for this test. Since this cannot be done with the PMDG 744, I'm gonna have to use an other trick to command that valve to its closed position. If one engine per wing is running above 50% N2 the ECS MISC CARD will close the FOV.I believe arming the FWD CARGO FIRE switch also causes the valve to close (perhaps this applies only in air mode). I'll give it an other try. I think I've seen some tables or graphs of the FOB flow rate somewhere.see you,delcom

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Guest hangar744

Again PMDG made my day, the beer is on me:-beerchug Just found a nice realistic touch/detail while desperately tried to pressurize the cabin in ground mode. Arming either FWD or AFT CARGO FIRE switch immediately shuts down PACK no.3. And that's exactly how it's done on the real kite. Memo message PACK 3 OFF...and the green flow line is removed from the ECS page.Guys...that's what I call attention to detail.Thanks PMDG,delcom

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"And that's exactly how it's done on the real kite. Memo message PACK 3 OFF...and the green flow line is removed from the ECS page."There was considerable discussion on this issue amongst the beta testers recently... Apparently there are quite a few variations in the real 744 world regarding pack shutdown in response to arming the fire switches. PMDG has only modelled one variation... and it seems to match yours, but don't get caught out working on foreign airliners ;)Cheers.Q>

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Guest hangar744

I couldn't pressurize the cabin on the ground so far. So I took her to the skies for a low level test flight. Established 4.0 psi differential and performed the test. It took a lot longer than I expected. ;-) Here are the results I plotted. The BLUE curve shows the PMDG 744.The RED curve represents an actual real world counterpart I tested about three weeks ago. Guys...use it only as reference. (No temp/QNH correction). enjoy it,delcomhttp://forums.avsim.net/user_files/190977.jpg

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7 minutes? That's just over the limit of acceptibility (according to the Maintenance Manual 05-51-08). How old was your 744? (or did someone flush the toilet during the test :( )Cheers.Ian R>

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Guest hangar744

Well...she is 17 right now. But I think what more important here is that the test was performed as a preliminary check just before FD03. This explains everything about the steep pressure curve . So yeah, probably she was leaking through a thousand wholes. Don't exactly know when the last C check was performed. A few days later she no longer looked like a Queen. An empty tube plus millions of pieces laying around. At this point we're into week three. She has four new legs already. We will give her the nose gear tonight. Hopefully the fuel tanks will be closed in a few days so we can start with the adjustment and testing phase. I'd say in two (give or take) weeks she is ready for the test flight. I'll record and plot the figures of a post-maintenance AMM 05-51-08 check. Let's see how the "health" difference will be.On the other hand I don't think even a new bird would keep up sixty plus minutes. But hey...I might be totally wrong, since I do heavy maintenance I don't get to meet the young ladies, lol.;) So guys...conclusion: The PMDG 744 pressurized cabin is really safe.I really had a lot of fun this weekend performing this test and preparing that chart. Still don't understand why I couldn't do it on the ground? Anyone?Q, would you be so kind to give it a go, please? The fuel is on me, truck's on the way. :-wave Testing is the best thing,delcom

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Q, would you be so kind to give it a go, please? The fuel is on me, truck's on the way. :-wave
Delcom, I have no doubt that your findings are correct ;).. I did notice, however, that you are using the cargo aircraft. They have quite a few extra valves I'm not familiar with. Also, they have less main deck doors for air to leak out of ... but I don't know if that's negated by the extra large ones in the nose and side :(Cheers.Q> イアン

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Guest D17S

I agree. Someone left a door open. Even the old DC8 freighters I mess with decay at ~ 500FPM from any pressure one might want to endure. We'd be in there for weeks waiting for the thing to leak down by itself. Great fun to really ramp the rate on the way back up. Fog city. The noobees freak . . . "We're on fire!""Well, go put it out."

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Guest hangar744

Hi again,I forgot to mention an important item in my previous post. The PMDG 744 outflow valves (in automatic mode) start closing when you pass 65 knots during takeoff roll. Just the way it's supposed to be. Thumbs up, PMDG.Uh...yeah Q, I found the FWD OVERBOARD VALVE flow characteristics charts. I knew I had them somewhere. So if you are interested, just let me know. Valve effective area, cabin altitude and flight level plotted against airflow through the FOV kinda things.regards,delocm

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