"Thickness" of B airspace boundary.

[GreatLakesFlying]

The airport I fly from (06C) is under the lateral boundaries of KORD's Bravo airspace. My airport is at 801 ft. The Bravo sector above us starts at 1900. TPA is at 1601.

When departing or approaching, I maintain 1700'. That gives me some separation from TPA and also a 200' buffer below the Bravo. Often, however, when there are several aircraft in the pattern and I need to overfly for my entry into the pattern, I climb to 1800 for safety.

That's 1800' on an old Archer's altimeter set to 06C's pressure. I've noticed that altimeter settings for 06C and KORD vary as much as 0.1 inches, i.e., I may be as much as 100 ft off in altitude.

So, when I am cruising at 1800' near 06C there is a good chance that I may actually be at or above 1900', i.e., inside the Bravo airspace.

I guess that there is some tolerance in the system and that controllers may anticipate someone's tail slicing through the Bravo airspace. What is the practical consideration here, how thick is the Bravo boundary in the situation I describe? (different altimeter settings, older altimeter in aircraft, etc)

Now that the airplane I fly has ADSB, I've gone to FlightAware and checked the telemetry. In some instances it shows me at 1950' and that's as bad as it has been, but technically I may have busted the Bravo. At the same time, my Foreflight log shows me between 1800-1820', and so was my indicated altitude on the altimeter.

 

[CC268]

I’d say about 2 mils

 

[mscard88]

I’d say about 2 mils

 

[GreatLakesFlying]

I’d say about 2 mils

Do I surrender my certificate to the nearest FSDO, or do I mail it back to FAA HQ, postage due?

 

[bradg33]

I've never bothered to look into the regulatory effect of it, but on my last tour of the local center (ZKC, Kansas City), the controller giving us the tour noted that from their perspective the altimeter in the airplane controls what altitude you are "at." If your altimeter says 1800ft (I'm assuming there's an implicit requirement that your altimeter be set using the local altimeter setting), you're at 1800ft even if your transponder is reporting 2200ft or ATC's computer is converting encoder output using a different altimeter setting.

 

[Tantalum]

transponder

Yeah.. that's also why you often have them say "indicating" not "at" because if all they have is the xpndr then it may not be *that* accurate. Granted, I've also heard them often ask "say altitude" to confirm that the guy is where he should be, probably in cases where the xpndr is way off. When I hear my own plane called out they always say "at X" in reference to my altitude, but I also file IR 90% of the time so they should have a positive ID on me, vs "not talking to him, indicating 3,500"

 

[Shawn]

Here is the thing...unless you are talking to ATC, they can not verify your altitude anyway.

There are not alarm bell and jets that get scrambled to locate the pilot for issuing an airspace bust if you wander into Bravo by 100' under your scenario. From the sounds of it, most controllers do not wanna deal with the headache and based on the tolerances listed...it is probably common thing.

 

[Jmcmanna]

06C is a little bit of a special case because it lies just underneath the final approach path for 9L and 9R and large air carrier airplanes can be as low as 2300’ right there...that is the concern.

C90 is not policing whether you are on the edge of the class B (vertically or laterally), but if you negatively impact the ORD operation, they will make an attempt to find you.

Recently we had someone who appeared to bust the class B over 06C and an air carrier got a TCAS RA and had to go around. They found the guy...I have no idea what happened after it went out of the facility.

 

[flyingcheesehead]

The airport I fly from (06C) is under the lateral boundaries of KORD's Bravo airspace. My airport is at 801 ft. The Bravo sector above us starts at 1900. TPA is at 1601.

Restaurant is at 821.

When departing or approaching, I maintain 1700'. That gives me some separation from TPA and also a 200' buffer below the Bravo. Often, however, when there are several aircraft in the pattern and I need to overfly for my entry into the pattern, I climb to 1800 for safety.

Why do you need separation from TPA when you're departing/arriving, unless you're flying through the pattern? Just don't fly through the pattern. When I'm arriving/departing there, I generally just stay at TPA until I'm out from under the shelf, and I don't fly through the pattern, I join it.

If it's because your CFI (who presumably was somewhere else, I can't imagine someone teaching this method at Schaumburg) taught you to overfly the field 500 feet over TPA and do the descending teardrop to the 45 to the downwind - Don't do that. Certainly not at Schaumburg, and I don't advise doing it anywhere else either.

Also, being at 1700-1800 when the traffic pattern is 1600 isn't really making you any more safe. Altimeters are allowed to be up to 75 feet off in either direction, so you could have one person flying at 1600 and one at 1750 - Or at least that's what their altimeters read, but they're actually at the same altitude. And that's if both of them have their altimeters on the same setting! If one person sets theirs on the ground at 06C and the other sets theirs to O'Hare, or worse yet if someone flew in from elsewhere and never reset it, the errors could be further compounded. So, IMO it would be better for you to remain at 1600 and not have the false sense of security that is also leading to you risking a Bravo bust.

So, when I am cruising at 1800' near 06C there is a good chance that I may actually be at or above 1900', i.e., inside the Bravo airspace.

I guess that there is some tolerance in the system and that controllers may anticipate someone's tail slicing through the Bravo airspace. What is the practical consideration here, how thick is the Bravo boundary in the situation I describe? (different altimeter settings, older altimeter in aircraft, etc)

Now that the airplane I fly has ADSB, I've gone to FlightAware and checked the telemetry. In some instances it shows me at 1950' and that's as bad as it has been, but technically I may have busted the Bravo. At the same time, my Foreflight log shows me between 1800-1820', and so was my indicated altitude on the altimeter.

There is no "thickness" to the boundary. And while indicated altitude is probably what matters, what the FAA will hang you with is the ADS-B reported altitude and/or the altitude indicated on ATC radar, which is not always going to be right on your indicated altitude. You can see what pressure altitude your plane is reporting to ATC on some transponders - What transponder does the plane you fly have?

Bottom line, what's on ATC radar and ADS-B is what matters. ForeFlight will log GPS altitude, which isn't what the Bravo is based on, so it will not be usable in your defense. So, now it's up to your word vs the FAA, and the FAA has radar recordings and ADS-B.

You've probably gotten away with it because you haven't gotten close enough to an airliner to cause a TCAS RA. If you were the guy in @Jmcmanna's story, how would you defend yourself? Don't give the FAA any rope to hang you with - Give the Bravo a wide berth.