Busting Class B, what?

[N941WR]

Originally Posted by nucleus

My conclusion is that the GPS doesn't allow for the tiering or altitudes of the airspace, that it just takes the biggest ring and extends it to all altitudes.

Not the case with my 496, it is good about letting me fly under the Class B but every now and then it tells me I've busted it.

Not a big deal, now that I know it will do that.

This is the first time I have ever flown with a GPS so I have lots to learn. (No need for a GPS in a 95 MPH T-Craft, so I never bought one before now.)

[scottg]

There is an altitude buffer setting. Download the user manual from Garmin and search for 'altitude buffer'.

My assumption is that this is designed to provide a warning prior to an airspace bust by allowing for an altimeter setting that may be slightly off.

BTW, I've noticed that my GPS altitude and barometric altitude are very, very close.

[LettersFromFlyoverCountry]

I have the same problem flying north of STP when it reported I was in Class B (floor of which is 2300, I believe). I just ignored it but I'll bet it was the Garmin 296 altitude being off. I'd noticed in the past that it would show me 200 or 300 feet higher than I really was. I just doublechecked the barometric pressure and relied on my altimeter.

It was a good check of my heart rate though.

[Alan Carroll]

Here is a link to a website that does a nice job of explaining the altimeter errors: http://mtp.jpl.nasa.gov/notes/altitu...udeScales.html

It confirms what Alex has posted - the altimeter should only read the same as the GPS when flying in a "standard" atmosphere. This is rarely the case in reality, because conditions seldom match the ISA atmosphere model. The main error comes from nonstandard temperatures. Warmer than standard temperatures cause the altimeter to read lower than true (or GPS) altitude, and the error gets bigger as altitude increases. The link above shows that the error is about 4% for temp 10?C above standard (roughly 40' error at 1000' msl, 400' error at 10,000' msl).

Going the other way, colder than standard temps cause the altimeter to read too high. There are correction tables available for cold-weather IFR flying, to ensure obstacle clearance (for example: http://bathursted.ccnb.nb.ca/vatcan/...rentTopic.html).

I too have gotten a scare from the 396 a couple of times while flying near Class B airspace, until I figured out what was happening. As posted earlier, the airspace boundaries are based on the indicated (altimeter) altitude, not true (GPS) altitude.

[Kevin Horton]

Originally Posted by Alex Caldwell

Tell me if this is bogus, but it sounds like it really all has a lot to do with the errors built into our altimeters - the same stuff that you have to know when you take your written test and have to memorize that mnemonic - "High to Low, Hot to Cold, Look out below". The GPS altitude is really closer to the "true altitude" because it does not change with non-standard atmosphere changes in pressure or temperature.

Correct. This is why our GPS will usually read a different altitude from our altimeter, and why we should ignore our GPS altitude when flying, unless we are trying to figure out how our altitude compares to some terrain.

But, some of the reported times when a GPS was squawking about Class B seem to have been when the aircraft altitude was quite a long ways from the Class B floor.

[L.Adamson]

Originally Posted by Bob Collins

I have the same problem flying north of STP when it reported I was in Class B (floor of which is 2300, I believe). I just ignored it but I'll bet it was the Garmin 296 altitude being off. I'd noticed in the past that it would show me 200 or 300 feet higher than I really was. I just doublechecked the barometric pressure and relied on my altimeter.

It was a good check of my heart rate though.

I've taken my Garmin 296 on many ground trips throughout the mountain west, to see how it compares to posted elevation signs. The Yellowstone area for instance, has many signs, as well as the continental divide.

When picking up WAAS signals, altitude was usually within 30' of the posted altitudes. For all I know, I can assume their might be slight errors in the posted altitude also.

Most error without WAAS, was around 120', as I recall. All in all, I'm just impressed how well these GPS's work these days, vertically!

L.Adamson

[Alan Carroll]

Originally Posted by Kevin Horton

But, some of the reported times when a GPS was squawking about Class B seem to have been when the aircraft altitude was quite a long ways from the Class B floor.

This might be partly due to the "altitude buffer" used in generating the alarm? (added to the altimeter error)

[LettersFromFlyoverCountry]

Originally Posted by L.Adamson

IMost error without WAAS, was around 120', as I recall. All in all, I'm just impressed how well these GPS's work these days, vertically!
L.Adamson

I agree. On the ground, I've found my 296 reads the altitude properly. In the air, not so much. I can't really explain it, and I haven't done enough testing on it -- I haven't flown since Feb. 06 -- but I know one time I was climing to pattern altitude off Flying Cloud and -- while both altitude and altimeter had the same indication during run-up -- the 296 was showing me at about 2200 while the altimeter was showing me at about 1910.

The tower called out my position to other traffic and said I was -- and exact memory deserts me here -- at about 1950.

I landed at Glencoe , a few miles away, and the two agreed again.

But the time I was referring to in the earlier message was doing a ring circuit around the Twin Cities -- there's about 6 different airspaces to go through with a half dozen reliever airports and it's a good workout to brush upon radio technique because you get several towers plus flight following etc...

Anyway, I was passing the Shoreview towers -- the big TV towers in this area -- north of St. Paul and the 296 was telling me I'd busted the Class B.

The base of the Class B in that location is 3,000. I was a little, as I indicated earlier, panicked when I saw the flashing warning....but then I looked out my window and I was below the top of the tower. The tower is 2438.

That's when I just said "screw it," I'll fly by my altimeter.

I'll have to dig out the old instruciton manual and see what this altitude buffer setting is all about.

[lucaberta]

Originally Posted by Kevin Horton

Correct. This is why our GPS will usually read a different altitude from our altimeter, and why we should ignore our GPS altitude when flying, unless we are trying to figure out how our altitude compares to some terrain.

correct me if I am wrong here Kevin, but I think that it's not only that.

Another source of difference in baro vs GPS altitude lies in the fact that the two reference planes are infact different, one being a geoid and the other one being an ellipsoid. The difference thus vary locally, sometimes being very small, other times being more important.

Also, since altitude is defined as vertical height above the reference plane, and being the two reference planes not necessarily complanar, the resulting altitude vector can be several degrees slanted between the two readings, and thus the error grows as the absolute altitude grows (error which can be in thousands of feet, was reading almost 38000' when cruising in a 757 a few days ago, and we were at FL350, so it would not only be a matter of altimete setting way different from 29.92").

Of course WAAS LPV approaches take all of these points into account, and project a nice glide slope down to our minimums, irregardless of the baro vs. GPS altitude discussion...

Ciao, Luca

[Kevin Horton]

Originally Posted by lucaberta

correct me if I am wrong here Kevin, but I think that it's not only that.

Another source of difference in baro vs GPS altitude lies in the fact that the two reference planes are infact different, one being a geoid and the other one being an ellipsoid. The difference thus vary locally, sometimes being very small, other times being more important.

Also, since altitude is defined as vertical height above the reference plane, and being the two reference planes not necessarily complanar, the resulting altitude vector can be several degrees slanted between the two readings, and thus the error grows as the absolute altitude grows (error which can be in thousands of feet, was reading almost 38000' when cruising in a 757 a few days ago, and we were at FL350, so it would not only be a matter of altimete setting way different from 29.92").

Yes, you are correct that the GPS's geoid is not exactly the same as MSL. To be honest, I have never dug into the details of how much the difference is, so I don't know if it is significant or not. Can you point us to a good online reference that shows how the WGS 84 geoid compares to MSL?