Where to post questions?

'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 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...

U.S. FAR 91.121(a) implies flying by reference to an altimeter whose output
can be adjusted from station readings or actual airport elevation. I doubt
either of your GPS's allow such adjustment settings, so you probably can't
fly VFR cruising altitudes by reference to them.

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

Regulations do not appear to allow setting an altimeter to anything other
than a barometric altimeter setting from a station within 100 NM, other
appropriate station, or to actual elevation of a departure airport.

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

Unless your altimeter is way out of calibration (in which case you probably
shouldn't be flying that airplane until the problem is corrected,) then
everyone else near your aircraft should in theory be flying with similar
offsetting temperature and pressure differences, so long as they are all
"off" in the same way.

(Unfortunately it isn't hard to think up cases where the above fails to
work out.)

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.

* 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.

* 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.

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.

* 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.

Mxsmanic wrote:

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

Babbling nonsense when talking about aviation GPS.



--
Jim Pennino

Remove .spam.sux to reply.

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.

Jim Logajan writes:

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

WAAS is independent of GPS. It's one of many GPS augmentation schemes.

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.

So if you depend on your WAAS GPS for altitude, you might collide with traffic
that is depending on barometric altitude.

Which is why WAAS was developed.

WAAS was developed to improve GPS accuracy overall, not to allow GPS to
replace traditional barometric altimeters.

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

Mxsmanic wrote:
Jim Logajan writes:

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

WAAS is independent of GPS. It's one of many GPS augmentation schemes.

WAAS cannot simultaneously be independent of GPS and an augmentation of it.
Only the second of your two quoted sentences is accurate in any useful way.

While your conclusion that he should use the altimeter set to the nearest
station setting was correct, the rationale leading up to it was based on an
invalid premise.

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...)