An airport tower closes at 1600 hours:
- Does the extension (class E from surface up) airspace becomes non existing?
- Can I fly through since the tower is closed?
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Does the extension (class E from surface up) airspace becomes non existing?
It depends on whether the Class D airspace at the surface reverts to surface-level Class E airspace, or to Class G airspace (usually with a Class E floor above, at 700' AGL or 1200' AGL), when the tower closes.
If the Class D converts to surface-level Class E airspace, then the extensions remain in place as surface-level Class E airspace.
If the Class D converts to Class G (usually with a Class E floor above, at 700' AGL or 1200' AGL), so do the extensions.
This is clarified by the following content in the "Airspace" entry in the "Legend" section of the "Chart Supplement" (formerly "Airport/Facilities Directory") :
NOTE: AIRSPACE SVC "TIMES" INCLUDE ALL ASSOCIATED ARRIVAL EXTENSIONS. Surface area arrival extensions for instrument approaches become part of the primary core surface area. These extensions may be be either Class D or Class E airspace and are effective concurrent with the time of the primary core surface area. For example, when a part-time Class C, Class D, or Class E surface area defaults to Class G, the associated arrival extensions will default to Class E at the same time. When a part-time Class C or Class D surface area defaults to Class E, the arrival extensions will remain in effect as Class E airspace.
The full content of the "Airspace" entry (item 30, on the page labelled p.26) in the "Legend" section of the Chart Supplement (Northeast region, issue effective beginning 3-24-2022) is attached below.
In the case where the Class D converts to Class G and the "extensions" therefore vanish, be aware that there is some slightly misleading content in the Legend of the Chart Supplement-- the Legend indicates that the actual Airspace entries for each airport also clarify whether the overlying Class E airspace begins at 700' or 1200' AGL, but that's not really true. You have to figure that out by looking at the sectional chart.
At any rate, you won't have any trouble telling from the Chart Supplement whether the core Class D airspace converts to surface-level Class E airspace (and the "extensions" therefore remain), or converts to Class G airspace (and the "extensions" therefore vanish.)
You should be able to tell the same information from the sectional chart itself. ***(Caution: this only works for airports that do have surface-level Class E extensions, not for those that lack surface-level Class E extensions.) Here's how the system is supposed to work:
If the Class D airspace reverts all the way to Class G, then the sectional chart is supposed to depict that there is a change in status of the core Class D airspace and the surface-level Class E extensions. Because both are reverting to Class G. This is accomplished via the use of the phrase "See Chart Supplement for D/E(surf) eff hrs".
If the Class D airspace reverts to surface-level Class E airspace, then the sectional chart is supposed to depict that there is a change only in the status of the core Class D airspace. Because the status of the "extensions" is not changing. This is accomplished via the use of the phrase "See Chart Supplement for D eff hrs".
(For airports where are no surface-level Class E "extensions" to the Class D airspace, the latter phrase is always used, hence the word of caution. In this case the use of this phrase on the sectional chart must not be taken as an indication that the Class D airspace reverts to surface-level Class E airspace, rather than Class G airspace, when the tower is closed. Instead, the pilot must consult the Chart Supplement.)
There have been some errors in the past when this system was not uniformly adopted, and the former phrase on the sectional chart was incorrectly applied to cases where the Class D airspace reverted to surface-level Class E airspace (and the "extensions" therefore remained as surface-level Class E airspace).1 Some of these inconsistencies would certainly have been present at the time that the question was originally asked. Most or all of these cases have apparently been recently corrected.2 In all cases, the Chart Supplement itself did correctly explain the status of the airspace.
Can I fly through since the tower is closed?
You don't normally need to talk to anyone to fly through Class E airspace -- including the surface-level Class E "extensions", or the core surface-level Class E airspace areas that actually include the airports for which they are designated -- unless you can't maintain the relevant VFR cloud clearance and visibility requirements, in which case you might consider requesting a Special VFR clearance. Note that FAR 91.155(c) prohibits operation below a cloud ceiling that is lower than 1000' AGL in Class E airspace that extends all the way down to ground level, unless you obtain a Special VFR clearance. (There is actually some ambiguity around whether or not Special VFR clearance would even be valid for a surface-level Class E "extension". For more on this, see the links near the end of this related ASE answer.)
1,2) See ACM Charting Group Minutes here -- https://www.faa.gov/air_traffic/flight_info/aeronav/acf/media/RDs/21-01-360_Contolled_Airspace_Effective_Hours.pdf . See also this related ASE answer.