What are the historical precedents of today's E2 and E4 airspace? From what previous airspace classes did these airspaces evolve? How do these precedents help us understand the significance of the distinction between E2 and E4 airspace today?
What we now know as Class D and E2 airspaces were once (before 1992) a single type of airspace called a "Control Zone". "Control Zones" first appeared on charts in the mid-1940's and consisted of a thin dashed circle enclosing an airport-- originally red, later blue. By the time of this 1974 test-prep publication, "Control Zones" had been defined to go up to the overlying "Continental Control Area" at 14,500' MSL.
Originally a "Control Zone" did not indicate a communications requirement for VFR traffic-- many non-towered airports had a Control Zone around them, and a few towered airports lacked them. Instead, all towered airports were automatically assumed to have an "Airport Traffic Area" around them, 5 miles in radius and 3000' tall, that could not be entered without first establishing radio communications with the tower. The purpose of a "Control Zone" was simply to impose-- all the way to the surface-- higher VFR cloud clearance and visibility requirements than would exist in "uncontrolled" airspace, for the protection of IFR traffic emerging from clouds.
Around September 1992, changes were made to the markings on the sectional charts in preparation for the upcoming September 1993 "alphabet" airspace reorganization, when the "Airport Traffic Areas" would disappear and the boundary of the communication requirement would be shifted to the edge of Class D airspace. All "Control Zones" that did not contain a control tower had their depiction changed from a dashed blue line to a dashed magenta line, and were identified on the chart legend as a "Control Zone without communications requirement". This was the immediate forerunner of today's E2 airspace. At the same time, to eliminate the future communication burden on VFR traffic (and towers) that would be created in cases where Class D airspace had a projection extending far from a towered airport, some of the projecting portions of Control Zones at towered airports had their borders changed from dashed blue to dashed magenta and were identified on the chart legend as "Control Zone extensions". These "Control Zone extensions" are the immediate forerunners of today's E4 airspace. The complete legend on the charts for the new dashed magenta line was "Control Zone at airport without control tower and Control Zone extension without communications requirement".
It is interesting to compare aeronautical sectional charts dating from shortly before the introduction of the dashed magenta markings, to charts effective immediately after the introduction of the dashed magenta markings. It is interesting to see how the Control Zones at airports with towers changed from their previous shapes. Some were made smaller. On some, any rectangular projections remained part of the blue dashed shape surrounding the airport, while on others, some or all rectangular projections were changed to magenta dashed lines that abutted against the blue dashed shape. The determining factor does NOT appear to simply be how far the projections projected away from the basic inner circular shape. Clearly, planners were preparing for the communications requirement that would soon be shifted from the old "Airport Traffic Area" boundaries, to the edge of the Class D (or higher) airspace.
Note however that according to the FAR's, the "Airport Traffic Areas" still served as the boundaries of the communications requirement until the September 1993 "alphabet" airspace re-designation.
Therefore it's not clear to me whether or not the distinction between a "Control Zone" and a "Control Zone extension" had any actual practical or regulatory significance in the September 1992 to September 1993 time period. Could it have simply been a charting convenience to help pilots, chartmakers, and others prepare for the regulatory changes that would go into effect in September 1993?
Certainly FARs 91.155c, 91.157a, 91.303c, 101.33a, and 103.17 continued to simply refer to "Control Zones" with no mention of "extensions" all the way up until the September 1993 airspace re-designation. Would these FAR's have applied to airspace that was charted as a "Control Zone extension", or only to airspace that was charted as a "Control Zone"?
Diving a little deeper into this question of the significance of the distinction between a "Control Zone" and a "Control Zone extension" prior to September 1993: in the version of the FARs effective Jan 1993, we read "71.1 Applicability The complete listing for all jet airways, area high routes, Federal airways, control areas, control area extensions, area low routes, control zones, transition areas, terminal control areas, airport radar service areas, postive control areas, reporting points, and other controlled airspace can be found in FAA Order 7400.7A, Compilation of Regulations, dated November 2 1992." Note the lack of the phrase "control zone extension" in this sentence. Would a careful reading of the November 2, 1992 edition of FAA Order 7400.7A show that the airspace that appeared on the charts as a "Control Zone extension" was still simply considered part of a "Control Zone" from a regulatory standpoint? Does the phrase "Control Zone extension" appear in this edition of FAA Order 7400.7A at all, or was that airspace simply treated as a completely integral part of the adjoined "Control Zone"? So that the distinction between "Control Zone" and "Control Zone extension" essentially existed on the aeronautical charts only, and nowhere in the actual regulatory documents? I haven't yet been able to obtain a copy of the November 2, 1992 edition of FAA Order 7400.7A, and don't know the answer to these questions.
The actual transition from the "Control Zone"/ "Control Zone extension" terminology to the modern "alphabet" terminology happened in September 1993. At this time "Airport Traffic Areas" ceased to exist, and all the airspace surrounded by a dashed blue line on the sectional became Class D airspace, and all the airspace surrounded by a dashed magenta line on the sectional became Class E airspace that extended all the way to the surface. This is when the awkward terminology that we currently find used to describe E2 and E3/E4 airspace in the "Airspace Designations and Reporting Points" document was first applied to what had previously been depicted on the charts as "Control Zones" (around non-towered airports), and what had previously been depicted on the charts as "Control Zone extensions", respectively. Considering that E4 airspace also goes all the way down to the surface, it's unfortunate that someone didn't come up with a better term than "Designated as Surface Areas" or "Designated as a Surface Area for an airport" to describe the E2 airspace. September 1993 is also when the language of FARs 91.155c, 91.157a, 91.303c, 101.33a, and 103.17 was changed to its current awkward form. Regardless of whether or not these FARs had been considered to apply to the airspace of the "Control Zone extensions" during the year or so that they were labelled in that manner on the charts, the language adopted for those FARs in September 1993 clearly matched the E2 airspace description in the "Airspace Designations and Reporting Points" document effective at the time (as well as at the present), and did not match the E3/E4 airspace description.
As a point of related interest, here is a list of activities that require prior authorization (or in the case of aerobatic flight, are prohibited) in E2 airspace but not E3/E4 airspace, along with the relevant FAR or other legislation:
Operating VFR without a SVFR clearance in certain adverse weather conditions -- FAR 91.155c
Ultralight flight-- FAR 103.17
Aerobatic flight-- FAR 91.303c
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
Also, FAR 91.157a allows ATC to grant a SVFR clearance in E2 but not E3/E4
As noted in the links below, the distinction between E2 and E3/E4 airspace is not always observed by ATC in actual practice in relation to the requirement for, and the possibility of granting, a SVFR clearance.
Related content on ASE--
In actual practice, do ARTC Center personnel generally follow the rule that a SVFR clearance may be granted in E2 airspace but not E4 airspace? (US) In the US, is there much variation among the various ARTC Centers as to where a SVFR clearance may be granted?