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Entering Class B airspace requires an explicit clearance. Entering Class C or Class D airspace requires only establishing two-way communications, upon which the pilot can still be explicitly refused entry.

So effectively, in both cases ATC maintains full control and can instruct a pilot not to enter their airspace. The only difference is whether the pilot asks for permission or simply announces an intent. This seems like a very fine semantic distinction which has no real-world meaning.

Why, then, does the FAA make this distinction? What practical impact does it have on a pilot?

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    $\begingroup$ I appreciate the word-smithing. I built upon them to tighten up the question just a little bit more. $\endgroup$ – Kenn Sebesta Jun 21 at 23:35
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For a large class B complex, the same approach control frequency may be working aircraft both inside and outside the B, with some of those outside the B in a class C or D.

Establishing communication with ATC is sufficient to enter the C or D unless the controller says no, but you still have to keep out of the B unless they say yes. If the rule were the same for all 3 (i.e. you may enter the B unless specifically told no), then most initial contacts would get a reply to the effect of "N123AB, radar contact, remain clear of Bravo." Lots of words used for lots of aircraft that may never intend to enter the class B.

The class C and D don't tend to need as much tight control as the class B, so "just" establishing communication is sufficient. But just because the controller is talking to you, doesn't mean that they're okay with you entering the class B.

If we think about it in terms of computer logic, it'd be simple for the controller's first reply packet to include bits to indicate 'cleared into class B', 'cleared into class C', and 'cleared into class D', all of which are set as desired and any of which could later be changed as necessary. But voice communication has nuances that digital processing lacks, and the amount of fast-moving (generally, airline) traffic in class B makes it desirable to have a more explicit clearance into that airspace required, and a stronger "stay out unless you have permission" requirement in place.

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    $\begingroup$ So the notion here is that Class B reached a tipping point where there is so much traffic, and so much detailed information required about said traffic, that ATC needed a way to initiate communication without de facto permitting traffic to proceed? $\endgroup$ – Kenn Sebesta Jun 22 at 2:24
  • $\begingroup$ Yeah, fair summary. They may want details (aircraft type - i.e. aprox speed, intended routing, etc) that take some communications, and even then may delay giving clearance until traffic permits. Class B really is a different animal than C or D. $\endgroup$ – Ralph J Jun 22 at 2:54
  • $\begingroup$ "the same approach control frequency may be working aircraft both inside and outside the B" — this is in no way unusual or any different from any other TRACON anywhere. Class C and Class D TRACON controllers are responsible for much more airspace than the charted C/D airspace. (In fact, Class B TRACONs are more likely to not extend much beyond the charted Class B airspace, at least the upper layer of it.) $\endgroup$ – randomhead Jun 22 at 3:52
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The answer has to do with ATC's separation requirements.

  • In Class D airspace, VFR aircraft are not afforded any kind of separation. Mandatory traffic advisories and safety alerts are provided between VFR/VFR and VFR/IFR traffic.
  • In Class C airspace, VFR aircraft are provided target resolution laterally ("dots don't touch") or 500' vertically between them and IFR traffic. No separation is provided between VFR/VFR traffic; again, mandatory traffic advisories and safety alerts are provided.
  • In Class B airspace, all aircraft are provided explicit milage-based radar separation regardless of flight rules.

In order for controllers to reliably provide milage-based radar separation, aircraft must be operating predictably and with no room for deviation. Therefore VFR aircraft, which are not usually given explicit heading and altitude instructions (at least not usually at the same time), are given explicit clearances when entering Bravo airspace. This means the controller knows what the aircraft are doing and can vector traffic to effect the proper lateral separation.

This is also why cloud clearance requirements are relaxed to simply "clear of clouds" when in Class B airspace. Because ATC is providing radar separation, the risk of an IFR/IMC aircraft suddenly appearing out of a cloud and colliding with a VFR aircraft is minimal; instead, the only major concern is that a non-IFR-rated pilot does not enter IMC.

To make the comparison with Class C airspace clearer, observe that VFR pilots will often receive a heading assignment when operating in Class C airspace, especially on initial departure. They may also (or instead) receive the instruction to "Maintain VFR at or above/below [XXX altitude]." This is because ATC has a responsibility to maintain defined separation between IFR/VFR traffic. Because the separation standard is only target resolution, and because Class C airspace is much smaller than Class B, a full-blown clearance to fly a specific heading and altitude is not necessary.

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Class B is expected to be a busier airspace. The difference between "ATC can deny clearance" and "you must get explicit clearance" is huge in that context.

Typically Class B surrounds major airports. If you just told ATC you're there and came on in, what if ATC was too busy to notice? The chances of a near collision incident or worse is quite high.

By requiring explicit permission to enter class B, the FAA is ensuring that the airspace controller knows who you are, where you are, and where you are going before you enter the airspace.

C and D are relatively lower traffic in general. In those airspaces, collisions are less likely and pilots are probably aware of any traffic announcing a position near them, so even if ATC drops the ball the pilots will call out any problem themselves.

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    $\begingroup$ This is a first attempt at an answer on Stack Exchange. I'm sure it's not ideal, but I'm happy to learn! $\endgroup$ – madscience Jun 21 at 23:46
  • $\begingroup$ I see what you're saying, but I struggle with "If you just told ATC you're there and came on in, what if ATC was too busy to notice." Since the requirement for having established two-way comms is that ATC must respond, there's no slipping through this crack. $\endgroup$ – Kenn Sebesta Jun 22 at 0:14
  • $\begingroup$ @KennSebesta. Not really; if the controller replies "N123AB squawk 1234 and ident" then you've "established radio communication" even though the controller still doesn't have you on his scope yet. (A reply, "aircraft calling, stand by" would not be established communication.) But the fact that he's replied to you with your callsign doesn't mean that he has all the info he'd want/need to issue a Bravo clearance. Sure, he could add "... and remain outside the Bravo" to lots of radio calls, but that much extra verbiage, over and over, becomes problematic after a while. $\endgroup$ – Ralph J Jun 22 at 1:25
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    $\begingroup$ Or the controller could reply "N123AB, standby..." That is established comms and you can come on in. $\endgroup$ – Michael Hall Jun 22 at 3:39
  • $\begingroup$ @KenSebesta. I get what you're asking but I wasn't sure how to make it more clear. Happily more experienced pilots (I'm a student in ground school) have provided more complete answers that hopefully answered your question fully. The accepted answer from Ralph J is excellent! $\endgroup$ – madscience Jun 22 at 19:35
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The difference boils down to the likelihood that you’ll be in the way of IFR traffic—and how easily ATC can solve that problem.

A typical class C or D airport may see only a few hundred (C) or a few dozen (D) IFR operations per day, so the odds are you won’t be in anyone’s way when you pass through. In the rare cases that you are, ATC can easily vector you to a different part of their airspace where you won’t be. So, the presumption is they’ll accept you.

In contrast, the biggest class B airports may see hundreds of IFR operations per hour, and while the airspace is much larger, there are so many planes in it there may be no way through that won’t put you in someone’s way. So, the presumption is they won’t accept you.

What this means to me as a VFR pilot is that I expect that I’ll be able to go through C/D, so that’s what I’ll plan. (It’s also so small and simple, it only takes a few seconds to replan off I’m wrong.) However, I assume I won’t be able to go through B, so I’ll plan my route around it and, if I happen to get a shortcut through, that’s just a bonus.

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  • $\begingroup$ I agree with the mentality of approaching a B vs a C or D (although I've maybe had much better luck than you with transiting Bravos), but IMO the language of the FARs is too carefully chosen to be this open to interpretation. Without any evidence beyond a lifetime of experience with technical writing, I hold that there must be a very specific and concrete meaning which motivates the difference between the two. $\endgroup$ – Kenn Sebesta Jun 22 at 1:12
  • $\begingroup$ @KennSebesta There’s also some holdover from the TCA/ARSA/TRSA rules when they adopted the alphabet system in the 90s, unlike other countries that wiped the slate clean, but I don’t see much room for interpretation here. $\endgroup$ – StephenS Jun 22 at 1:20
  • $\begingroup$ What I'm driving at is that from the pilot's perspective, a presumption one way or the other would be a very loose way to interpret these rules. I fly under a Class Bravo and presume I will always get in without any issue. If you presume the opposite, then it's fair to say the regulation is so loosely written that we are coming to contrary conclusions. Although that wouldn't be the first time this happens with the FARs, I hope to find a better explanation. $\endgroup$ – Kenn Sebesta Jun 22 at 2:39
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    $\begingroup$ @Kenn, the "specific and concrete" difference is the separation services you're provided, as detailed in my answer. $\endgroup$ – randomhead Jun 22 at 3:53

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