What altitudes does Center have jurisdiction over?

Hello everyone. I have a question regarding center frequencies and altitudes. When flying into an active center frequency, do you join the frequency even if you’re at a higher altitude? For example, I’m flying through Memphis center at FL350. Under Memphis center, it has 0ft → 18000ft. I originally thought that you would only join at or above 0 feet or at or below 18000 feet so I just flew through it without contacting anybody. Center contacted me to join the frequency so now I’m confused as to what 0ft → 18000ft means because she wanted me on the frequency at a higher altitude. It could be because she saw a potential conflict between me and another aircraft because she immediately said to stay at FL350 after I checked in.

Hi there, Trey! Center’s airspace extends from the surface (0ft, in this case) to FL600 (60000ft). Approach and Departure frequencies have jurisdiction from the surface to FL180 (18000ft). This means that, in the absence of Approach/Departure, Center may want you on their frequency as soon as you depart. Take a look at this graphic from the ATC Manual:


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You should join Center when you’re above FL180 (18,000 feet), in most cases.

Without an Approach frequency, Center takes over their responsibilities. So they talk you all the way down to the approach (down to the ground, which is why it shows up at 0ft)

If you’ve been cruising for a while without disturbing your device, you will be marked as “away” or “inactive” and Center controllers will not disturb you.

Thanks guys! I understand now. You learn something new every day!

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