[rjcthree]

How 'good' is the altitude reported by GPS (on a 496)?

I am panel planning, and as far as I can tell, for my night VFR 9A, my Dynon will be primary Alt ind, the 496 secondary Alt ind.

I plan on having a steam ASI secondary to the D180 ASI, since the GPS can't give me a good airspeed that I can trust on approach.

Thanks, Rick
9A fuse, panel planning, starting to think about running wire.

rest of panel:
D180, HS34, AP74 (all dynon), 496, SL40, GTX327, Hobbs, oil pressure idiot light, intercom, CO monitor, round ASI, switches, breakers. Longer term (read first annual) the ASI gets replaced by another Dynon, networked(maybe, I want to fly it for a year, see if I really need that much info on constant display - with smart warning settings, I think maybe not.)

 

[n5lp]

How 'good' is the altitude reported by GPS (on a 496)?...

The 496 altitude is a GPS altitude and tends to be quite good for actual height above sea level. Height above sea level is not what the barometric altimeter shows though. The altimeter, if set correctly, will show near actual MSL at the airport it is set for. At all other altitudes, it will most likely be in error, which is usually not important when everyone uses the same data for that area.

For a backup, it may pretty good, but it is not the same as the instrument for the purpose.

 

[N941WR]

Larry brings up a good point. All the planes out there are flying based on their altimeters, not GPS, which could cause you a few problems if flying by GPS altitude.

Your transponder reports based on your altimeter, not the GPS. Thus if you were flying a GPS altitude, it is possible to bust class Bravo airspace when flying above or below a shelf. it is also possible to be off enough to be cruising at the wrong altitude.

 

[N184JG]

Larry brings up a good point. All the planes out there are flying based on their altimeters, not GPS, which could cause you a few problems if flying by GPS altitude.

Your transponder reports based on your altimeter, not the GPS. Thus if you were flying a GPS altitude, it is possible to bust class Bravo airspace when flying above or below a shelf. it is also possible to be off enough to be cruising at the wrong altitude.

The transponder reports pressure alt. at 29.92 only.

 

[cholladay]

Our 496 gets the nearest altimeter setting and uses it. I think it gets it from the XM weather. Our 496 reads the same as the altimeter.
Or at least the one I fly does.
chad

 

[N941WR]

The transponder reports pressure alt. at 29.92 only.

Correct, but the point is, this would be different than the GPS.

Our 496 gets the nearest altimeter setting and uses it. I think it gets it from the XM weather. Our 496 reads the same as the altimeter.
Or at least the one I fly does.
chad

This assumes the GPS has XM weather turned on. If no, well, it's anyone's guess.

 

[rjcthree]

Ok, now I'm confused

I understood that the 496 and it's siblings gave you GPS based MSL - nothing to do with referencing Baro data or local static pressure - if that was the case, simple cabin pressure variations brought on by ventilation could swing in or out of the vfr lanes/class C airspace.

And I am talking a VFR backup here. +/- 100 feet is fine(and the goal), +/- 1000 feet is not.

Rick

 

[N395V]

Lifted from Wikipedia MSL

To extend this definition far from the sea means comparing the local height of the mean sea surface with a "level" reference surface, or datum, called the geoid. In a state of rest or absence of external forces, the mean sea level would coincide with this geoid surface, being an equipotential surface of the Earth's gravitational field. In reality, due to currents, air pressure variations, temperature and salinity variations, etc., this does not occur, not even as a long term average. The location-dependent, but persistent in time, separation between mean sea level and the geoid is referred to as (stationary) sea surface topography. It varies globally in a range of ? 2 m.

Traditionally, one had to process sea-level measurements to take into account the effect of the 228-month Metonic cycle and the 223-month eclipse cycle on the tides. Mean sea level does not remain constant over the surface of the entire earth. For instance, mean sea level at the Pacific end of the Panama Canal stands 20 cm higher than at the Atlantic end.

Despite the difficulties, aviators flying under instrument flight rules (IFR) must have accurate and reliable measurements of their altitudes above (or below - see Schiphol Airport) mean sea level, and the altitude of the airports where they intend to land. That problem can compound when landing on an aircraft carrier in a gravity anomaly. In aviation mean sea level is increasingly being defined according to the reference ellipsoid defined by the World Geodetic System. Compared to a geoid, an ellipsoid is simpler to model mathematically and therefore lends itself to use with the Global Positioning System.

 

[mburch]

Here's a previous answer to this question that comes straight from the experts:

http://www.vansairforce.com/community/showpost.php?p=166965&postcount=5

mcb

 

[L.Adamson]

Here's a previous answer to this question that comes straight from the experts:

Regarding the text from that Garmin statement:

What this means is that your GPS derived altitude is almost always more accurate than your altimeter. However, since everyone uses a pressure-based system to determine altitude, you need to fly using the pressure altimeter to ensure the required vertical separation.

I'm always comparing my Garmin GPS's to known elevations such as the Great Divide, National parks, and summits of mountain highways. The GPS is usually always within 0 - 20'; which is more accurate then the 75' +/- altitude deviation allowed for altimeters.

Of course we have to assume, that these elevation markers were set with instruments in good calibration. But, as Garmin say's, we all need to be flying along at the same pressure settings, regardless of what instrument is closer to actual elevation, to maintain seperation.

L.Adamson

 

[rjcthree]

So in summary . . .

So as I digest all of this, I draw the following conclusion(s):

I can use my internal-battery backed up 496 as a secondary legitimate altitude indicator. A periodic comparison of the D180 vs the GPS is a good idea to see how it trends. I can eliminate the traditional round altimeter from the panel, unless I wanted a tertiary indicator(not).

Thanks for you help and the thoughts.

Rick