Cabin Altitude Corrected

[Tom Goodrick]

The other day I was thinking about how it would probably be unsafe for me to travel on an airline due to my heart condition. The 8,000 ft cabin altitude at cruise would not be good for my heart that has lost 30% of its muscle. So how do all the ambulance jets carry patients who may have heart deficiencies? I looked at cabin altitude using one of my gauges. It was much higher than it should have been with the jet flying at 30,000 ft.

I decided it was time to fix those gauges. They started out with a good, proper formula. But then that formula became corrupted as I tried to adjust it for loss of power - shutting down one engine. The result was that while being right in the special case of flight with an engine out, it was wrong all the time in normal flight. A consequence was that it was tied to a specific engine for which a particular bleed air pressure was observed at cruise. Using the gauge in a different aircraft made the gauge erroneous.

So I decided to make the gauge right during all normal. The formula is a simple one that just makes the cabin pressure ratio to 8000 ft the same as the ratio of actual altitude to the "8000 ft altitude" of the cabin - a common spec that gives the actual altitude at which the cabin has a pressure altitude of 8000 ft.

In FLYING JETS by Linda Pendelton, she states the common program for pressure is a simple proportion.

The gauge for many high-performance jets is CAB_ALT51 which should give an 8,000 ft cabin when the aircraft is cruising at 51,000 ft. These aircraft include the LJ31, :J45, LJ60, Citation 680 (Sovereign) Gulfstream IV and the new Gulfstream 100 (formerly IAI Astra).

To fix the gauge, (do this without running FS9) change the gauge CAB_ALT51.xml to CAB_ALT51.txt so you can edit it using Note Pad. The line that begins and ends with <String> and ends with </String> is the line that does the calculation and prepares it for display. Change it so it looks like the following:

<String>%((A:PLANE ALTITUDE, feet) 8000 * 51000 /)%!5.0f!</String>

This multiplies the plane altitude by 8000 and then divides it by 51000. That gives the cabin altitude the proper proportion.

Here is a check during climb to 51,000ft.

ALTITUDE___CAB ALTITUDE
0__________0
10000______1570
20000______3136
30000______4705
40000______6275
50000______7842
51000______8000

During the descent I checked the effective descent rate. At an actual descent rate of 2000 fpm, the cabin descent rate was 324 fpm. At 3000 fpm, the cabin descent rate was 470 fpm.

I have usually used 2000 fpm but 3000 fpm would be fine, keeping the airspeed at a safe value.

It is clear that a private jet flying at 20,000 ft or 30,000 ft would give the heart patient a safer ride. Now it is possible that the pressure could be set higher in the cabin, up to the max pressure differential. That's a calculation for another day.

All other CAB_ALT gauges can be corrected the same way, regardless of how the original calculation was made. The number after CAB_ALT is the 8000 ft altitude. Use that number in place of the 51000 in the calculation above.

[flaminghotsauce]

Cool. This is good for realism, but would be really fun is if there was a negative effect for not obeying the cabin altitude. Like the sceen dimming away as one "loses consciousness" or something. Some of those aircraft with the copilot should have that copilot slump forward and pass out or something, maybe start talking gibberish.

I usually don't fly that high, but when I do I don't even bother with the cabin altitude. There's no damage in ignoring it.

[Tom Goodrick]

The Cabin Altitude indication is intended to show how the altitude in the cabin of a pressurized aircraft changes when the pressurization system operates normally. If "you don't obey it," then either you did not turn on your pressurization system or you do not have one.

One poorly done aspect of Flight Simulator is that they give you several aircraft - the Mooney Bravo and the two Caravan models - that can fly well above 12,500 ft but normally would not do that when carrying passengers because oxygen masks would be required for all the passengers, providing oxygen from individual tanks or from a large tank with a distribution and regulation system. When you fly IFR with ATC in FS9, you hear radio call to the Mooney and to the Cessna's (Caravans). In real life they would seldom fly that high because being up where oxygen is required is dangerous, particularly if passengers must manage their own oxygen masks.

One aspect of flight above 12,000 ft in an unpressurized aircraft is that descents must be performed no faster than 500 ft per minute to avoid injury to the passengers. This requires considerable planning. As I showed above, when you have a pressurization system the cabin sees a slower descent rate than the actual airplane does so you can come down much faster - 1500 fpm in a P Baron, 2500 fpm in a turboprop or 3500 fpm in a jet. This makes it much easier to fly at the higher altitudes in these planes.