Jim Logajan wrote:
Mxsmanic wrote:
WhenTurtlesFly writes:

'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...

No. The altimeter is more accurate than the GPS units. The fact that
the two GPS units agree simply means that they are both subject to the
same error when operating in the same place at the same time.

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

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

No. The altimeter is more accurate than the GPS units.

WAAS is accurate to 25 ft vertically 95% of the time. Standard aneroid
altimeter have no adjustment for changes from non-standard temperature. A
temp difference of ~4 degrees Celsius from standard atmosphere would show a
500 ft difference.

The temperature correction is affected by height. The rot is 4 feet per
thousand feet for each degree C of difference from standard. A 500 ft
difference at 4 degrees would only be true at 30,000 feet. So at my 250
foot high airport, it would have to be -35C to get my altimeter to be
50 feet high. I don't think I've ever seen it more than 20 feet off when
set to the broadcast setting on the ground.

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

Yes. Fly the altimeter. The risk of being 500 feet off is virtually
nil if you've set the altimeter correctly to begin with.

GPS provides very poor vertical accuracy compared to a barometric
altimeter. GPS is designed for lateral accuracy rather than vertical
accuracy.

Which is why WAAS was developed.

--
Don Poitras