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

In article ,
says...

'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?

Dunno why you ask. It's simple. Just pretend you have no GPS. In
short, if you are VFR, you fly according to the reported local area QNH.

The consequences, I guess, are that you will not otherwise be correctly
relative to other VFR traffic.

--
Duncan.

"Stephen!" wrote:
Jim Logajan wrote in
:

His new portable would almost certainly use WAAS when available and
would not be off by 500 ft.

It could very well be 'off' by that much quite easily. Keep in mind
that the earth is neither smooth nor a sphere. Depending on where you
are on the theoretical sphere that the GPS uses for its calculations,
its altitude calculation could be off from actual AGL by quite a bit.
I saw that quite frequently on Guam where I would be in a boat and the
GPS would swear we were a couple hundred feet below sea level.

I don't believe WAAS is yet available in Guam.

In article ,
WhenTurtlesFly wrote:

(until theairliner
comes by with the radar altimeter...)

btw - The radar altimeter isn't used at altitude.

On 7/12/2010 8:26 PM, WhenTurtlesFly wrote:
Thanks for all the thoughtful replies! Somewhat gratifying to know
there is some discussion over my confusion.

I'll fly altimeter and hope everyone else is equally off (until the
airliner comes by with the radar altimeter...)




Radalts are used in the approach/landing phase. Some don't register
above 2500 AGL. Do you really fly?

Brian W

betwys1 wrote
Radalts are used in the approach/landing phase. Some don't register
above 2500 AGL. Do you really fly?

My C-172 partner is a Private Pilot with over 1000 hours of flying
experience and I doubt very seriously if he has ever heard of a
Radio Altimeter.

Bob Moore