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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 much more common situation for untowered airports.


What is the primary reason that the non-towered airports KACV and KTVL both feature E4 Class-E-to-surface "extensions" to the E2 Class-E-to-surface airspace that surrounds the airports?

Note 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.

KACV: California Redwood Coast Humboldt County airport near Arcata -- http://vfrmap.com/?type=vfrc&lat=40.978&lon=-124.108&zoom=10

KTVL: South Lake Tahoe airport -- http://vfrmap.com/?type=vfrc&lat=38.894&lon=-119.995&zoom=10

This is extremely uncommon. These appear to be the only two airports in the entire US that are depicted this way, though there are three other airports that actually have identical airspace configurations but are (perhaps mistakenly) depicted in a different manner on the sectional charts (see note at end of this question).

Possible answers to this question include--

1) For some reason, it was considered undesirable to require a SVFR clearance for operations below a 1000' ceiling in the airspace within the lateral boundaries of what became the E4 extensions. FAR 91.155c requires a SVFR clearance for operations below a 1000' ceiling within the lateral boundaries of E2 airspace -- see Does FAR 91.155c apply to class E surface extensions?

2) For some reason, it was considered undesirable to allow authorization of a SVFR clearance in the airspace within the lateral boundaries of what became the E4 extensions. FAR 91.157a authorizes SVFR operations within the lateral boundaries of E2 airspace -- see Does an SVFR clearance extend to Echo surface extensions? and Does an SVFR clearance extend to Echo surface extensions?

3) There was a desire not to require prior authorization for operation of unmanned free balloons within the lateral boundaries of the airspace that became the E4 "extensions". FAR 101.33a requires prior authorization for unmanned free balloons operating below 2500' within the lateral boundaries of E2 airspace.

4) There was a desire not to prohibit aerobatic flight in the airspace within the lateral boundaries of the airspace that became the E4 "extensions". FAR 91.303c prohibits aerobatic flight within the lateral boundaries of E2 airspace.

5) There was a desire not to require prior authorization for operation of ultralight aircraft within the lateral boundaries of the airspace that became the E4 "extensions". FAR 103.17 requires prior authorization for ultralight vehicles operating within the lateral boundaries of E2 airspace. See related answer to related question Which parts of class E airspace can an ultralight (part 103) fly in without prior ATC authorization? .

6) There was a desire not to require prior authorization for operation of sUAS (drones) within the lateral boundaries of the airspace that became the E4 "extensions". FAR 107.41 requires prior authorization for sUAS operating within the lateral boundaries of E2 airspace but not E4 airspace. For more on this see http://goldsealgroundschool.com/uav-library/surface-E-authorizations.pdf . Note also that the E4 airspaces-- including those at KSGU, KTPL, and KBIH (see below) -- are not included on this FAA interactive map as areas requiring prior authorization under FAR 107.41 .

Note -- it is really a matter of some uncertainty whether the regulations listed above that pertain to the E2 "Surface Areas", do or do not also pertain to the E4 Class-E-to-surface "extensions". For example, many air traffic control centers will state that they do have the authority to issue a Special VFR clearance in a Class-E-to-surface "extension". Follow the links given above for more on these topics.

7) For some reason, in the context of IFR flight, the airspace inside the "extensions" is controlled by a different agency than the airspace inside the inner circles. (This is the situation that exists in relation to E4 "extensions" to class D airspace-- see related answer to related question Does an SVFR clearance extend to Echo surface extensions? .)

8) Regulatory guidance may exist stating that the airspace protecting the instrument approaches should be designated as E4 "extensions" rather than as part of the E2 airspace surrounding the airport in any case where the airspace in question extends more than a certain distance beyond the basic E2 circle?

Note-- the actual airspace descriptions for the airspaces discussed here are as follows-- the pages given below are from the document "Airspace Designations And Reporting Points", dated August 13 2018, "https://www.faa.gov/documentLibrary/media/Order/JO_7400.11C.pdf"

KACV -- Arcata Airport, CA

E2 described on page E-127, E4 described on page E-211

KTVL -- South Lake Tahoe Airport, CA

E2 described on page E-135, E-4 described on page E-220

Three other airports are described in an identical manner in the August 2018 "Airspace Designations and Reporting Points" document, but each is (perhaps mistakenly) depicted by a single dashed magenta border on the most recent sectional charts. These airports are--

KSGU -- St. George Regional Airport, UT http://vfrmap.com/?type=vfrc&lat=37.036&lon=-113.510&zoom=10 E2 described on p. E-84, E4 described on p. E-190

KBIH -- Bishop Airport, Bishop CA http://vfrmap.com/?type=vfrc&lat=37.373&lon=-118.364&zoom=10 E2 described on p. E-130, E4 described on p. E-211-212

KTPL -- Temple, Draughon-Miller Central Texas Regional Airport, TX http://vfrmap.com/?type=vfrc&lat=31.152&lon=-97.408&zoom=10 E2 described on on p. E-127, E4 described on p. E-209

Also note that many Class D airspaces with E4 extensions also have separate descriptions of the inner dashed circle in the E2 section of the "Airspace Designations and Reporting Points" document. However, these represent cases where the tower is not open 24 hours a day. In such cases, when the tower is closed, the D airspace usually (but not always) reverts to E2 airspace, hence the separate description of that airspace in the E2 section of the "Airspace Designations and Reporting Points" document. That can't be what's going on with any of the five airports described above.

As an aside, it would be good for the chartmakers to standardize on the notation used at KACV and KTVL rather than the notation used at KSGU, KTPL, and KBIH, since there appear to be some functional differences between the E2 and E4 airspaces, as noted above.

Which among the answers 1 through 8, or what other answers, seem the most plausible reason or reasons why KACV and KTVL-- as well as the other three airports noted here-- feature the unusual configuration (for a non-towered airport) of E4 "extensions" in addition to the E2 airspace surrounding the airport?

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  • $\begingroup$ Material in comments has now been moved to question. The point was just that some E2 airspaces do include very long projections that are designated as part of the E2, not as E4 "extensions" -- see for example KSHR --Page E-92 in the "Airspace Designations" document --The E-2 airspace extends 17.6 (nautical) miles southeast of the airport-- and also 16 miles northwest of the airport-- no E4 is present-- airspace last amended 5/31/12 vfrmap.com/… $\endgroup$ – quiet flyer May 7 '19 at 19:41
  • $\begingroup$ As an aside, AIM 3-2-5 d1 gives this general criteria for designating a piece of airspace protruding from a Class D airspace area as an E4 "extension" or as part of the Class D airspace-- "1. Class D surface area arrival extensions for instrument approach procedures may be Class D or Class E airspace. As a general rule, if all extensions are 2 miles or less, they remain part of the Class D surface area. However, if any one extension is greater than 2 miles, then all extensions will be Class E airspace." $\endgroup$ – quiet flyer May 13 '19 at 6:16
  • $\begingroup$ Possible item of peripheral interest--re Class E surface extensions to Class D airspace-- " d. 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." -- FAA Order "Order JO 7400.2L Procedures for Handling Airspace Matters" --faa.gov/documentLibrary/media/Order/… -- $\endgroup$ – quiet flyer Feb 18 at 15:27
  • $\begingroup$ Looks like "community" popped it up to the top of the stack, so I'll make the edits-- $\endgroup$ – quiet flyer Feb 18 at 16:28
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Option 7 cannot be the answer-- I called the ARTC Centers controlling the E2 and E4 airspaces around these airports and in every case the staffer told me they controlled both the E2 airspace and the E4 airspace.

The staffers also told me that they make no distinction between E2 and E4 airspace in terms of granting SVFR flight, but that doesn't necessarily eliminate the possibility that the people who originally planned the airspace configuration may have been operating under the understanding that Special VFR clearances should not be required or authorized E4 "extension" airspace. So options 1) and 2) seem not to be ruled out by the information stated above.

Options 4, 5, and/or 6 all seem like possible explanations as well. Option 3 seems unlikely, because unmanned free balloon operations are rather rare.

In truth, it is really a matter of some uncertainty as to whether the regulations listed in the question that pertain to the E2 "Surface Areas", do or do not also pertain to the E4 Class-E-to-surface "extensions". Follow the links given in the original question for more on this topic.

Option 8 seems unlikely, considering the rarity of this airspace configuration, and also considering the very long E2 projections that exist at some untowered airports such as KSHR (Sheridan, WY). Guidance similar to that suggested in option 8 does exist for Class D airspace, but apparently not for Class E2 airspace.

It is the case that at KTVL-- but not at KACV or the other 3 airports mentioned here-- the description of the E4 airspace in the "Airspace Designations and Reporting Points" document has a note to see the A/FD for effective hours, while the E2 airspace has no such note. The sectional chart has a similar notation, with an arrow pointing specifically to the E4 airspace.

The current A/FD entry for KTVL simply reads "Airspace: Class E".

It's unclear whether or not there ever was an intent to allow for the possibility that the E4 airspace at KTVL would have limited effective hours while the E2 airspace would be effective 24/7. Alternatively, the note in the airspace description for KTVL regarding the hours of operation of the E4 airspace may have simply been a mistake and not the real reason for designating an E4 extension that was distinct from the E2 central region.

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  • $\begingroup$ @DeltaLima thanks for pointing out the notation re limited hours of operation of the E4 at KTLV in your earlier answer; I have not found any new information as of yet – $\endgroup$ – quiet flyer May 15 '19 at 16:42
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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.

Image from Sectional Chart Legend

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  • $\begingroup$ This answer seems to overlook the opening sentence in the 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 much more common situation for untowered airports." $\endgroup$ – quiet flyer Jan 22 at 19:27
  • $\begingroup$ Also, the illustration doesn't really show an "extension", at least not in the same sense the question was asking about, which referred to a rectangular projection of airspace that is separated (on the chart depiction) by a dashed line from the main chunk of controlled-to-surface airspace that actually surrounds the airport. $\endgroup$ – quiet flyer Jan 22 at 19:29

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