Below sea level

"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

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"

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)