The 727 and for that matter 707 old style (as in the CS model) pressure control panels were very simple to operate, but took a bit of art on the F/E's side to make smooth.

The concept is simple. Air In - Air Out.



The engines supply high pressure bleed air through a pressure control valve on the engine. The F/E uses the pressure control panel to send signals to the outflow valves to control the amount of pressure in the plane's cabin. In the drawing, I show the air as water, but the concept is the same. The differential pressure is the difference of the air pressure inside the cabin to outside the cabin, displayed as P.S.I.



Here is a look at a real 727 pressure control panel. The knob on the lower right is where you select the altitude you want the cabin to go to. In this picture the cabin is selected to something less than 1,000 feet below MSL and the little window in the center top of the instrument shows the airplane altitude at max differential pressure. It indicates about 19,200 feet. Max differential is just over 9 psi. The knob just above the cabin altitude select knob is the barometric correction for local pressure. The baro window at about 2 o'clock shows 28.35 inches of Mercury (pretty low outside pressure - standard is 29.92). The other knob on the lower left is the rate of change selector. The arrow indicates the position of the selector - in this case almost max rate. If you align the arrow with the white triangle under the word RATE it should give you around 500 FPM.

So, how does it work?



Here is a look at the Captain Sim 727 pressure panel. It consists of 4 parts. The manual pressure control on the lower right, the automatic pressure control, lower left and two gauges above. The gauge on the above right is the rate of climb or descent indicator (VSI) and the one to the upper left is a combined cabin altitude and cabin differential gauge.

This is how the F/E would operate this system.

Since there is no ground venturi switch on this basic system the panel is operated just a little different to avoid large pressure bumps. After engine start, one or both packs can be started for air conditioning before the doors are closed. Just before the doors are closed the pack should be turned off. The pack valves are designed to open and close slowly to help with squeezing the ears. If the plane had a ground venturi fan, it would hold the outflow valves open and really aid in controlling the pressure bump. But is doesn't so tough it out!

Once all the doors are closed - cargo and cabin - the F/E can turn on one pack at a time and give the system time to react. You could select an altitude, on the automatic panel, to something above the current field elevation, but there is little muscle in the system to react since the cabin and the plane are at the same altitude. As the pack valve opens, there will be a small down surge in the cabin pressure and this can be seen on the VSI. After the pressure settles down you can open the second pack valve. Now as the plane taxis out for takeoff any addition of thrust from the engines will result in a down surge in the cabin - nice system, Hun! Most pilots were aware of the pressure system and would try to avoid doing this to the F/E, since it also effected them as well. As you took the runway for takeoff the F/E would check the differential pressure to make sure the red placard was being followed. You are limited to a max differential pressure of .125 psi on the ground. This is so the doors can be opened in an emergency. As the power was increased for takeoff the F/E would move the rate selector to minimum rate and maybe turn the cabin altitude selector up above the field elevation, but there was not much the poor F/E could do at this point. There would be a pressure bump as the plane was rotated for takeoff and airflow over the outflow valves changed. Not much the F/E could do here either. As the plane started to climb the F/E would select the desired cabin altitude for cruise plus a few thousand extra feet to avoid getting too close to the automatic pressure relief of 8.4 psi. Once established in a climb the F/E would adjust the rate knob to obtain around 500 fpm climb of the cabin. If the plane was light and climbing fast, the F/E would have to increase the climb rate of the cabin so as no to get caught by the climbing plane. If that were to happen, the cabin would climb with the plane and all the passengers and the other two pilots would curse the poor F/E to tears. This last sentence was an example of the negative pressure relief system.

At top of descent the F/E would slowly start the cabin down to the destination airport field elevation. Again, you needed to keep the cabin ahead of the plane in descent or the same thing would happen and more cursing would ensue. If you took off from a sea level airport and your destination was Denver, you might not have to do too much with the cabin altitude as the cabin was probably very near the field elevation. To change cabin altitude either up or down required slow and careful movement of the cabin select knob to avoid the pressure bump. Most F/E's got very good at this system and kept everybody happy. If you had a leaky plane, idle thrust might not have enough bleed air to keep the cabin from climbing during descent. F/E's would ask the pilots to push up one of the throttles just a bit to get a little more bleed air and keep the cabin going down.

The manual controller worked just like the auto controller, but you had to apply more attention to the device since any power change resulted in a different rate, and you needed to keep an eye on the differential and cabin altitude since there was no automatic level off at the correct altitude. The manual controller and automatic controller are connected, but the manual controller had more muscle and would override the auto controller.

I hope this little explanation helps to understand the old fashioned pressure system. It was fun to make it work nice, but I for one was not unhappy to see it go either.  

Lou

P.S. the latest version of the CS727 & 707 cabin pressure systems do not work as they should for whatever reason. A fix is in the plan.

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Dutch wrote:

So, to sum up, it's not the status of whether you're on the ground or not as I had guessed.  The pressurization system is working (the system is adjusting the cabin altitude towards the selected goal by the chosen rate) when either or both of the packs are on and the airplane is sealed up.  

Negative pressure is not allowed. A relief valve will see to that.

The system does not start to adjust the cabin just sitting there on the ramp. Sure, you could start the cabin down and increase pressure, but remember the limit of .125 psi. It is after you takeoff that the system starts to do its thing.
The altitude you select in the auto controller and the rate will start the cabin to climb as the plane climbs, but at a much slower rate. The goal is to climb the cabin at a smooth steady rate of lets say 300 to 500 fpm and arrive at the selected cabin altitude before the plane gets to its cruising altitude. Example: aircraft at 35,000 feet, cabin at 7,000 feet - differential pressure around 8 psi.

When no packs are on, or the doors are open, the goal becomes the outside air pressure.  With packs off but cabin sealed, use the rate selected.  If a door is open, it's much more rapid.

With no packs on and doors open inside is the same as outside. With packs off and all doors closed technically you get a slight negative pressure from the equipment cooling fan, but that is too much detail.

Manual overrides auto and sets rate independent of the normal rate dial - unless manual switch is in the auto range.

Correct! With the Manual needle in the 12 o'clock position the auto controller is boss. If you move the manual knob out of that middle position the auto controller is overridden and slowly the manual controller takes over.

Pressure differential can not be negative and not exceed 9 PSI (what happens if it does?)

Yup! That is true. If in auto control and 8.4 is neared, the auto relief valve starts to work to cap the differential. If you exceed 9. something, the manual relief valve comes into play and the correction is a lot bigger. Not a good place to be with passengers on board as there will surly be some laundry issues.

I love the level of detail in your reply, but I'm not going to try and implement all that.   At this point I just want a reasonably accurate implementation.  

We bump up against the fact that as simmers we are a one-man show trying to run a three-man airplane.   When I'm flying I can't be riding herd over this pressure controller to be smoothing out pressure bumps -- I'd probably crash while staring at the FE panel.  I've stalled once and crashed while testing!  Was annoying...

When I fly the 727 or 707 of Captain Sim, I don't worry too much about the F/E panel. I keep an eye on the fuel on a long trip and cross feed when needed, but as you said - I did enough of that F/E stuff to last a life time.

How about this for a workable mini-checklist.

1.  After engines are started and cabin sealed, set pressure goal 1,000 feet or so above field elevation and rate at 300-500FPM.

2.  Once established in the initial climb phase, set pressure goal to what you'll want it to be at cruise and let it start rising.

3.  At top of descent, set pressure to 1,000 feet above field level and rate to 200-300 FPM.  Try to avoid letting the airplane get ahead of the system, but let the controller itself prevent negative differentials on the way down.

4.  After landing set pressure to field elevation so as not to induce a surge when the doors get opened.

This may be a little more semi-automatic than the real thing, but still operated in basically the same way.  It should keep the altitude, rate of change and differential numbers within a reasonable range, without the huge amount of work, which is good enough for me while we wait for Captain Sim to fix it officially.

1. OK
2. OK
3. Set the auto controller to the landing field elevation. Keep ahead of the plane at ALL times or we are back to the Laundromat issue. These relief valves are pretty savage. Nothing subtle in their corrections.
4. If you do 3 correctly you have done 4!  

Lou 

Dude, you've done it! it works for me as you described ) Thanks! Now i got to get used to not to skip it in checklists

P.S. Can you program xml ? Can you add trim motor sound when evelator trim key pressed?

I'll be happy to help them if I can.  I doubt they are interested my code because it is written in XML and their code is in C - and besides it appears they are trying to use the built-in FSX pressurization system whereas I just replaced it.

My advice would be to look into not ever using the "pressure dump" feature of FSX because it appears to be broken.  From what I saw in my testing, if that is removed the CS system should work.

Dutch