What is the primary reason that the non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI have E4 extensions to E2 airspace?

Question

NOTE-- this question is not about why it was desired to protect the airspace that ended up within the E4 "extensions". Rather, this question is about why that was done by designating that airspace as an E4 "extension" rather than simply as part of the E2 airspace that surrounds the airport, which is the more common situation for untowered airports.

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What is the primary reason that the (U.S.) non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI all feature E4 Class-E-to-surface "extensions" to the E2 Class-E-to-surface airspace that surrounds the airports?

Why was the airspace not simply all designated as E2 Class-E-to-surface airspace, as was done at SHR?

Is there some established guidance that areas of Class-E-to-surface airspace projecting beyond some given distance beyond the airport whose approaches they protect should be designated as E4 airspace rather than E2 airspace?¹ Or is it up to the discretion of the team designing the airspace in each specific case?

There is arguably some regulatory ambiguity around the operational significance of the distinction between E2 airspace and E4 airspace. Certain FARs pertaining to eligibility for Special VFR clearance, VFR operations under a 1000' ceiling without a Special VFR clearance, ultralight operations, unmanned aircraft operations, aerobatics, and more, might arguably be construed to apply to E2 airspace but not to E4 airspace. (For more on this, see this related ASE question, and the other related linked questions.) Was the FAA designing the airspace around these airports with an eye to minimizing the amount of airspace to which these FARs would be applicable, to lift the regulatory burden on certain aviation activities? And/or to limit the amount of airspace in which pilots would be eligible to be granted a Special VFR clearance?

Or was there some other consideration that was the primary reason that the airspace at these airports was designed to have an E2 "Surface Area" with E4 "extensions"?

Bonus question-- why was the airspace portrayed differently on the VFR sectional charts in the first two examples (ACV and TVL) than in the last three examples (SGU, BIH, and SIT/PASI)?

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Note that for ACV and TVL, on the depiction on the sectional charts, there is a dashed magenta line separating the rectangular E4 "extensions" from the circular E2 airspace around the airports.

ACV-- California Redwood Coast Humboldt County airport near Arcata CA

Airspace description in FAA JO 7400.11E-- pp. E-130, E-212²

TVL-- Lake Tahoe Airport, South Lake Tahoe CA

Airspace description in FAA JO 7400.11E-- pp. E-136, E-221³

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Note that for SGU, BIH, and SIT/PASI, on the depiction on the sectional charts, there is no dashed magenta line separating the rectangular E4 "extensions" from the circular E2 airspace around the airports. Yet in all three of these cases, the rectangular projections are in fact designated as E4 "extensions", as distinct from the E2 inner circles, just as is the case at ACV and TVL:

SGU-- St George Regional Airport, St George UT

Airspace description in FAA JO 7400.11E-- pp. E-82, E-193

BIH-- Bishop Airport, Bishop CA

Airspace description in FAA JO 7400.11E-- pp. E-131, E-213⁴

SIT/PASI-- Sitka Rocky Gutierrez Airport, Sitka, AK

The 2/25/21 Juneau sectional chart portrays a recent airspace change effective 11/5/20 (pictured below)-- the new airspace definitions may be seen here (E2) and here (E4, after an error was corrected). The airspace now features a very large E4 "extension". The old airspace configuration, which at present may still be seen on vfrmap.com, featured an L-shaped area of E2 airspace, with no E4 extensions.

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Note that there are other examples of (U.S.) airports that feature large expanses of Class-E-to-surface airspace, with no control tower in the vicinity, where the entire airspace was simply designated as E2 airspace, with no E4 "extensions". One of the best examples of this is SHR:

SHR-- Sheridan County Airport, Sheridan, WY

Airspace description in FAA JO 7400.11E-- p. E-91

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Footnotes:

1. E.g. something similar to this guidance regarding E4 extensions to Class D airspace: "If all arrival extensions are 2 NM or less, they will remain part of the basic Class D area. However, if any extension is greater than 2 NM, then all extensions will be Class E airspace."-- from FAA Order JO 7400.2L "Procedures for Handling Airspace Matters". Also appears as AIM 3-2-5 d1.

2. All images are clipped from screenshots of the 2-25-2021 editions of the relevant VFR sectional charts, accessed via this FAA webpage. All are intended to be to same scale.

3. Note that at TVL, but not at ACV, the chart bears a notation "See NOTAMs/Supplement for Class E(sfc) effective hours", with an arrow pointing specifically to the E4 "extension". The current Chart Supplement, however, simply states "Airspace: Class E". To the best of my knowledge, this has been the case ever since the E4 airspace was first created at this location. The E4 airspace description for TVL in the "Airspace Designations and Reporting Points" document JO 7400.11E does include the note "This Class E airspace area is effective during the specific dates and times established in advance by a Notice to Airmen. The effective date and time will thereafter be continuously published in the Airport/Facility Directory. " Perhaps there was a desire to facilitate making the E4 extension part-time for some reason in the future, by simply changing the Chart Supplement? In all the other E2 and E4 airspace descriptions referenced in this question, there is no indication that any of the airspace is intended to be in effect less than 24/7.

4. The hang gliding symbol within the E2 Class-E-to-surface airspace at BIH is rather "interesting", considering the language of FAR Part 103.

Answer 1

Bonus question-- why was the airspace portrayed differently on the VFR sectional charts in the first two examples (ACV and TVL) than in the last three examples (SGU, BIH, and SIT/PASI)?

After submitting an inquiry through an on-line portal, I recently spoke to a cartographer with the Aeronautical Information Services, who said in his opinion the depiction of the airspace at ACV was a mistake, and should instead resemble the airspace at SGU, BIH, and SIT/PASI. He is taking steps to make the change, and expects it to go into effect for the next edition following the 2-25-2021 edition. TVL presents a more complex case because the airspace description in J.O. 7400.11E does indicate that the E4 extension may not always be in effect 24/7, even though the present Chart Supplement does indicate that all the Class E airspace at this location is in effect 24/7. The cartographer indicated that he would take steps to see if there should be a change in how the airspace at TVL is depicted as well.

Was the FAA designing the airspace around these airports with an eye to minimizing the amount of airspace to which these FARs would be applicable, to lift the regulatory burden on certain aviation activities?

This seems unlikely.

Was the FAA designing the airspace around these airports with an eye to minimizing the amount of airspace to which these FARs would be applicable, to... limit the amount of airspace in which pilots would be eligible to be granted a Special VFR clearance?

This seems unlikely too. While it doesn't speak directly to the intent of the airspace designers, it is the case that in early 2019, I called the ARTC Centers controlling the E2 and E4 airspaces around these airports (except SIT/PASI which did not yet have E4 airspace at that time) and in every case the staffer I spoke to told me that in actual practice, they controlled both the E2 airspace and the E4 airspace. The staffers also told me that they made no distinction between E2 and E4 airspace at these locations in terms of granting a SVFR clearance.

What is the primary reason that the (U.S.) non-towered airports ACV, TVL, SGU, BIH, and SIT/PASI all feature E4 Class-E-to-surface "extensions" to the E2 Class-E-to-surface airspace that surrounds the airports?

Why was the airspace not simply all designated as E2 Class-E-to-surface airspace, as was done at SHR?

This answer is unable to address those aspects of the question at this time; it remains unknown.

But, regardless of the reason, it is a positive thing for those flying small Unmanned Aircraft under Part 107 or the 2018 "Recreational Exception", and arguably for ultralight and aerobatic fliers as well. For more, see this related ASE answer.

Answer 2

The extensions are aligned with the runway for instrument approaches.

The following is an excerpt from the Aeronautical Information Manual (AIM, Chap 3. Airspace, Section 2. Controlled Airspace)

e. Functions of Class E Airspace. Class E airspace may be designated for the following purposes:

1. Surface area designated for an airport where a control tower is not in operation. Class E surface areas extend upward from the surface to a designated altitude, or to the adjacent or overlying controlled airspace. The airspace will be configured to contain all instrument procedures.

   (a) To qualify for a Class E surface area, the airport must have weather observation and reporting capability, and communications capability must exist with aircraft down to the runway surface.
   (b) [...]

2. [...]

3. Airspace used for transition. Class E airspace areas may be designated for transitioning aircraft to/from the terminal or en route environment.

   (a) Class E transition areas extend upward from either 700 feet AGL (shown as magenta vignette on sectional charts) or 1,200 feet AGL (blue vignette) and are designated for airports with an approved instrument procedure.
   (b) The 700-foot/1200-foot AGL Class E airspace transition areas remain in effect continuously, regardless of airport operating hours or surface area status.