PCool wrote:
Thanks Marc,

is it correct to say that the Pressure Altitude is an altitude calculated
starting from a pressure value, following a sort-of a rule as for ICAO-ISA ?
I guess official IGC loggers read the exact pressure as garmins and suunto
watches (!) and then they apply some calculations and name this result as
"altitude".

Yes

Do they do this without looking at what the GPS say?? Not even for an hint?

Yes, the calibration is fixed at the time the recorder leaves the
manufacturer, and subsequent visits to the calibration lab simply
provide you with the data to manually correct the original calibration.

What is the formula used by all IGC loggers for doing this, then?

The details can be found here, in the section Standard Atmosphere and
Altimetry:

http://williams.best.vwh.net/avform.htm#Altimetry


It's beyond my comprehension why if we are talking about pressure which is
always measured in the same way (right?) then this value has different
meanings and cannot be simply converted like with QNE-QNH-QFE.
ICAO-ISA is sort of a more complicated QNE, right? (question!)

The ISA model assumes a standard lapse rate (0.0065°C/m) below the
tropopause (11.0 km), the real atmosphere is more complicated, which is
what causes the error. Altimeters are mechanical computers which do a
simple ISA to indicated altitude conversion, flight recorders do it in
software (but use the fixed 1013.2 hPa altimeter setting), in the end
they all have the same errors relative to actual height on days when the
lapse rate differs from the standard (which is every day). Calculating
QNH requires an inverted application of the ISA conversion, the weather
guys do it in the privacy of their offices.

On garmins you have a pressure sensor just like on a Colibrì, then this
sensor is used to compensate the gps and vice-versa, according to the patent
they have registered. By the way Marc could you understand anything useful
out of it?

Yes, they are describing in very mechanical terms (which is how you get
a patent on a software process) how to use GPS altitude to continuously
recalibrate the pressure sensor, such that you obtain actual height (but
not ISA/QNH indicated altitude) without the short term noise normally
present in GPS altitude. This is what was discussed earlier in the thread.

After 4 years there are again the same questions on this matter so I guess
it's not very clear to everybody (me too).

I have to think about it myself every time it comes up, I can never
remember if altimeters read high or low on hot days. And, I still can't
keep my Q codes straight, which I no doubt demonstrated above...

Marc