Busting Class B, what?

Twice now I've been flying close to Charlotte's Class B airspace but not in it and my 496 has told me that I have just entered it.

In both instances I have been under the Class B airspace by 500 feet and well outside of it. Both days were VFR and I knew where I was at all times. My Altimeter had the current AWOS setting and in one case I was talking to a controller, flying over a class D airport.

What is going on here, have any of the rest of you seen this?

I see it in Minne with my 396

Hi Bill,

When I fly to KSTP, close to the MSP bravo, I get it all the time. I know where I am and how high I am and am usually on with STP tower. They WILL let you know if you are too high. I have not checked my GPS alt at that time, but doubt I am too high by that measure either. Not sure what to make of it.

Where does the 496 get its altitude information? I assume its a fixed 29.92? If its not being corrected for current pressure, perhaps it though you were 500 feet higher?

GPS altitude

My Anywhere Map's GPS derived alt. is always different from the altimeter reading. I've always wondered which one is right. You'd think the magic satelite box would be more accurate with enuff sats locked on. Anyone know the answer to this?

billnaz said:

My Anywhere Map's GPS derived alt. is always different from the altimeter reading. I've always wondered which one is right. You'd think the magic satelite box would be more accurate with enuff sats locked on. Anyone know the answer to this?

Click to expand...

GPS altitude is not the same thing as the barometric altitude we use in aviation. The difference could be several hundred feet, with the difference increasing with height above ground and with temperatures that differ from standard temperature.

If you want to measure the height of a mountain, GPS altitude will be more accurate than barometric altitude. If you want to determine whether you are in Class B or Class C airspace, barometric altitude will be more accurate than GPS altitude.

Kevin Horton said:

GPS altitude is not the same thing as the barometric altitude we use in aviation. The difference could be several hundred feet, with the difference increasing with height above ground and with temperatures that differ from standard temperature.

If you want to measure the height of a mountain, GPS altitude will be more accurate than barometric altitude. If you want to determine whether you are in Class B or Class C airspace, barometric altitude will be more accurate than GPS altitude.

Click to expand...

AFAIK, all ATC cares about is indicated baro altitude, not true altitude. If you have the right setting in the box, your static system (including encoder!) is accurate, and it says you're clear of Class B, then you should be clear. Remember that the only way ATC can determine your altitude is Mode C, and they correct that for baro changes.

I've seen the indication before and it's caused me a little panic too.

TODR

GPS Altitude

It would seem to this semi-rookie that altitude is altitude. Why, with WAAS active, does the GPS tells me I'm 600' higher than my altimeter? It's much more accurate in lateral positioning so how can the altitude be so far off? Garmin claims an accuracy of 3-7 meters in altitude and 3-5 meters in horizontal positioning.

Another maybe dumb question along these lines, when ATCC tells you the altitude they have for you does it vary according to your altimeter setting or is Mode C being reported to them and they determine your true altitude? Had all of these issues come up Monday flying across Nevada. It was confusing.....

TODR has it right

I'm sure Kevin can explain it much better than I ever could, but bottom line ATC wants everyone to have the same relative reference point for altitude, that way they can maintain separation. Indicated baro alt may be way off of GPS alt and that is OK as long as the flyers up there have the same ref point. The GPS only knows what it calcs from the sat data. It is not using the same ref point as we flyers are.

I hope I am close to being right on this.

Quote from somewhere on the web:

"The indicated altitude is the altitude shown on the altimeter when it is using the correct altimeter setting. It will be identical to absolute altitude when the plane is sitting on the ground, but, unless the temperature lapse rate is exactly the same as ISA, it will become increasingly inaccurate with altitude: on a cold winter day, the altimeter can overread by 1,000 ft or more at normal cruising altitudes, so that pilots are flying much lower than they think; on a hot summer day, the altimeter can underread by 1,000 ft or more at normal cruising altitudes, so that pilots are flying much higher than they think. Since all altimeters experience the same error, and the error diminishes near the ground, the difference between indicated altitude and absolute altitude does not normally cause a problem for crusing and approaching: as long as everyone is flying at, say, 8379 ft MSL, it does not much matter if they think that they're at 9000 ft. The one exception is flying around mountains in the winter, where the altimeter error can cause collision with terrain.'

Pretty sure this is an accurate description of the errors in our altimeters...

If your GPS gives you a terrain warning, you better trust it! Really trust it in the winter time!

I can still hear the King's repeating "High to Low, Look Out Below!"

Does the GPS Even look at Altitude

I have noticed that when I am flying over or under restricted airspace that both my handheld Garmin and the panel mounted Apollo are alarming that I have entered the airspace. This happens even when I am 1000 feet or more over or under. One time I was 2200 feet over controlled airspace and both were alarming.

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.

I don't like this explanation, I expect better from our GPS's, but it is the only one that seems to fit the circumstances.

Nucleus

nucleus said:

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.

Click to expand...

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.)

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.

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.

Altitude errors

Here is a link to a website that does a nice job of explaining the altimeter errors: http://mtp.jpl.nasa.gov/notes/altitude/AviationAltiudeScales.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/fir/moncton/WeeklyTopics/Archives/20040104/CurrentTopic.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.

Alex Caldwell said:

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.

Click to expand...

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.

Bob Collins said:

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.

Click to expand...

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

Kevin Horton said:

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.

Click to expand...

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

L.Adamson said:

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

Click to expand...

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.

Kevin Horton said:

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.

Click to expand...

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

lucaberta said:

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").

Click to expand...

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?

What about Panel mount GPS

Everyone here has talked about hand helds. Take one Arrow, 2 garmin 530's and SF's Class bravo... Flying at 5500 msl near Livermore when the garmins went ballistic, saying 'you have entered class bravo"... it makes the pucker power multiply. I knew where we were. so... Anyway if it can happen...