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Old July 13th 10, 02:29 AM posted to rec.aviation.piloting
betwys1
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Posts: 23
Default Where to post questions?

On 7/12/2010 12:34 PM, WhenTurtlesFly wrote:
'Cause I have one- Filed to cruise at 8500. Two GPS's (old simple one
in the plane and my new portable) say I'm at 8500, but pressure
altimeter says I'm at 8000. I of course set my altimeter to departing
airport elevation, which should calibrate it to current pressure and
nonstandard temperature, right? Do I...

* fly what seems to be accurate and correct readouts on both of my
GPS's? Then risk my altimeter being off and my Mode C reporting
incorrectly...

* fly GPS's and adjust altimeter to read the GPS altitude, even though
the altimeter setting is not what is reported?

* fly the altimeter set at reported conditions, have my Mode C report
accurately, but run the risk of being off VFR altitude?

Thanks




If you are VFR, getting Flight Following, or talking to a Center,
they will be expecting you to be flying a heading of 180 to 359
(semicircular rule) at that altitude, using the Kollman setting they
give you, or a current value from a station en-route. Their computers
can take care of density versus pressure considerations, and can massage
the standard day altitude reported by your transponder
against the altitude you report verbally.

Important to remember that the pressure versus altitude indicated in the
cockpit has no strong relation to your ACTUAL altitude - the graph of
pressure vs altitude used by barometric altimeters is computed for mid
latitudes for an assumed temperature lapse profile. The GPS altitude has
its own inaccuracies, but that's a different ball game

Brian W