Below sea level

Hi,

How is standard temperature and pressure defined below sea level? How does
DA increase/decrease below sea level? I know that the gravitation force
starts decreasing in a *linear* fashion below sea level, but how that affect
the temp, pressure, and DA is beyond me. Any books, or URLs that could help
me out here?

Thanks!

Hilton

"Hilton" wrote in message
ink.net...
How is standard temperature and pressure defined below sea level?

Same as above sea level. Take the standards at sea level, apply a standard
adjustment, viola.

How does DA increase/decrease below sea level?

Same as above sea level, except in the opposite direction. Below sea level,
pressure altitude decreases, which decreases density altitude. Of course,
temperature has the same effect regardless of altitude; high temperature
increases density altitude, low temperature decreases density altitude.

I know that the gravitation force
starts decreasing in a *linear* fashion below sea level

As far as I know, the difference in gravity has to do with the amount of
mass under you, and is negligible with respect to temperature or pressure.
Same thing would be true of high-elevation terrain.

but how that affect the temp, pressure, and DA is beyond me.

I do not believe that the change in gravitation force is significant for the
purposes of piloting an aircraft.

Basically, nothing unusual happens just because you went below sea level.
"Mean sea level" is simply a convenient reference point. Just as you could
set your reference point to what we now call 4000' MSL, and then find that
density altitude simply decreases below that according to pressure altitude,
and temperature increases or decreases density altitude in the same way it
does at any altitude...the exact same thing happens below sea level.

I haven't seen any calls to adjust density altitude to account for higher
gravitational forces at higher altitudes, nor would I expect to see any call
to do so at extremely low altitudes. Note that we also don't compensate in
our calculations for changes in gravity due to changes in our flight
altitude above ground level.

As far as us pilots are concerned, gravity is exactly the same everywhere.

Pete

Seems to me that the places where you could fly below MSL are very
hot. Wouldn't that affect the aircraft more than the altitude, which
in any event would not be very many feet below 0 MSL?


-- all the best, Dan Ford

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On 2005-06-13, Peter Duniho wrote:
"Hilton" wrote in message
ink.net...
How is standard temperature and pressure defined below sea level?

Same as above sea level. Take the standards at sea level, apply a standard
adjustment, viola.

On a point of pedantry, what does a stringed musical instrument, a
little larger than a violin, have to do with the subject at hand? :-)

I know that the gravitation force
starts decreasing in a *linear* fashion below sea level

As far as I know, the difference in gravity has to do with the amount of
mass under you, and is negligible with respect to temperature or pressure.
Same thing would be true of high-elevation terrain.

Especially since airports below MSL aren't that far below MSL anyway
(IIRC, the 'deepest' airport in the continental US is only about -200 MSL,
but the highest, Leadville, has an additional 9000 ft of rock below it).

I've been to a below MSL airport - Thermal (Palm Springs), California.
I didn't notice any difference. But it was kind of neat watching a
correctly set altimeter go below 0 feet :-)

--
Dylan Smith, Castletown, Isle of Man
Flying: http://www.dylansmith.net
Frontier Elite Universe: http://www.alioth.net
"Maintain thine airspeed, lest the ground come up and smite thee"

Actualy, the places I know of are not very hot but rather cool and
humid.
I'm serious here, it is a small country called The Netherlands which is
for about one third below actual seal level.

-Kees

"Hilton" wrote in message
ink.net...
Hi,

How is standard temperature and pressure defined below sea level? How
does
DA increase/decrease below sea level? I know that the gravitation force
starts decreasing in a *linear* fashion below sea level, but how that
affect
the temp, pressure, and DA is beyond me. Any books, or URLs that could
help
me out here?


http://www.faa.gov/atpubs/SWO/chapter_14Sect2.htm

but it only has the standard pressures down to about -2700 feet, no
temperatures. Increments of .01 inches of mercury.


The following is much more coarse.... every 500 feet:

http://www.sablesys.com/baro-altitude.html

but it shows pressures AND Temperatures down to -5000 feet

On Mon, 13 Jun 2005 05:34:13 -0400, Cub Driver
wrote in
::

in any event would not be very many feet below 0 MSL?

Bad Water, the lowest point in the western hemisphere, located in the
Death Valley is 282' below sea level.
http://www.virtualtourist.com/travel...ater-BR-1.html

The lowest place in the world is the Dead Sea at 1,395' below sea
level: http://www.extremescience.com/DeadSea.htm

Furnace Creek, minus 210.

Jim


(IIRC, the 'deepest' airport in the continental US is only about -200 MSL,
but the highest, Leadville, has an additional 9000 ft of rock below it).

Dylan Smith wrote:
On 2005-06-13, Peter Duniho wrote:
Same as above sea level. Take the standards at sea level, apply a standard
adjustment, viola.

On a point of pedantry, what does a stringed musical instrument, a
little larger than a violin, have to do with the subject at hand? :-)

Darn! You beat me to it :-)

Actually, it's an American custom that you might not have picked up
in your time here. We usually have our Koch chart taped to the back of
our violas (the instrument, not the flower).

John Galban=====N4BQ (PA28-180)

Seems to me that the places where you could fly below MSL are very
hot.

I made it a point to fly over Death Valley on the way back from SFO,
just so that I could fly below sea level. I wanted to see the
expression on my altimeter's face.

It looked bewildered.

It was August; temp was well over 100 F. I decided not to land.

vince norris