Cabin Altitude & Differential Pressure Equation?

[PA32]

Some of the Aircraft Flight Manuals I have contain a "Cabin Altitude for Various Airplane Altitudes" graph in them. Typically, the Aircraft Altitude is on the y-axis, going from 0 to about 50,000 feet; and the Cabin Altitude is on the x-axis. Then there are a series of diagonal lines on the graph which are labeled 0 Diff Press (delta p), 1 PSI Diff Press, 2 PSI Diff Press, on up through about 10 PSI delta p. (Which is about as good as the best pressurization system gets on the fanciest bizjets.) The 0 delta p line is perfectly linear (of course), and the other delta p lines, below about 5 PSI delta p, are "close to" linear at the lower altitude ranges (below about 25,000 feet). Above this, the lines get progressively more non-linear.

Can anyone tell me what the equation is that determines the values on this chart? i.e., if I did not have the chart handy, and if I knew my current aircraft altitude (in feet), and my current cabin differential pressure (in PSI), and wanted to calculate my predicted cabin altitude (in feet) based on these factors, what equation would I use?

 

[mfb]

The 0 delta p line is perfectly linear (of course)

This is not trivial, it depends on the scaling of the axes.

Can anyone tell me what the equation is that determines the values on this chart? i.e., if I did not have the chart handy, and if I knew my current aircraft altitude (in feet), and my current cabin differential pressure (in PSI), and wanted to calculate my predicted cabin altitude (in feet) based on these factors, what equation would I use?

Pressure outside roughly follows an exponential function, see Wikipedia for an introduction.

 

[NatSusan]

Thank you for sharing this information about the cabin altitude graph in aircraft flight manuals. I am not an expert on this topic, but I can offer some insights based on my understanding.

The equation used to determine the values on this chart is likely specific to each aircraft model and its pressurization system. It takes into account factors such as the aircraft's altitude, cabin differential pressure, and other variables that affect cabin pressure.

To calculate your predicted cabin altitude, you would need to consult the flight manual for your specific aircraft model and use the equation provided. Alternatively, you could also use a digital cabin altitude calculator, which is a tool designed to calculate the predicted cabin altitude based on the current aircraft altitude and cabin differential pressure.

I hope this helps. Perhaps someone with more expertise on this topic can provide more detailed information.