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Sectionals say to contact the approach center of a class C airspace within 20 miles. Is this a suggestion or a rule? enter image description here

What is the maximum distance to contact them via radio, other than signal limitations, if one exists?

What is the maximum distance to receive radar service from class C?

Note that class C radar service and flight following from an ATC center are not the same thing as it pertains to my question.

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There is no strictly defined "maximum distance" beyond which you cannot receive radar service from an approach-control-whose-primary-airport-is-a-Class-C.

Terminal areas around a Class C airport generally comprise a 5-mile inner ring, 10-mile outer ring, and 20-mile outer area. The Approach Control is guaranteed to own the airspace laterally bounded by the outer ring of the Class C, extending upwards to their delegated ceiling (usually 10-to-12 thousand feet MSL) whether Class C or Class E; they are all but guaranteed to control the Class E airspace with 20 miles as well.1 Within that usually-20-miles and usually-10-kft cylinder of airspace, they will provide Class C services to all participating aircraft, even if the aircraft happen to be in Class E airspace—which is obviously more statistically likely, given the relative airspace volumes! However in Class E airspace you're not required to talk to them, whereas in Class C you are.

The reason the box says "CTC KNOXVILLE APP WITHIN 20NM" is that 20 miles guarantees you're within the "outer area" and thus Approach's airspace. If you contacted them at 30 miles out, you may be Center's airspace, in which case Approach would need to redirect you to Center, and then Center would hand you back to Approach a few minutes later. That's a waste of both your time and the controllers' time, so the chart reminds you of the minimum range to help you avoid that.

There is no theoretical maximum, and many Approach facilities do in fact control more (sometimes far more) airspace than the minimum. In this beyond-20-mile area you will not receive Class C services, but the airspace still belongs to the Approach rather than to the Center. However, the FAA does not publish the actual sector boundaries, so the best practice is to assume no more than the minimums—or get Flight Following for your entire trip, in which case you don't have to worry about it at all.


1The "procedural outer area" is, by default and in most cases, defined by a 20-mile ring around the primary airport. But in congested or mountainous areas—e.g. OAK, MDW, or SBA—this may not always hold true... Screenshot of the Chicago Terminal Area Chart showing magenta text enclosed in a magenta-bounded box.  The text reads: CTC CHICAGO MIDWAY APP WITHIN 15 NM ON 119.45 269.125 SERVICE AVAILABLE IN CHARTED CLASS C ONLY.

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  • $\begingroup$ 20 miles from the airport or outer airspace ring? $\endgroup$ Apr 10, 2019 at 19:34
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    $\begingroup$ @RyanMortensen All distances are from the primary airport. If you draw a circle at 20 miles, it (usually) passes through the "CTC APP" boxes, which is a subtle clue. $\endgroup$
    – StephenS
    Apr 10, 2019 at 20:53
  • $\begingroup$ That makes sense, but I never knew that. Thanks! $\endgroup$ Apr 10, 2019 at 22:02
  • $\begingroup$ @Ryan 25-30 miles out shouldn't usually be a problem, and IMO sooner is better. If necessary the controller can get a pointout from the controller who owns the airspace you're in, but if you're near to the boundary and they don't need to issue any immediate control instructions it isn't a big deal to let you run for a few miles. Neighboring controllers will recognize the squawk bank and know you're inbound to XYZ approach's airspace. $\endgroup$
    – randomhead
    Mar 9, 2021 at 20:55
  • $\begingroup$ Coming back to this answer, I really must take exception to several of your points, especially There is no such thing as "Class C radar service". I think I understand what you are trying to say, but as @Ryan said in his question, "Class C service" is a very well-defined concept and is not the same as basic VFR radar service—namely, when providing Class C service ATC will ensure positive separation between IFR-VFR traffic, while basic radar service only provides traffic advisories and safety alerts between IFR-VFR traffic. $\endgroup$
    – randomhead
    Mar 15, 2022 at 21:45
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Is this a suggestion or a rule?

Rule, if you are VFR and landing there. If IFR, or VFR with flight following, you will be told when to contact approach when arriving from elsewhere, and approach will hand you off to tower. If just passing by, I would at least monitor them to help with situational awareness of traffic in the area.

What is the maximum distance to contact them via radio, other than signal limitations, if one exists?

I'd be contacting them within at least 10 miles, before you enter the Class C airspace. You can try them as far out as you can receive the ATIS. For example, I was flying VFR into Binghamton, NY (BGM) and called them out about 20 miles after picking up the ATIS - and they told to enter right base for the active runway and I was cleared to land! Slow day I guess, not much other traffic.

What is the maximum distance to receive radar service from class C?

Usually 10 miles. If VFR and departing the area, they'll often ask you to advise them when you are leaving the Class C. If flight following was requested, they can hand you off to approach quite soon, like 500 to 1000 ft AGL.

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