Gents:I flew from KMSP to KSLC the other. So, from an airport with an altitude of <1000 ft to an airport with an altitude of 4227 ft.I set the pressurization panel up as follows:Flt Alt: 35000 ftLand Alt: 4250Selector: AUTOAfter takeoff, the cabin altitude began to rise as expected and stopped at 5000 ft or so.But when I descended into KSLC, I expected the system to keep the cabin pressure around 4250 ft, but it didn't; it decreased to sea level at the time I landed. Shouldn't the system try to shoot for 4250 ft?Later that day, I continued my flight from KSLC to KCVG. This time, when I entered the aircraft the cabin altitude correctly read 4250 ft. I set up the same as before, expect the Land Alt was obviously less at 1000 ft or so. Bu t after T/O, the cabin altitude increased all the way to 8000 ft. Shouldn't the system just increase the pressure differential to try and hold about 5000 ft cabin alt?Also, the other day I was screwing around with the manual setting and opening and closing the manual valve. I got an OFF SHED DESCENT warning. What is this? I can't find any documentation.Thanks,James

Off Sched Descent Light: Illuminates if you initiate a descent without tripping the cruise relay (trips at 1000 feet before planned cruise alt I believe). Pressurization controller should set cabin alt to land at orginal take-off airport elev.The other stuff must be a bug. The controller should aim for a cabin alt/ flight alt differential pressure of 7.45 psi below 28K, and 7.8 above 28K-Bryan

>Pressurization controller should set cabin alt to land at orginal >take-off airport elev.Thanks for the feedback Bryan. Don't you mean that the pressurization controller should set cabin alt to land at the Land Alt setting?? Otherwise you could get some over- or under-pressurization situations.James

"After takeoff, the cabin altitude began to rise as expected and stopped at 5000 ft or so."Hi, James, according to the Maintenance Manual, a typical cabin altitude would be 8000', so I'm not sure if this is correct. At cruise altitudes less than 18,500 feet, the cabin altitude is set automatically to the landing airport altitude, so unless you are cruising at less than 18,500' and your landing airport altitude is 5000', I don't think your 5000' cabin altitude is correct."But when I descended into KSLC, I expected the system to keep the cabin pressure around 4250 ft, but it didn't; it decreased to sea level at the time I landed. Shouldn't the system try to shoot for 4250 ft?"It should ramp down the cabin altitude from 8000' to slightly below your set landing altitude. Cargo fires aside, during descent, the maximum rate of change of cabin pressure is normally 350 slfpm (sea level feet per minute). During takeoff and landings, there can be an uncomfortable pressure bump. It has something to do with the airflow around the outflow valve at nose-up attitudes (with the aircraft on the ground). This effect is reduced by increasing the cabin pressure slightly above airport pressure.There are special procedures for airports higher than 8000'. A Cabin Altitude of 10,000' will trigger a warning horn (which can be cancelled on the real aircraft)."Also, the other day I was screwing around with the manual setting and opening and closing the manual valve. I got an OFF SHED DESCENT warning. What is this?""Normally" this occurs if the pressurization system has detected that you have started a descent before reaching your set cruise altitude. However, this should not happen in MAN mode (only AUTO or ALT). By normally, I mean according to aircraft system logic. It is not normal to start descending before you have reached your cruise altitude (usually this would only happen in emergencies)Cheers.Ian.

James,Nope, "take off" airport elev is correct. If you don't reach your planned cruise alt, it's assumed that you've had a problem and that you have to turn back to your departure airport.Bryan

Thanks for the feedback guys. I understand now.However, I always cruise in the FL's, usually above 30,000, and I'm pretty sure that the cabin alt has been around the 5,000 ft mark. And it certainly doesn't seem to be ramping down to the land airport altitude. Of course, this is most noticeable when you fly from a low alt airport to a high alt airport or vice versa.James

Oops... My apologies.... I misread the text in my manual.... I thought the typical cabin altitude was 8000', but it's actually the "maximum for most flights".However, the Maintenance Manual is not exactly helpful in explaining what you should see. It says that the cabin altitude is held constant on the one hand, but that the pressurization system follows a "schedule" which shows that the differential* pressure should be at 7.8psi between 28,000 to 37,000 feet. However, if you keep the cabin pressure constant, the diff pressure surely has to vary.* (For beginners) Differential Pressure = the pressure difference between the outside air and the (inside) cabin air. Too great a pressure differential will rupture your fuselage (if it didn't have overpressure relief valves).I was looking at some altitude/pressure tables a few minutes ago and, if I'm reading these correctly, the outside pressure at 35,000' is 7.0406 inches of mercury (which is approx. 3.5psi). If a differential of 7.8 psi is scheduled (as mentioned in the maintenance manual), the cabin pressure must be around 11.3psi (3.5psi plus 7.8psid). This equates to a cabin altitude of 7,150' (approx). Not 8,000', but certainly not 5000'.Hopefully the experts can clarify this?Cheers.Ian.

The maximum certified altitude for the B737NG is 41000 ft. The pressure at this altitude approx. 2.5 psi. (ISA). Your maximum pressure differential will be reached at this altitude. Any lower altitude will probaly result in a lower cabin altitude. 8000 ft is OK for all ages. (Although night vision detoriates as low as 5000 ft).Hope this helps.BTW the formula fro pressure at altitude is:(Absolute temp. - temp lapse / abs temp) to the power 5,25 x 14.7 [answer in psi]Hope this helpsWil

Answer to Bryan with regards to setting the landing altitude.Hi Bryan, I have read your post and what you claim is not exactly standard Boeing procedure.The panel says Landing (altitude). And guess what? That is what you set. The system has a memory for the takeoff altitude. As long as the trip relay is not activated takeoff altitude is stored or in other words when you reach cruise altitude the landing altitude set in the window is then scheduled minus 300 ft. During an aborted climb the digital pressure controller returns to takeoff altitude minus 300 ft automatically. This is not unique to the NG. The old -200 had this implemeted as well.Wil

"BTW the formula fro pressure at altitude is:(Absolute temp. - temp lapse / abs temp) to the power 5,25 x 14.7"What are the units of pressure in your equation, Wil? psi? By absolute temp, you you mean Kelvin? Is one of the "abs temp"s in your equation supposed to be absolute height? (Otherwise where would you insert the height?).Thanks.Cheers.Ian.

Did a little testing. I recorded the info on the pressurization panel during a few recent flights. Here it is:Dep--------Flt Alt-------Cabin Alt------Land Alt-----DP-----ArrKCVG-------21000---------4000-----------650----------3.5----KDTWKDTW-------37000---------5500-----------50-----------6.8----KJFKKDEN-------33000---------9000-----------1000---------3.2----KMCISo, obviously it doesn't work quite right. I think that the controller just shoots for a cabin altitude about 4000-5000 ft higher than the departure airport altitude. I'll let someone else do the math to determine if the DP is about right.On the KDEN-KMCI flight, the cabin altitude was correct on landing (~1000 ft) which was one of the probs I wrote about in my original post; so, maybe I just messed up before. I'll keep watching this.Anyway, this is obviously not a very big issue, but it would be cool to see the Pressurization Controller try to maintain 8000 ft as noted in the post above.James

"Did a little testing. I recorded the info on the pressurization panel during a few recent flights. Here it is:Dep--------Flt Alt-------Cabin Alt------Land Alt-----DP-----ArrKCVG-------21000---------4000-----------650----------3.5----KDTWKDTW-------37000---------5500-----------50-----------6.8----KJFKKDEN-------33000---------9000-----------1000---------3.2----KMCII'll let someone else do the math to determine if the DP is about right."According to our manuals, James....Differential Pressure ScheduleFlight Altitude.......................... ScheduleSEA LEVEL TO 18,500'........Landing Field Elevation18,500 to 28,000'...........7.45psid (Vs 3.5 in your KCVG-KDTW 21,000' example) 28,000 to 37,000............7.8psid (Vs 3.2 in your KDEN-KMCI 33,000' example)37,000 AND ABOVE............8.35psid (Vs 6.8 in your KDTW-KJFK 37,000' example... or should this be in the 28,000 to 37000 range?)According to a standard Pressure/Altitude chart I downloaded (based on Sea Level being 29.92inches of mercury... or 14.7psi) At 21,000', the ambient px should be 6.47psi (in a standard atmospheric environment)At 37,000', the ambient px should be 3.14psiAt 33,000', the ambient px should be 3.80psiUsing the above chart... (I guess I can use this to calculate the Cabin Pressure)....At 4000', the cabin pressure should be 12.69psiAt 5,500', the cabin pressure should be 12.41psiAt 9,000', the cabin pressure should be 10.51psiI'll let you do the maths regarding "Cabin Pressure minus ambient pressure = Differential Pressure" ;-) I don't know if any temperature compensation has to be made here.Does anyone have a MSFS pressure Vs altitude chart? Also, some typical real airplane values would be useful here to see if I'm calculating this correctly.Cheers.Ian.

Ian:I think what we're basically getting to here is that the pressurization system doesn't work quite right. Thanks for the data that you've provided, and I hope that the PMDG guys are reading this post. There's no reason to fix something as petty as this via a patch, but it sure would be nice to have this incorporated into the -800/900 (hint, hint).James