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Pressure Altitude and Terminology



 
 
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Old November 27th 04, 10:14 PM
Icebound
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Default Pressure Altitude and Terminology

In a previous thread:

"Icebound" wrote in message
...


... a look at the altimeter provides you with *indicated*
altitude. It only shows *pressure* altitude if the altimeter setting is
set
to 29.92


"Mike Rapoport" wrote in message
k.net...


All altitudes shown on an altimeter are "pressure altitudes" (as opposed
to
true altitudes) since the altimeter is an air pressure gauge marked in
feet.



"Stefan" wrote in message
...


...the altimeter *always* indicates pressure altitude.
To get useful information, you must set the altimeter appropriately, of
course. Appropriately may or may not mean 1013.



"Julian Scarfe" wrote in message
...

You two are just arguing about terminology. ...



"Andrew Gideon" wrote in message
online.com...

Perhaps I'm missing something basic, but don't we specifically adjust the
kollsman window to convert values provided by a barometric altimeter to a
true altitude (at least below 18,000')?



In Aviation, I would think that there is not much room for deviations of
terminology.

Pressure Altitude
Density Altitude
Indicated Altitude
Actual Altitude (True? altitude)

Each of these mean something specific and different. The definitions and
terminology are not negotiable.

----------------------------------------

*Pressure Altitude* has a very specific definition... and per se, has
nothing to do with your altimeter.

It is the altitude at which your airplane is (flying or standing), if and
only if the atmosphere met the conditions of the standard ICAO atmosphe
.... the atmospheric pressure at sea level is 29.92 inches of mercury (1013
mb)
..... the temperature at sea level is 15 degrees Celsius.
.... standard ICAO lapse rate prevails. (Hence the temperature at your
location would be the appropriate temperature for that level per the ICAO
atmosphere.)

In other words, it is an instant conversion FROM the actual *pressure* that
your aircraft is (flying or standing) at, TO a number which is totally
hypothetical (called the Pressure Altitude), but which represents that
pressure EXACTLY under known atmospheric conditions.

Now, because our altimeter is calibrated for the ICAO atmosphere, and
because it converts pressure to an altitude display, it turns out we can use
the altimeter to make this *pressure-to-altitude* conversion for us. When
the conversion is made correctly with the Kollsman window ("altimeter
setting") set to 29.92 (1013), then and only then is the resulting altitude
display known as *pressure altitude*.

Why do we care about pressure altitude?

Because aircraft performance numbers in the POH are based on *pressure
altitude*.
Also, for certain performance criteria, we need to know *Density* altitude.
*Density* Altitude if a correction applied to *Pressure* altitude to account
for the temperature. In order for the E6-B computer to do that correctly,
it needs a proper *Pressure* altitude to start.



In North America, most of us do not fly at *pressure* altitudes.... The
pressure at sea level is rarely 29.92 (1013). So, below 18,000 feet, we
adjust the altimeter by means of the "altimeter setting". When the real
pressure is higher, we adjust so that we fly a little lower... when the real
pressure is lower, we adjust so that we fly a little higher. We do this so
that obstacle clearance is more-or-less the same, no matter what the
pressure.

I say "more-or-less" because such an adjustment still does not take account
of the differences between our actual environment and the ICAO atmosphere
for which the altimeter is calibrated.

Thus we are flying at an *indicated* altitude; an altitude that has been
adjusted to account for the current pressure by means of using the
*altimeter setting* adjustment, but still not the true, actual altitude.

We DO fly at "pressure altitudes" above 18,000 feet, because obstacle
clearance is not an issue for the most part (and it is difficult to maintain
the altimeter setting current at the speeds that we use up there). We DO
fly at pressure altitudes in the sparsely-settled areas of Canada, the
Arctic... because, again, it is difficult to maintain a reliable current
altimeter setting, due to insufficient weather stations.


Because the altimeter instrument does not account for true atmospheric
temperatures, *Indicated Altitude* is only *accurate* at the surface of the
station whose sea level pressure corresponds to the *altimeter setting*.
We don't really care that it is not the *actual* altitude because we all
follow exactly the same procedures and rules, and so our vertical resolution
is assured.


Knowledge of our *actual* altitude may be an issue where we have critical
obstacle clearance requirement, especially in very cold temperatures. To
get our *actual* altitude, a temperature correction has to be applied to the
*indicated* altitude, to account for the difference between the ICAO
standard atmosphere and the actual conditions.

Pressure Altitude
Density Altitude
Indicated Altitude
Actual Altitude

The definitions and terminology for aviation are not negotiable.




 




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