In the US, practically speaking, how does the distinction between the E2 and E3/E4 subtypes of Class E airspace affect pilots?
The FAA has generally construed the phrase "surface area of controlled airspace designated for an airport", and other similar phrases, to include E2 airspace but not E3/E4 airspace. It is difficult to explain in a concise way why this should be so-- see the links below for more. E3/E4 airspace is referred to in the FAR's, AIM, and other regulatory documents by other phrases, all of which include the word "extension". The key distinguishing feature of E3/E4 airspace is that it does not actually surround the airport whose approaches it protects, hence it is an "extension" to the other controlled airspace that actually surrounds the airport. Specific examples of E2 airspace and E3/E4 ("extension") airspace are given below.
What are the practical effects of this distinction for pilots?
Here is a list of activities that are prohibited (or in cases marked with double dots, require prior authorization) in E2 airspace but not in E3/E4 ("extension") airspace, along with the relevant FAR or other legislation:
Operating under a cloud ceiling of 1000' or less, while operating VFR with no SVFR clearance -- FAR 91.155c
Taking off, landing, or entering an airport traffic pattern when ground visibility is at less than 3 miles, while operating VFR with no Special VFR clearance -- FAR 91.155d
Aerobatic flight-- FAR 91.303c
- Ultralight flight-- FAR 103.17
- Operation of an unmanned free balloon below 2000' AGL-- FAR 101.33a
- Operation of a small Unmanned Aerial Vehicle (e.g. commercial drone) -- FAR 107.41
- Operation of a recreational small Unmanned Aerial Vehicle (e.g. typical hobbyist drone or model airplane) under the "Recreational Exemption" -- language passed by Congress in October 2018
And here is a list of activities that are allowed only in E2 airspace, not in E3 or E4 ("extension") airspace:
- Exercising the privileges of a SVFR clearance to substitute relaxed visibility and cloud clearance requirements (remain clear of clouds, and one mile visibility is required for taking off or landing) in place of the standard VFR Class E visibility (3 miles) and cloud clearance (500' below, 1000' above, and 2000' horizontally) requirements, below 10,000' MSL-- FAR 91.157a
Caution: as noted in the links below, in actual practice, the distinction between E2 airspace and E3/E4 ("extension") airspace is often ignored by ATC in relation to the requirement for obtaining, and the possibility of receiving and exercising, a SVFR clearance when weather conditions are poor. In relation to requiring, and granting, SVFR clearances, ATC often treats E3/E4 ("extension") airspace as if it is actually E2 airspace. There are variations from one ATC facility to another in this regard. This is explored further in some of the links below.
In light of the above list, when pondering the significance of the restrictions that would be imposed on some airspace users by treating E3/E4 ("extension") airspace as if it were actually E2 airspace, bear in mind that some of the E3/E4 "extensions" are quite large in area, and bear in mind that just like the E2 airspaces, they are defined to have no upper ceiling. There are cases where pilots of ultralight gliders have transited through E3/E4 ("extension") areas as high as 17,000' MSL on a good soaring day-- obviously far above any approach path. This would technically not be allowed without prior authorization if the airspace were E2 rather than E3/E4.
How to tell E2 airspace from E3/E4 ("extension") airspace on a sectional chart?
Here are some examples of E4 ("extension") airspace-- look specifically at the airspace within the dashed magenta line that does NOT surround the airport whose approaches are being protected. That is the E4 ("extension") airspace. The airspace within the dashed magenta or dashed blue line that DOES surround the airport is E2 or D airspace:
On the other hand, none of the airspace enclosed by the dashed magenta line here is E4 ("extension") airspace. It is all E2 airspace. There is no dashed magenta line that does NOT enclose the airport:
E3 and E3a airspace is depicted like E4 airspace but the dashed magenta "extension" abuts Class C or Class B airspace, respectively, which is depicted differently than the dashed blue or dashed magenta circles in the examples above.
Here is an example of E3 ("extension") airspace--the dashed magenta boundary extending toward the northeast:
Unfortunately there are a few exceptions to this clear distinction between how E2 and E3/E4 ("extension") airspaces are usually depicted. KSGU, KBIH, and KTPL all have inner E2 circles with E4 extensions, yet all are depicted on the sectional chart with a single dashed magenta border enclosing the entire airspace, just like KONP above. No inner dashed magenta circle is shown. Perhaps these were simply accidental charting errors, or perhaps these were the result of someone in the charting division believing that there was no functional difference at all between E2 and E4 ("extension") airspace. Thankfully-- especially for aerobatic pilots and pilots of ultralight aircraft-- cases like these three are extremely rare. See the last of the Aviation Stack Exchange links below to see depictions of these three cases on the sectional charts, and further discussion.
A footnote-- following FAA practice, I've used the word "extension" to identify E3/E4 airspace. One might ask why I haven't provided a similar identifier for E2 airspace. The FAA sometimes uses the simple phrase "Surface Area" or "Class E surface area", with no additional qualifiers, as an identifier for E2 airspace. Other times additional qualifiers are added, as noted in the first sentence of this answer. I find all of this to be inherently confusing, since E3/E4 ("extension") airspace also touches the surface, and the additional qualifying words don't seem to add any real clarity for anyone who isn't thoroughly familar with the FAA's "Airspace Designations and Reporting Points" document. Hence my preference for relying on the identifier "E2" rather than the phrases used in the FAR's and the AIM.
Related content on ASE--
What are E2 and E4 airspace? -- this link gives a brief description of the eight subtypes of Class E airspace.
What are E2 and E4 airspace? -- use this link to understand the difference between how E2 and E3/E4 airspace are depicted on an aeronautical sectional chart.
Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization? -- note that the same language discussed here, or similar language, applies to all the other FARs that in practice, make a distinction between E2 and E3/E4 airspace, even though they contain no explicit reference to E2, E3, or E4 airspace. So see this link if you want to learn more about the "why" behind this answer.
This post from an outside (model airplane) website gives a fairly brief explanation of why E2 airspace has come to be treated differently than E3/E4 ("extension") airspace by the FAA. Only the first part of the content, down to the first long line of asterisks, is really relevant to full-scale aviation-- https://www.rcgroups.com/forums/showpost.php?p=41892695&postcount=5