Busting Class B, what?

[N941WR]

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?

[petehowell]

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.

[G-force]

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?

[billnaz]

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?

[Kevin Horton]

Originally Posted by billnaz

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?

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.

[the_other_dougreeves]

Originally Posted by Kevin Horton

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.

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

[RVadmirer]

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.....

[petehowell]

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.

[Brantel]

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!"

[nucleus]

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