FSX [Pressurization]

I recently came across the [Pressurization] section in the FSX FA-18 aircraft.cfg file even though the aircraft doesn't appear to have any pressurization gauges on the panel. I thought I'd try to figure out the mathematics of this section and came up with the following:

[Pressurization] ;FA-18 cabin alt 0 ft for flt alts from MSL to 11,000 ft, increasing to design cabin alt 8,000 ft at flt alt 23,000 ft

design_cabin_pressure=10.908350 ;= PSI at design cabin alt 8,000 ft (PSI = millibars * 0.0145)

max_pressure_differential=4.960 ;= PSI at 8,000 ft - PSI at 23,000 ft, also = PSI at MSL - PSI at 11,000 ft

My calculations indicate that the FA-18 would reach the given max_pressure_differential of 4.960 and design cabin altitude of 8,000 ft at a flight altitude of about 23,000 feet, above which the pressure_differential should remain at 4.960 while the cabin altitude increases by 1,000 ft for each 1,000 ft of flight altitude above 23,000 ft. At flight altitude 35,000 ft (12,000 ft above design flight alt 23,000 ft) the cabin altitude should be about 20,000 ft (12,000 ft above design cabin alt 8,000 ft). Can anyone tell me if this is correct ?

I thought I'd add the [Pressurization] section to my B727 aircraft.cfg even though there are no working pressurization gauges on the 727 panels I use.

[Pressurization] ;B727 cabin alt 0 ft for flt alts from MSL to 22,000 ft, increasing to design cabin alt 7,900 ft at flt alt 42,000 ft

design_cabin_pressure=10.969350 ;= PSI at design cabin alt 7,900 ft (PSI = millibars * 0.0145)

max_pressure_differential=8.500 ;= PSI at 7,900 ft - PSI at 42,000 ft, also = PSI at MSL - PSI at 22,000 ft

and to the HS748 which does have working pressurization gauges on the panel (which worked fine before I added this section to the aircraft.cfg):

[Pressurization] ;HS748 cabin alt 0 ft for flt alts from MSL to 12,500 ft, increasing to design cabin alt 8,000 ft at flt alt 25,000 ft

design_cabin_pressure=10.908350 ;= PSI at design cabin alt 8,000 ft (PSI = millibars * 0.0145)

max_pressure_differential=5.500 ;= PSI at 8,000 ft - PSI at 25,000 ft, also = PSI at MSL - PSI at 12,500 ft

Flying the HS748 at flt alt 30,000 ft (5,000 ft above design flight alt 25,000 ft) the pressure differential read 5.500 and the cabin alt 13,000 ft (5,000 ft above design cabin alt 8,000 ft).

Concorde appears to be:

[Pressurization] ;Concorde cabin alt 0 ft for flt alts from MSL to 32,000 ft, increasing to design cabin alt 6,000 ft at flt alt 60,000 ft

design_cabin_pressure=11.772405 ;= PSI at design cabin alt 6,000 ft (PSI = millibars * 0.0145)

max_pressure_differential=10.700 ;= PSI at 6,000 ft - PSI at 60,000 ft, also = PSI at MSL - PSI at 32,000 ft.

I'm sitting in the sun, so I'm not going to get the calculator out. And i only read the first half of your post.

Just one thing I spotted.

Lets say you are at 35.000 ft , and cabin alt is 8.000 ft. Then the alt differential is 27.000 ft.

Btw, to my surprise I easily pulled the fa-18 up to 45k yesterday, with full fuel.

That was the fa-18_combat though, an addon. Freeware.

Looked other worldly up there:)

il88pp

You said

"Lets say you are at 35.000 ft , and cabin alt is 8.000 ft. Then the alt differential is 27.000 ft."

I said

"At flight altitude 35,000 ft (12,000 ft above design flight alt 23,000 ft) the cabin altitude should be about 20,000 ft (12,000 ft above design cabin alt 8,000 ft)."

Think you might have had too much sun.

most airliners maintain a cabin altitude of 8000 ft during cruise.

That means the pressure in the cabin is not sea level pressure, but the pressure you would have in ambient air at 8000 ft.

At that pressure air is still perfectly breathable.

Airliners maintain this pressure during the climb when they pass 8000 ft, up to cruise at for example 37.000 ft.

It seems you read somewhere a aircraft (f18) has a cabin pressure (/cabin altitude) of 8000ft when flying at 23.000

You have assumed that when flying higher the cabin altitude will rise as well.

That assumption was wrong (at least in arliners). When the plane climbs further, the cabin altitude will not rise further. (The airpressure inside does not decrease further.)

The pressurisation system just kicks up a notch, and keeps the airpressure at a comfortable 8000ft.

(and that is why I said: "Lets say you are at 35.000 ft , and cabin alt is 8.000 ft. Then the alt differential is 27.000 ft."

(I will leave conversion to psi to you, that would take me to long to look up.)

Here's a good read, wiki,

http://en.wikipedia.org/wiki/Cabin_pressurization

I found this in the FA-18 E/F NATOPS manual. Pub Number A1-F18EA-NFM-000

7.6.3.1 Cruise Check.

1. Cabin altimeter - MONITOR

Aircraft Altitude Cabin Altitude

Less than 8,000 feet Ambient

8,000 to 24,500 feet 8,000 feet

Greater than 24,500 feet Alt x 0.4 (rule of thumb)

Caution

A slowly increasing cabin pressure altimeter may be the first or only

warning of a gradual loss of cabin pressurization

The FA-18 A/B/C/D model manual (A1-F18AC-NFM-000) reads:

"1. Cabin pressurization/temperature - MONITOR

During cruise, check cabin pressurization/temperature control. Pressurization shall remain at 8,000

feet up to 23,000 - 24,000 feet altitude. Above 23,000 to 24,000 feet altitude, cockpit pressurization

shall follow schedule in figure 2-37.

AIRCRAFT ALTITUDE CABIN ALTITUDE

30,000 feet 10,000 to 12,000 feet

40,000 feet 15,000 to 17,000 feet"

, by the way.

Not much help for the rest of the planes, but at least it's something.

Maybe you can do the math off that, if you want. I don't know why, but space I keep inserting to separate the columns under each heading, Aircraft Altitude Cabin altitude

for example, don't show up so you have to read them kind of carefully...

Pat☺

I don't know why, but space I keep inserting to separate the columns under each heading, Aircraft Altitude Cabin altitude

for example, don't show up so you have to read them kind of carefully...

Pat☺

You need to wrap that text in code tags to preserve the formatting.

AIRCRAFT ALTITUDE              CABIN ALTITUDE
30,000 feet                    10,000 to 12,000 feet
40,000 feet                    15,000 to 17,000 feet

peace,

the Bean

Stringbean,

If you ever become unavailable to the flight sim community for ANY reason whatsoever, an incredible wealth of knowledge will have been lost.

Thank you very much for the help. I appreciate it!

Pat☺

I said earlier "23,000 feet, above which the pressure_differential should remain at 4.960 while the cabin altitude increases by 1,000 ft for each 1,000 ft of flight altitude above 23,000 ft.". This was based on the result from the flight in the HS748 and at the time seemed to be logical and would have put your cabin altitude at 30,000 ft at flight altitude 45,000 ft.

The maths seem to indicate this is incorrect. If the pressure differential is at max 4.960 at flight altitude 45,000 ft, the cabin altitude should be between 18,000 and 19,000 ft not 30,000 ft.

This seems to agree with the FA-18 manual quoted below.

What I said above was obviously incorrect. I assumed 'airliner' but obviously the rules for airliners do not apply to the F18.

Pantom_Tweak knows much more about he F18 then I do.

apologies for giving wrong info.