# [PATCH 2/2] Add initial rudimentary no-fly time calculation

Robert C. Helling helling at lmu.de
Fri Feb 8 02:51:25 PST 2013

```On Fri, 8 Feb 2013, Linus Torvalds wrote:

> On Fri, Feb 8, 2013 at 8:39 PM, Robert C. Helling <helling at lmu.de> wrote:
>>
>> After briefly staring at the patch I can say it looks good to me. But I have
>> two questions:
>
> And I have answers. Not that everybody necessarily will *agree* with
> them, but hey..
>
>> 1) It is not clear to me what the reference pressure really should be, I
>> find http://en.wikipedia.org/wiki/Cabin_pressurization not too helpful: When
>> they express it as an altitude, what do they assume about temperature
>> (always 15 degrees or dropping by some amount (0.0065 K) per meter? Also,
>> shall we protect against usual cabin pressure or shall we actually protect
>> against pressure drop in an emergency of the plane (in that case I presume
>> oxygen masks help the average passenger sufficiently against hyoxia but not
>> divers against inert gas forming bubbles at sudden drop of pressure). But I
>> don't know if this makes any relevant difference.
>
> we just came to Sydney from what the *real* pressure in the cabin of
> the plane was.
>
> The answer was that the poor pilot didn't even know. He basically sets
> the pressure not as a real pressure, but as a "altitude in feet". But
> of course, the actual cabin pressurization doesn't really work in
> altitudes (the real altitude is something totally different), so it
> has to really do this with a pressure sensor and compressor, and in
> the end the only thing it can do is to use the standard pressure
> formula.
>
> Which Dirk looked up on Wikipedia, and is what we calculate. So I'm
> pretty sure that we do exactly the same calculation as the pressure
> logic in the plane does.
>
> And it had better be standard temperature, because the temperature in
> the cabin certainly doesn't change.

I did some more googling for this question. It's not easy to find a good
answer as it seems to be the standard in aviation to meassure pressure in
feet. What I found (also regarding what to expect in an emergency)

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFAR.nsf/0/fed94f31539484ab852566720051aa5d!OpenDocument&ExpandSection=2#_Section2

And the table in
http://www.theairlinepilots.com/forum/viewtopic.php?t=505&sid=7c0f1f60624502d74507f9eb045ab589

Given we hard code the height as something (8000ft) we could equally well
hard code 753mbar.

Furthermore, I learned that what the cabin pressurization really seems to
do is to maintain a maximum pressure difference to the outside (wher if
course it is much colder than 15degC).

> As to protecting against pressure drop - it's clear that nobody really
> does the catastrophic pressure thing. You can't. If you're actually
> flying at 33000 feet, and get complete loss of pressure in the main
> cabin, you'll *never* get Buhlmann to give you an acceptable pressure
> by waiting at sea-level, because your pN2 will never go below 0.78. No
> matter how long you wait.

Not neccessarily. With the Buehlmann a and b factors (leave out gradient
factors for the moment) there is a tolerance for an over pressure (partial
pressure of inert gas can be higher than ambient pressure). For example
you can do saturation diving (i.e. an infinitely long dive) at almost
10depth and ascend without deco obligation, as this pressure drop is
within tolerance. I don't know what is the ceiling for a "saturation dive"
at surface pressure (which is what you are hinting at) but I would expect
it to be almost 0bar (as you can go from 2bar to 1bar without deco). You
nofly times and I woudl say this is one.

>> 2) I am not sure, the whole gradient factor thing makes any sense at
>> altitude: GF_high is supposed to be applied at the surface (and that's what
>> is in the formulas) but when you extrapolate to even lower pressures, you
>> get effective gradient factors that are even higher (possibly above 100%).
>> To be safe on that side, I would use GF_low=GF_high=whatever to make the
>> depth/ambient pressure dependence of the gradient factor go away. But I have
>> no good intuition about what a reasonable GF at altitude would be.
>
> Heh. I made the same logical mistake initially, but then corrected
> myself: we are never actually calculating any decompression at any
> lower pressures.
>
> All actual deco calculations are done at sealevel, using normal air.
> No deco calculations at all for lower pressure. Because the whole
> point is, we are *not* in the plane until *after* the no-fly time.

That is not exactly what happens. In your patch it says

+               if (tolerance < flight_pressure)
+                       break;

And in the calculation of "tolerance" we use the gradient factor at the
tolerated depth (or better ambient pressure) not the one where we
currently are. And that is up at 8000ft. That is implicit in the three
line formula in deco.c.

If you want to check for yourself: Play around with GF_low. If we would
really only be at the surface pressure (for the calculation) the results
should be independent of GF_low (and only depend of GF_high). If what I
claim is correct, lowering GF_low (i.e. making it more conservative at
depth) will _shorten_ the nofly time (as we are extrapolating outside the
GF_low -- GF_high pressure interval).

>
> So no, we don't get confused about GF_high at low pressures, because
> we literally only ever use surface pressure, never lower pressures.

> So think of the "no-fly" time as just a surface interval. Which is
> exactly what it is, and it uses the exact same calculation too.

Well, except that the end condtion is about being allowed to go up 8000ft
above sea level and this is where an ambient pressure outside the interval
comes in.

> Which is exactly why I wanted feedback - can I convince somebody
> *else* who knows what they are doing that it is correct?

At least you did not convice me.

Best
Robert

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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics
Scientific Coordinator
Ludwig Maximilians Universitaet Muenchen, Dept. Physik
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