Where to post questions?

[WhenTurtlesFly]

'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

[Jim Logajan]

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.)

[Mxsmanic]

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.

[Jim Logajan]

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.

[[email protected]]

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.

[a[_3_]]

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.

[Mxsmanic]

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.

[Don Poitras]

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

[Jim Logajan]

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.

[WhenTurtlesFly]

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...)

[betwys1]

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

[Dave Doe]

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.

[Jim Logajan]

"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.

[Bob Noel[_6_]]

In article >,
WhenTurtlesFly > wrote:

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

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

[betwys1]

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

[Bob Moore]

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