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Cabin Pressure Departing KDEN

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Departure from the 5440ft MSL field results in a cabin altitude over 10,000 at cruise. I've looked throught the history of this forum, variations of this topic have come up before but I'm left wondering if this is just a "accepted" limitation of the simulation. I've been flying the 737-700 for a couple of months now on SWA routes and finally starting visiting KSLC and KDEN when I noticed this glitch. Here is a data sample for a typical departure:ALT CABIN ALT dP5440 6500 010,000 7500 0.215,000 8500 0.720,000 10,000 1.025,000 10,000 1.630,000 11,000 2.235,000 12,000 3.040,000 12,000 3.8I think the cabin alt controller sim ignores high altitude field elevations, note that the highest dP (differential pressure) is much less that it should be at FL400. Pretty much the same thing happens departing KSLC at about 4,200ft MSL. All else functions as expected, and the walkaround preflight is unremarkable.Any feedback from PMDG?Thanks


Dan Downs KCRP

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Guest Ray51

I tried both the Pax and cargo versions out of KDEN and had no problems with cabin altitude. At FL370 I had a cabin altitude at 6800 and a delta P of 7.9 The only way I could get cabin altitude to near your readings was to place my outflow valves in manual and open them slightly. At least for my computer and PMDG installation this is not a problem.Ray

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Ray,I think we are talking about the NG here. (I might be wrong though, has happened on occasion before... ;-))Cheers,


Mats Johansson
PMDG Flight Test Dept
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Correct, the 737 w/ 700/800/900 upgrade. Anyone get the same results? I've been carefull to try to eliminate pilot error.


Dan Downs KCRP

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I'm looking for someone who would try to duplicate my cabin pressure glitch following a KDEN Denver Intl departure. Any 737-700 drivers out there who take the 20 min to climb out to FL400 and note the cabin alt and diff P for comparison with my observation would be appreciated. If it isn't duplicated then I know I've got an issue.Thanks,


Dan Downs KCRP

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Guest Ray51

I tried the 700 and the 900. Both had trouble with cabin pressure. I could not get cabin pressure up even by manually closing the outflow valves. After playing with the outflow valves and changing altitude, down to 10,000 ft, I was able to get cabin altitude to 5000 and a diff pressure of about 8 psi. Once pressure built up the system was able to maintain pressure even up to FL410. I would write a maintainance ticket on the outflow valves not functioning properly. After closing the doors, I am going to try and manually lower cabin pressure to 4000 ft prior to take off and see if the system can keep up.Ray

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Guest Ray51

Tested the 700 and 900 at KDEN. Both a/c started with a cabin alt of 7,000 ft, a diff pressure of 0 psi, and a cabin change rate of -.3 I closed the outflow valves and lowered cabin alt to 5k and returned control to auto. Climbed to FL410 at an average climb rate of 2000 fpm. At FL410 cabin alt was 9,000 ft, diff pressure was 5 psi, and cabin change rate of -.2I tested the 700 out of KDEN again. The starting cabin readings were the same. This time I didn't lower cabin alt. but left control in auto and took off. I again climbed to FL410 at an average rate of 2,000 fpm. The cabin alt climbed to 8,000 ft and the cabin change rate went to 0. It appears the system was trying to hold 8,000 ft but couldn't do it. The cabin change rate quickly went to +.3 and stayed there until a cabin alt of 10,000 ft. When I reached FL410 the cabin alt was 10,000 ft, the diff pressure was 4.4 psi and the cabin change rate was -.2It appears the outflow valves control is sluggish. I also noted that the outflow valve postion and cabin change rate does not appear to correalate. With a duct pressure of 30+ psi and the outflow valves closed I saw a cabin change rate of any were from +.3 to a -1.5 Since this is a measure of the differantal pressure between the cabin pressure and the outside pressure the rate will vary with rate of climb and a/c altitude but with the outflow valves closed and full duct pressure I was surprised to see cabin altitude rising.Clearly the pressurization system as modeled is barely able to provide control of cabin altitude under all conditions. When the system falls behind, such as in my case were cabin altitude starts at 7,000 ft, the system struggles to mainatin 8,000 ft cabin altitude. I also never got a cabin altitude warning horn or ECAS message even when I let cabin altitude get to 40,000 ft. Ray

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Thank you very much... very good look for all. At least I know the problem is reproduceable. Maybe next minor rev update by PMDG will solve this as well as the VNAV constraints bug.Gee, if they add something of value that we'd pay for then most everyone would be happy.Thanks again,


Dan Downs KCRP

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