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Many Class C airspaces do not have a full-time tower and the airspace reverts to Class E or G when the tower is closed. The wording in CFR Part 91 § 91.225 doesn’t specifically say whether ADSB is required when the tower is closed and I can’t find any guidance from the FAA or AOPA on the matter.

(d)…no person may operate an aircraft in the following airspace unless the aircraft has equipment installed that meets the requirements in paragraph (b) of this section:

(1) Class B and Class C airspace areas

(3) Above the ceiling and within the lateral boundaries of a Class B or Class C airspace area designated for an airport upward to 10,000 feet MSL;

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    $\begingroup$ As when the tower is closed the airspace is E/G, then your question boils down to "is ADSB required in class E/G". Right? See also. $\endgroup$
    – mins
    Nov 29, 2021 at 18:39
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    $\begingroup$ @mins Not exactly. The question boils down to whether the reversion to Class G or E also changes the requirement for ADSB or whether the Class C as depicted on the chart is in effect no matter what the status of the tower. $\endgroup$
    – JScarry
    Nov 29, 2021 at 21:47
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    $\begingroup$ @JScarry: RE "whether the Class C as depicted on the chart": the chart like the chart supplement also notes what happens, e.g. Mobile Class C from the sectional. If you've meant something else, then it might be worth adding to the question body. $\endgroup$
    – ymb1
    Nov 30, 2021 at 1:36
  • $\begingroup$ Are there any Class B or Class C airspaces that would be excluded by the phrase designated for an airport? Why did they stick that in there? Another ASE question in the making-- $\endgroup$ Nov 30, 2021 at 15:11
  • $\begingroup$ @quietflyer According to faa.gov/documentLibrary/media/Order/JO_Order_7400.11F_508.pdf there are no Class C currently that do not have an airport. I suppose it’s possible that when military facilities are shut down the Class C airspace could remain without a tower for a while. $\endgroup$
    – JScarry
    Nov 30, 2021 at 18:34

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If an airspace is only class C when the tower is in operation, then it is no longer class C airspace when the tower is closed. In that case, the rules applicable to the class E or G airspace apply rather than class C, because the airspace is not class C at that time.

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  • $\begingroup$ I would concur. The tower being open or closed has no bearing on the ADS-B requirement, except insofar as it affects what class of airspace is in effect. $\endgroup$
    – randomhead
    Nov 30, 2021 at 2:02

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