Having the NGX at FL370 , I decided to close both bleed valves to simulate cabin pressure loss. I was expecting a very quick drop in pressure, and was suprised to see pressure loss was minimal. Opening the outflow valve manually to 100%, increased the rate of pressure loss. Just wondering, does anyone know that this is the way the real aircraft would respond.---This is my first post (hope it got to the correct area !). T/Y

Having the NGX at FL370 , I decided to close both bleed valves to simulate cabin pressure loss. I was expecting a very quick drop in pressure, and was suprised to see pressure loss was minimal. Opening the outflow valve manually to 100%, increased the rate of pressure loss. Just wondering, does anyone know that this is the way the real aircraft would respond.---This is my first post (hope it got to the correct area !). T/Y

You answered your own question there! The outflow valves do just that, release pressure from the pressure hull. If they're closed, pressure will remain constant. The packs simply pressurize.By the way, there's an NGX forum. http://forum.avsim.n...32-pmdg-737ngx/

Thanks for the reply Zach. I realized that the outflow valve would definitely increase the rate of depressurization of the cabin when opened to 100%. What I was curious about is whether the rate of pressure loss in the cabin would be very slow. During the sim test I performed (both bleeds closed), the loss of pressue took several minutes. Though I have been studying the 737 for about 10 years, I was always under the assumption that at high altitudes, the loss of an engine would very quickly 'drop the masks' in the cabin. Kinda suprised me. However, I guess with the large volume of cabin air, compared to the small outflow valve, this is probably correct. (and besides, the NGX is PMDG-has to be correct !!!)

However, I guess with the large volume of cabin air, compared to the small outflow valve, this is probably correct.

Exactly!As for the rest, I'm thinking that you may not fundamentally understand the pressurization process. If you do, I apologize. Here's a basic simplified run down. Pressurization begins on the ground (typically) with the outflow valves completely closed. The outflow valves open after a certain pressure differential is reached between outside pressure and cabin pressure (during the aircraft's climb to cruise). At that point the aircraft is opening the outflow valves to achieve a particular "climb rate" within the cabin (pressure is decreased slowly) until the desired cabin altitude is reached.