Cabin pressurization

[camg]

Does anyone know of a freeware gauge or a freeware aircraft with functioning cabin pressurization? I already have a dual-needle cabin press gauge that shows cabin altitude and differential pressure, but it is a "dumb" gauge in that I have programmed the cabin altitude and diff. press. to read out as a function of the aircraft altitude.

Now I want to make it more realistic by having a second gauge to set and indicate cabin climb at a selected rate--just like most pressurization systems.

I am flying mostly the MU-2, Piper Meridian and Cessna P210 of late and I have Cab Press gauges in all of them reflecting each one's max differential pressure and max altitude for an 8000 ft. cabin.

Cam

[Tom Goodrick]

I have several of those "dumb" cabin altitude gauges on my panels. (Just download any pressurized aircraft from my site and you will get one.) But it just shows an increase in altitude in proportion to the ratio of altitude to "8,000 ft cabin altitude" for the particular aircraft. It is also based on Bleed air in jets and another parameter in prop planes so that if you shut down one or both engines the cabin altitude rises. It does not allow you to set the rate of climb as in real aircraft. It does require some care during descent to avoid abrupt pressure surges. The gauges are set for each aircraft based on "8,000 ft cabin altitude" which is in many aircraft specs. I have been using these gauges for years.

[n7rg]

try jetstream 31 panel

baepanelproject.com/

[camg]

Thanks for the suggestion!

I loaded the three cabin press system gauges to my Cessna P210.

The gauges look great; the problem is they don't work like a real pressurization system. They don't even work as described in the Jetstream 31 panel manual.

I checked the XML files of all these gauges to find out why. A couple call for a system variable called: "Pressure mode", and a couple call for "Altitude pressure rate (percent)". Pressure mode (Boolean value) works, but it doesn't appear the "APR" is functioning in these gauges.

Does anyone know where I can get a list or readout of all the system variables available in FS2004?

On the Internet, I ran across reference to Microsoft ESP software this is specifically for aircraft panels and gauges. I have no idea how this software relates to FS2004 or FSX. But I found a list of system variables available in ESP. These include: PRESSURIZATION CABIN ALTITUDE, PRESSURIZATION CABIN ALTITUDE GOAL, PRESSURIZATION CABIN RATE, PRESSURIZATION PRESSURE DIFFERENTIAL, and PRESSURIZATION DUMP SWITCH.

If these variables are also in FS2004 it would give us a chance to build a genuine cabin press system. In it you would set a cabin target altitude, and cabin altitude climb rate. Then the cabin would pressurize as desired after takeoff.

[Tom Goodrick]

I don't know anything about that "panel language." But the variables available to anyone - even developers of panel languages - are listed in the two SDK's that Microsoft has released for FS9. ("SDK" stands for Software Developers' Kit"). The Aircraft Container SDK or "ACSDK" consists of all aircraft-related parameters that can be set in any of the files to establish the aircraft configuration. The panel SDK has lists of variables contained within the sim that developers can read and use in gauges and other control parameters that can be entered into the sim to change the motion of the aircraft. Some of the parameters for pressure you mention are not listed in the SDK but can be derived from SDK data using the simple Gas Law relations of physics.

In the case of an aircraft pressurization system, pressure varies linearly with altitude, either inside or outside of the airplane. Pilots understand altitude better than pressure. Thus to display either condition all you need is an altimeter and a rate of climb indicator. The cabin altitude is simply proportional to the 8000 ft value in the same ratio as the actual altitude is to the altitude known as the 8,000 ft cabin altitude (a term meaning the outside pressure altitude at which the cabin has a pressure equal to the standard presure at 8,000 ft). Neither the pilot nor the gauge programmer needs to be concerned with pressure. Just use altitude. For extreme accuracy (more than the FS treatment deserves) you could use a different altitude than the indicated altitude which will, of course, be based on 29.92 in hg for any aircraft operating above 18,000 ft instead of based on the actual msl baro pressure at that spot.

In FS9, you want the cabin altitude to rise and to descend at a given rate and then you want it to hold a given value at a given altitude that is proportional to the ratio of altitude to 8,000 ft altitude. That is what my gauge does. Because you can set the autopilot to climb at a particular real rate, you can esily calculate from the specs what real rate corresponds to a desired cabin climb rate. Then you set the autopilot to climb at that rate. You don't need to worry about what the pressurization system does if you conform to this guide.

The only additional consideration needed in an FS cabin altimeter is whether or not all engines are running. If an engine has a problem, the gauge must show a higher cabin altitude because you must get the aircraft down to a safe altitude for the passengers. That is what my gauges have been doing for many years.

Starting with the Cessna 340 in 1972, manufacturers have recognized that pressurization systems need to be more automatic and involve less pilot activity than they did previously. I am sure most systems today are pretty automatic. It helps for the pilot to climb at a suitable rate.

Let's look at the math. Consider that a cabin climb rate of 500 fpm is best for the compfort of most passengers (who are not pilots and are not used to pressure changes).

The MU-2B has an 8kft altitude of 27300 ft. Therefore its max climb rate should be: 500*(27300/8000) or 1706 fpm.

The Citation II has an 8kft altitude of 43000 ft. Therefore its max climb rate should be: 500*(43000/8000) or 2688 fpm.

Any pilot who can't do this math should be disqualified. It sure isn't rocket science.

[n7rg]

Vickers Viscount panel vc813pl3.zip

This one is very well thought out panel with Full documentation, comprehensive manual and checklist.I have been thinking about adapt parts of it to my
L-188 Electra VC panel. I would like to know you think of the
cabin pressurization system

www.surclaro.com/downloads_c25_150_dateD.html

[Tom Goodrick]

That 4.5 Mb panel file is more than my system can handle, especially when added to a decent aircraft model. Such a system may well work accurately. I don't care. There are people whosimulate in detail the electrical systems, hydraulic systems and fuel systems. I don't care about that level of flight simulation, especially in old and obsolete systems like in the Viscount. As I said, the older pressurization systems demanded some attention. That was part of the job of the flight engineer. To get him out of the cockpit, much of the pressurization system was automated.

I believe we should fly with a cabin altitude gaueg in the panel of pressurized aircraft so that 1) the sim pilot knows there is a difference between the air outside and the air inside and 2) we have the ability of simulating engine emergiencies which become pressurization emergencies requiring the pilot to get the plane to breathable air before resumming the flight with reduced power to an airport for service.

[n7rg]

Hi camg
I came across this, Cabin Pressurisation system working gauges FS9. at Freeflight Design Shop

www.aerodynamika.com/cgi-bin/yabb/YaBB.cgi?num=1262180518

Freeflight Design Shop, is A good source for Information

Rich

[Bill Von Sennet]

Hi Tom,

This is the code for cab_alt 51 which is the gauge used in your twin jet panel that I am using.

- <Gauge Name="CAB_ALT51" Version="1.0TG">
- <Element>
<Visible>(A:Circuit general panel on, bool)</Visible>
<Visible>(G:Var2) 0 == (A:Avionics master switch, bool) &&</Visible>
- <Element>
- <Text X="50" Y="16" Bright="Yes" Length="5" Font="Arial" Color="%('#FF0000' '#00FFFF')" Adjust="Center" VerticalAdjust="Center" Multiline="No" Fixed="No">
<String>%((A:TURB ENG1 BLEED AIR, psi) (A:TURB ENG2 BLEED AIR, psi) + -253.814 / 1 + (A:PLANE ALTITUDE, feet) *)%!5.0f!</String>
</Text>
</Element>
</Element>
- <Mouse>
<Tooltip>%Cabin Altitude TG</Tooltip>
</Mouse>
</Gauge>

How would I modify it for a jet with a pressure differential of 9.3 vs a differential of 8.9 ?

[Tom Goodrick]

A much more common spec for pressurized aircraft than pressure differential, is the "8,000 ft Cabin Altitude" which is the actual aircraft altitude at which the cabin experiences the same pressure as at 8,000 ft.

My gauges are all based on an "8,000 ft cabin altitude." This would agree with the pressure differential but is a different way of expressing it. The "51" in the title of my gaueg means it is set for an aircraft whose 8,000 ft cabin altitude is 51,000 ft. That may well be the differential of 8.9 psi you mention. I don't have any gauge for an altitude higher than that because that is the max altitude allowed for any civilian jet in the US. Normally, the
"8000 ft altitude" is the same as the certificated max altitude of the aircraft.

But if you can get me the "8,000 ft cabin altitude" (or the certified ceiling) for your aircraft, I can give you a cabin altitude gauge for it that fits. Until then the Cab_alt_51 gauge will be pretty good.

[Tom Goodrick]

My Cab_Alt gauges fit the following:
Piston aircraft: P210, 15, 20, 21, 23, 25S
(S for single engine). 15 means 8K alt is 15k.
Turboprop: 25TP, 27, 30, 41T
Jet: 30J, 41, 43, 45, 51, Prem

The purposes of a Cab Alt gauge in FS are:
1) to let you know what altitude the cabin is experiencing;
2) to give you guidance on how quickly you can descend.

If you try to cut the engine power too drastically and descend quickly, the gauge will show a high cabin altitude which should tell you you are hurting your passengers. Do not takw too uch power off at once. Decrease power in increments of about 15 to 20%. Let the cabin pressure respond gradually.

All my Cabin Altitude gauges are based on an engine parameter. The parameter depends on the type of engine - piston, turbo prop or jet. in addition to sudden power reduction, the cabin altitude will respond to shutting down and engine. If you are flying at a high altitude and must shut down one engine, you must get to a lower altitude promptly as the cabin pressure is reduced quickly.

Try one before you decide you must have a different type of pressurization gaueg. This type of gauge does place a burden on the pilot but does not simulate actual gauges and pressurization controls. Most modern pressurization systems require little pilot involvment. Thus my gauge will suffice.

[Bill Von Sennet]

Phenom 100 41,000

Hawker 400XP unknown max ceiling. Sea level cabin to 24,000 9.1 psi The cruise data shows 41,000


King Air C90GT 30,000 max ceiling, but at that altitude Flying magazine shows a cabin altitude of 12,500' 5 psi

Most pilots would prefer an 8,000' cabin to keep from loosing brain cells.

[Tom Goodrick]

If you have any of these in a FLYING article, look at the spec summary carefully. Regardless of what they say in the article, they usually list an "8,000 ft cabin altitude" which is what you need for the gauge. I'll send you a Cab_Air_25TP for the King Air C90 and a Cab_Air_41 for the Hawker 41. Fact is, I'll send you the whole bunch and let you decide.

A statement like "sea level cabin to 25,000 ft" generally means nothing because the pressurization system is generally set up to be proportional to the outside pressure. This gives the 8k cabin at max certified altitude. If you were to set it for sea level cabin, it would change the cabin pressure rate above that altitude.

The value of pressurization is not just in the pressure at a high cruise altitude but in the low proportion of altitude rate of change when climbing or descending. Jet climb rates are normally above 3000 to 6000 fpm in climb and 2000 to 3000 fpm in descent. This would be very painful without pressurization. The pain is the reason pressure rates of change are not adjusted ad hoc by pilots. The pain can be disabling. The rate of change shown by these gauges can be high enough to cause trouble if you cut power too much suddenly. When working properly, the pressurization system will keep the cabin altitude rate of change less than 1/3 the actual altitude rate of change.

I cannot claim my gauge is realistic in showing the pilot's control over pressure. The gauge mainly shows the cabin altitude changes when the pilot has the pressurization system on automatic control. I'll work up a table of cabin altitudes versus real altitudes to show how the systems can work.

For jets, "Certified Ceiling" is determined during FAA certification tests and depends on many different things, often related to stability or controlability. The altitude at which the cabin is at the pressure equivalent to an 8K altitude for the Mitsubishi Diamond II (the forerunner of the Beech/Hawker 400) is 48,000 ft. But the Certified Ceiling of that plane is 41,000 ft. I use the Cab_Alt_51.xml gauge on this aircraft. Perhaps the _45 would be better.

[Tom Goodrick]

I thought it would come in handy some day: the book Flying Jets by Linda D Pendleton covers this topic well in language aimed at pilots rather than engineers or physicists [McGraw Hill, 1996].

First she gives a table of pressure in psi versus altitude:

Alt kft P psi
0 14.7
1 14.17
2 13.66
3 13.17
4 12.69
5 12.23
6 11.78
7 11.34
8 10.92
9 10.5
10 10.11
15 8.29
20 6.754
25 5.45
30 4.36
35 3.46
40 2.72
45 2.14
50 1.68

Then she gives the following example: A Lear 36 has a max pressure differential of 9.4 psi.** If this aircraft is operating at 45,000 ft, add the 9.4 to the value at 45,000 ft ( = 2.14) to get 11.54 and look at the table for the altitude having a pressure of 11.54. We find it is between 6,000 ft and 7,000 ft. That is what the cabin altitude will be. For an aircraft to have an "8,000 ft cabin altitude at 45,000 ft, it needs a max differential of 8.75 psi.

I have tables showing data for altitudes every 1k ft to 65k ft but we really don't need to flood the page with numbers. You get the point.

Pendleton explains that, for jets, bleed air supplies plenty of air continuously into the cabin "box" from which there is a continuous outflow. The outflow cn be regulated insuch a way as to keep a suitable pressure differential up to the max differential. The outflow controller can use various functions but the most common is simply to maintain a linear growth in differential pressure with altitude from zero at the surface to the max differential at the operating altitude. This is exactly what my gauge does. She shows a chart for aircraft and cabin altitudes over a typical jet operation. You could make similar chart using the gauge.

Pendleton further explains the FAA requirements for control and for gauges in pressurized aircraft. There must in general be three gauges: one showing differential pressure, one showing cabin pressure and one showing cabin rate of climb. I am happy with just the cabin altitude. I'll think about making a cabin rate gauge but I suspect most people would think it just clutters up the panel.

Her book is a good basic reader for people learning to fly jets and would not be out of the question for an avid flight simmer. Its retail is $41 but I paid a little less than that. (Amazon might have a used copy.)

**It may be of interest that about 14 years ago, the FAA re-certified the Lear 35/36 series for flight only up to 41,000 ft because a few had mysteriously gone missing while cruising at higher altitudes.

[Tom Goodrick]

Here is The Table we have been needing. If you do not know the "8000 ft altitude," but you know the max cabin pressure differential, this table will tell you the 8000 ft altitude.

For an actual altitude shown in the first column, the table shows the differential pressure required to put the cabin at 8000 ft.

Suppose your plane has a max differential within .05 psi of 8.32 psi. The table shows the 8000 ft cabin altitude is 41k ft.

ALT kft 8K Diff
8 0.00
9 0.41
10 0.81
11 1.20
12 1.57
13 1.93
14 2.28
15 2.62
16 2.95
17 3.27
18 3.58
19 3.87
20 4.16
21 4.44
22 4.71
23 4.97
24 5.22
25 5.46
26 5.70
27 5.92
28 6.14
29 6.35
30 6.55
31 6.75
32 6.93
33 7.12
34 7.29
35 7.46
36 7.62
37 7.77
38 7.92
39 8.06
40 8.20
41 8.32
42 8.45
43 8.56
44 8.67
45 8.78
46 8.88
47 8.97
48 9.06
49 9.15
50 9.23
51 9.31

Unfortunately, this table is not unique. It is based on data from an old textbook of mine - about 4 decades old. There has been a new "Standard Altitude Table" just about every time someone has measured it. I wrote a space flight and Atmosphere entry sim for NASA years ago and had to include four different "atmosphere's" or "standard atmosphere profiles" to give everyone their favorite. They were named based on the year the measurement took place. The variations are not large but might make a difference of a thousand feet or so in the "8000 ft altitude." These differences might be related to the part of the world sampled, the time of year, etc. To assume the Earth has a uniform and constant atmosphere is, of course, ridiculous.