On Jul 12, 1:34*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

--
WhenTurtlesFly

Everyone in the airspace should be flying with reference to the
(correctly set) altimeter and you should too. The fact that you may
know your exact altitude by other means is not relevant when you're
cruising en route to somewhere -- the idea behind using the same
reference instrument is traffic avoidance.

You'll have a measure of the altimeter's accuracy when you're at an
airport and set the altimeter to the local barometric pressure. As
best I remember, the altimeter should be within 75 feet of the
reported field elevation -- be sure you know the field elevation where
your airplane is, airports are mostly not level, and the minor changes
in field elevation can eat into your error budget.