VFR Minimums

No pilot may operate an aircraft under basic VFR when the flight visibility is less, or at a distance from clouds that is less, than that prescribed for the corresponding altitude and class of airspace.

Airspace questions can seem tricky on knowledge tests, since they require memorization.

There aren't any VFR minimums in Class A airspace, because VFR traffic is not permitted.

Three (3) statute miles of visibility is the most common requirement, which applies to class B, C, D, E, and G — always, or at least at times.

The number "3-152" is a common mnemonic. In addition to three miles of visibility, aircraft must remain 1,000 above clouds, 500 feet below clouds, and 2,000 feet horizontal from clouds. This applies to class C, D, and and E airspace.

Class B has one variable: There are no cloud requirements, other than that VFR traffic must remain "clear of clouds."

Class E has one variable: Above 10,000 feet, minimum visibility increases from three (3) to five (5) miles, and the cloud-distance requirements are "111" — 1,000 feet above, 1,000 feet below, and one (1) mile horizontal. These increases account for aircraft traveling at higher speeds when at higher altitudes.

Therefore, "three and clear," "3-152," and "5-111" are the only three cloud-and-visibility requirements of VFR controlled airspace.

Class G Airspace (and its exceptions…)

Which brings us to Class G — airspace that does not have any controlling authority, but that does have VFR minimums, and it has six variables. FAA tests often include Class G questions because of this complexity.

Class G also is the only class of airspace with separate day and night VFR minimums.

The rules covering Class G minimums are more complex than the rules for all other airspaces combined. Examining specific use-cases is the best way to retain the information for testing purposes. After the test, you can always consult FAR 91.155.

Class G airspace isn't depicted on a VFR sectional the way other airspaces are. Instead, we tend to see it by knowing where it is not. This leaves us with two questions:

• Where does Class G originate at the surface?
• Under what circumstances does Class G terminate?

Class G starts at the surface, unless a higher class of airspace has been established at the surface. Class B, C, and D airspace starts at the surface. Class E airspace can be designated to start at the surface.

Class G terminates:

• Usually: At 1,200 feet AGL;
• Sometimes: At 700 feet AGL, if class E has been lowered in the vicinity of an airport;
• Rarely: Above 1,200 feet AGL, if it underlies a non-standard floor of Class E airspace.

Class B and C airspaces are easy to spot on VFR sectionals. Class D airspace is depicted with a dashed blue line, while Class E Surface Airspace is depicted with a dashed magenta line.

Per the Aeronautical Chart User's Handbook, "Class E airspace exists at 1,200 AGL unless designated otherwise." A common example of "otherwise" is class E airspace that begins at 700 AGL adjacent to (or overlying) airports with instrument approaches. This floor is depicted on the sectional with a magenta vignette.

Southwest Oregon Regional (KOTH) offers a look at these varying airspaces. Class D originates at the surface, in the immediate vicinity of the airport. Class E Surface Airspace is established to the east and west as arrival extensions, which support instrument approaches. And Class E with a floor of 700 AGL is established in the airport's general vicinity.

Thus, Class G exists where these airspaces do not exist: below 700 AGL within the magenta vignette — but not in the airspaces that originate at the surface. External to the vignette, Class G extends vertically to 1,200 AGL.

Have a look at the triple-line over the Pacific Ocean. The serrated magenta line is the boundary of a Military Operation Area (MOA), while the colocated blue serrated line is the boundary of a Warning Area. The uneven blue line between them marks a boundary between where Class E has a floor of 1,200 AGL (east) and a floor of 5,500 MSL (west).

Just to the south of the controlled airspaces associated with KOTH is a smaller airport called Gederos, which is private. This is the sort of airport where Class G minimums come into effect. Gederos has a field elevation of 280 MSL, which means that Class G extends to 1,480 MSL (1,200 AGL) when calculated in the vicinity of the airport.

A Minor Detail at Best: Why is Class G adjacent to or overlying airports that have one or more published instrument approaches? If the approach terminates above 700 feet AGL, then the FAA may decide to reduce Class E airspace from 1,200 to 700 AGL, and in many cases this likely is true.

However, in some cases (such as KLKV Lake County, Ore.), an RNAV approach has a decision altitude (DA) of 250 AGL. Therefore, it appears that there are cases where approaches are permitted to continue into non-controlled airspace, since LKV does not have Class E airspace at the surface.

Rules & Exceptions

Let's return to "3-152," because it's the most common visibility and cloud-distance requirement in Class G regulations. Unless an exception is identified, Class G is 3-152 below 10,000 MSL.

Daylight Benefit: Only one (1) mile of visibility is required for daytime operations when operating below 10,000 MSL. At night below 10,000 MSL, Class G is 3-152.
   Mnemonic: "It's easy to see in daylight (below 10,000 feet), so only one mile of visibility is necessary."

Low-Altitude Benefit (Day): Aircraft conducting daytime operations at or below 1,200 AGL must remain "clear of clouds," just as in Class B airspace. Again, at night below 10,000 MSL, Class G is 3-152.
   Mnemonic: "It's easy to remain clear of clouds below 1,200 AGL because there aren't many clouds there anyway on VFR days. But if it's night, I can't see the clouds well enough to stay clear."

Traffic Pattern Benefit (Night): FAR 91.155 states that if visibility is less than three (3) statute miles but not less than one (1) statute mile during night hours, an airplane in an airport traffic pattern within 1/2 mile of the runway may operate clear of clouds. Thus, when at traffic-pattern distance and altitude, the Daylight Benefit (one mile visibility) and Low-Altitude Benefit (clear of clouds) are in effect during night operations.
   Mnemonic: "The airport's beacon at night is artificial sunlight. Within 1/2 mile of the beacon at night, I'm only required to have one (1) mile of visibility and remain clear of clouds."

Practically speaking…

In reality, it's unlikely that you will ever operate an aircraft in Class G airspace and not use "one-and-clear" during daylight hours. Knowledge tests can include questions about Class G airspace that is more than 1,200 AGL, but in fact virtually all of this airspace is over oceanic waters and in Alaska. Variable Class E floors over remote areas within the Lower 48 have been phased out over the past two decades.

Class G test questions also like to mix the 1,200 and 10,000 feet values to trip you up. You can get a questions on required visibility above 1,200 feet, or cloud clearance requirements below 10,000 feet. The 10,000 figure is for daylight visibility (1 mile), whereas the 1,200 figure is for clouds (remain clear).

All traffic patterns at Class G airports use the "one-and-clear" minimums for both day and night operations.

What if the airport traffic pattern is in Class E airspace that has a 700 AGL floor? Inevitably, the airport will have Class G airspace from the surface to 700 AGL, which means the complete traffic pattern exists in two types of airspace. The regulations state that one-and-clear operations "may be conducted in Class G airspace below 1,200 feet above the surface." Thus, Class E airspace minimums override Class G traffic pattern benefits. If you come across this scenario on a test question, don't consider the airport to be a Class G field.

Not including airport traffic patterns, night cancels Class G exceptions. It's as simple as that. At night, Class G does not become Class E, but it conforms to Class E — 3-152 becoming 5-111 at 10,000 MSL.

But if 10,000 MSL is within 1,200 feet of the surface, such as in the vicinity of Lake County, Colo. (LXV, 9,934 feet), then 5-111 is deferred until aircraft are operating at or above 1,200 AGL. An aircraft operating above LXV and below 11,134 MSL can fly one-and-clear during the day, and also at night in the airport's traffic pattern. (LXV is the highest airport in the conterminous United States and one of the few locations where this exception applies.)

Underlying, Overlying, and Overlapping Airspace

When a positively controlled airspace is established under another layer of positively controlled airspace, it may be assigned one or more non-standard termination levels. This is Underlying Airspace, where vertical movement from the lower airspace will encounter more restrictive airspace — either immediately, or at some point above a vertical gap.

Class C Airspace

Class C airspace uses a value of "T" to indicate that a sector of airspace terminates when it makes contact with overlying Class B airspace. The "T" is generic (and possibly short for "terminates"), since multiple sectors of Class B above a sector of Class C airspace may have varying floors.

Using Oakland International's (OAK) Class C airspace as an example, we can see that the central area has a "T" termination. Over the San Francisco Bay, OAK's central Charlie airspace terminates at 2,099 MSL, since San Francisco International's (SFO) Class B airspace has a 2,100 MSL floor.

To the west, over San Leandro, OAK's central Charlie airspace terminates at 3,999 MSL, where SFO's Bravo has a 4,000 MSL floor.

And to the northwest, over Alameda, OAK's central Charlie airspace terminates at 2,999 MSL, where SFO's Bravo has a 3,000 MSL floor.

The northwest and southeast shelves of OAK's Charlie airspace also have "T" terminations with variable upper limits. Both sectors have 1,500 MSL floors.

Class D Airspace

Class D uses a different convention: If the vertical limit of the airspace is up to, but not including, the floor of overlying airspace, then the Delta upper limit is shown in the standard two-digit box used for all Delta airspace, but the value is preceded by a minus symbol.

If we look at the Class D airspace associated with Hayward Executive (HWD) to the southeast of OAK, we can see that the airspace's upper limit is "-15" — i.e., 1,499 MSL. This is because most (but not all) of the Delta airspace is capped by the 1,500 MSL floor of OAK's southeastern Charlie shelf.

There is a very small area of Delta airspace that isn't under the Charlie shelf (around Castro Valley). However, in this case the entire Delta terminates at 1,499 MSL. Above this altitude over Castro Valley, the airspace is Class E from 1,500 to 3,999 MSL, becoming SFO's Bravo airspace at 4,000 MSL.

Thus, while a negative value associated with the vertical limit of a Delta airspace suggests that the whole of the airspace is capped by more-restrictive airspace, this isn't always the case.

There are three Delta airspaces to the south of Dallas-Fort Worth (DFW): Arlington Muni (GKY), Grand Prairie Muni (GPM), and Dallas Executive (RBD). All three have upper limits that are charted as negative values. However, only the northern portions of GKY and GPM make contact with the 2,000 MSL floor of DFW's adjacent Bravo shelf. To the south, both airspaces have a 1,001-foot gap between Delta and Bravo, which is Echo airspace (albeit with under-Bravo-shelf restrictions).

Most of the RBD airspace makes contact with the overlying Bravo shelf. This is conventional underlying/overlying airspace, since RBD's Delta terminates at 2,999 MSL and the DFW Bravo shelf in this sector has a floor of 3,000 MSL.

But there is a very small segment of RBD's Delta airspace, to the northwest, that intrudes into a DFW Bravo shelf with a 2,000 MSL floor.

This small area isn't overlying/underlying airspace, but instead overlapping airspace. In this case, two airspaces exist in the same location.

Overlapping Airspace

Per 14 CFR 71.9 "When overlapping airspace designations apply to the same airspace, the operating rules associated with the more restrictive airspace designation apply." It's as simple as that.

Let's get a look at the Class C airspace at Portland International (PDX) and the Class D airspace to the east at Portland-Troutdale (TTD).

Most of the TTD airspace is overlapped by two PDX Class C shelves. The Delta airspace to the east terminates at 2,500 MSL, where it becomes Class E from 2,501 MSL to FL 180.

Since TTD's Class D is charted to terminate at 2,500 MSL, this then is the upper limit to the east.

Where the Class D intersects with Class C, per § 71.9, it terminates at either 1,699 MSL or 1,999 MSL.

If just one Charlie shelf overlapped the Delta airspace, it might make more sense to chart TTD's Delta in a different manner — for example, "-20" if the base of all overlying Class C airspace started at 2,000 MSL. But this then would reduce the upper limit of the Delta to the east, and there may be good reasons to retain 2,500 AGL there, which is the standard height for Delta airspace.

Bear in mind that Delta airspace isn't segmented in the same manner as Bravo or Charlie. Airspace designers only get to work with one Delta sector that has one charted upper limit.

"Upward from above"

This won't be on a Private Pilot knowledge test, but Class Bravo has an "upward from above" convention on aviation charts. Bravo floors that extend upward from above a certain altitude are preceded by a "+".

This is prominent in the Bravo airspace around New York City's passenger terminals (JFK, La Guardia, and Newark). Here we can see that Bravo shelves begin at 501, 801, and 1,201 MSL.

Reasons for a non-standard Bravo floor are unclear. The Aeronautical Chart Users' Guide only advises us that they exist.

Could the "+" be required for all Bravo sectors that start at low altitude? For example, 1,200 AGL and below? This doesn't seem to be the case, since one of Seattle's Bravo segments above Boeing Field (field elevation 22 feet) starts at 1,100 MSL — not 1,101 MSL.

One possible reason for this would be so that ATC can assign 500, 800, and 1,200 MSL altitudes to helicopters on published helicopter routes, and thus keep any transition time within the Bravo airspace to a minimum.

Unlike VFR Transition Routes, helicopter routes do not have charted altitudes. The helicopter chart notes that "altitudes will be assigned when contacting air traffic control."

Miami's Class B also uses the "+" for one sector's floor value. MIA doesn't have an associated helicopter-route chart, but the segment with a "+10" floor abuts the shoreline. It's a "cut-out" from the central portion of the airspace that originates at the surface.

The purpose of the "+10" seems more clear in this location. It's reasonable to expect air traffic here, and the chart notes both a seaplane base and hang-glider activity under the shelf.

Why not put the floor of the Bravo airspace exactly at 1,000 MSL here? It appears so that traffic under the shelf can use, or be assigned, a 1,000 MSL cruising altitude without a Bravo clearance.

This suggests that a non-standard "+" Bravo floor is established when the floor has a low altitude (1,200 AGL or less) and a substantial amount of non-terminal traffic is expected under the floor. It appears that these non-standard floors only exist in New York and Miami's Bravo airspace.

(Note: The string of red dots on the Miami TAC is not aeronautical information, but instead only to connect the X44 information block to the location of the seaplane base.)

Special VFR

Per § 91.157 "Special VFR weather minimums," pilots are allowed to operate below the established weather minimums in "controlled airspace designated to the surface for an airport." This type of operation commonly is referred to as "Special VFR," or just "a Special."

Special VFR requires:

• an ATC clearance;
• a flight visibility of at least one (1) statute mile;
• if landing, a ground visibility of at least (1) statute mile;
  • If ground visibility is not reported, flight visibility may be used for purposes of landing;
• that aircraft must remain clear of clouds;
• operations only "between sunrise and sunset," i.e. during daylight hours.
  • Note that the 1.1 definition of "night" is not used in this case, so civil twilight does not apply.

There is an exception to the daylight requirement. Special VFR can be conducted between sunset and sunrise (i.e., at night), but only if:

• the person being granted the ATC clearance meets the applicable requirements for instrument flight (per Part 61);
• the aircraft is equipped for instrument flight (per 91.205).

Some airports within controlled airspace don't permit Special VFR. If so, this will be noted in the Chart Supplement, and "NO SVFR" will be included in the information block on the Sectional/TAF.

Why is Special VFR limited to controlled airspace? The obvious reason is because a Special requires an ATC clearance, and ATC doesn't provide services in non-controlled airspace.

A Special VFR in Class G would provide marginal benefits. During the day, Class G below 10,000 MSL requires only one mile of visibility, and below 1,200 AGL aircraft must remain clear of clouds. At night, Class G is "one-and-clear" in airport traffic patterns.

A Special VFR in these cases would provide a bit more operating room, since it reduces the weather minimums to "one-and-clear" within the airspace designated for an airport, rather than below 1,200 AGL (day) or the traffic pattern (night).

However, without airspace designated for the airport, or a controller to provide a clearance, pilots operating at Class G airports have somewhat more-restrictive weather minimums than pilots operating under Special VFR in controlled airspace.

Special Use Airspace: AC 210-5B

While the Aeronautical Information Manual and the Pilot's Handbook of Aeronautical Knowledge cover Special Use Airspace, Advisory Circular 210-B, "Military Flying Activities" (Aug. 8, 1990), is an authoritative document.

For those willing to dig into the details, 14 CFR Part 73 offers everything that's stated for legal purposes.

Prohibited and Restricted Areas

A common student question is "What's the difference between a Prohibited Area and a Restricted Area if I can't enter either airspace?"

They key distinction is that flight is not allowed in either type of area without permission, but if entry is permitted into a Prohibited Area, such can only be granted from the controlling agency. However, flight into a Restricted Area can be permitted from either a controlling agency or a using agency.

Per 14 CFR 73, a controlling agency is "the FAA facility that may authorize transit through or flight within a restricted area in accordance with a joint-use letter…."

A using agency is the "agency, organization, or military command whose activity within a restricted area necessitated the area being so designated."

Thus, while the FAA can clear aircraft into a Prohibited or Restricted Area, the military only has legal authority to clear aircraft into a Restricted Area — which makes sense, since Restricted Areas typically include military flight operations hazardous to non-participating aircraft. Prohibited areas, on the other hand, are closed for reasons of national security.

Or to put it another way: There's always something within a Prohibited Area. For example, P-51 on Hood Canal in Washington State is centered on the Trident submarine base associated with Naval Base Kitsap.

By comparison, we're more likely to find nothing within a Restricted Area, since it's reserved for things like artillery firing, aerial gunnery, and guided missiles. R-5701, in Oregon, includes more than 120 square miles of featureless dirt. In the center of this barren expanse are the white concentric circles of what appears to be precision target (with a 3,000-foot diameter).

Military Training Routes

While aircraft operating below 10,000 MSL are not permitted to fly at indicated airspeeds greater than 250 knots, the regulation does not apply to military aircraft operating on Military Training Routes (MTRs). AC 210-5B notes that MTRs are "established to accomodate low-altitude training operations that must be conducted at speeds in excess of 250 KIAS below 10,000 feet MSL."

You read that correctly: "must be conducted…" In principle, aircraft on MTRs can operate at any airspeed below Mach 1.

MTRs are thin lines on aviation charts, but AC 210-5B notes that only the route's centerline is depicted, and that the normal route width is 5-10 miles from the centerline. MTR route widths can vary from two (2) to 20 miles.

Civilian pilots should cross MTRs with due caution, modifying courses to cross all MTRs at right angles when practicable.

Flight Service Stations (FSS) can brief pilots on MTR times of use, altitudes, and route widths, but this is "only on pilot request" and not included in any of the three routine briefing formats (Abbreviated, Standard, and Outlook).

Transponder Codes

Most pilots know that "Mode C" is required in restrictive airspace, but overall awareness often doesn't go beyond that. Student pilots need to watch out for FAA knowledge test questions that refer to transponder modes — it's easy to get lost in the jargon.

Without transponders, the only way that ATC can see aircraft is via primary radar, as an aircraft's presence interferes with electromagnetic waves that are illuminated in the atmosphere.

Transponders are a component of secondary radar — technically, the Air Traffic Control Radar Beacon System (ATCRBS) (AIM 4-5-2). The system requires an interrogator and a transponder, along with a radar scope that can display returns from primary radar and ATCBRS.

Within ATCRBS, transponders can operate in modes. The military has five transponder modes, which are denoted with the nautral numbers 1 through 5. Meanwhile, civilian operations use letters instead of numbers.

Mode A is the transponder mode that sends four discrete numbers (0-7) to the interrogator for display on the radar scope. This corresponds to the military's Mode 3, so this feature sometimes is described as Mode A/3 or Mode 3/A.

Mode C is the transponder mode that transmits altitude information, also for display on the radar scope. The military's Mode 3 also includes this function. And Mode C doesn't replace or interfere with Mode A. Thus, when operating a Transponder in Mode C, civilian pilots actually are operating in mode A and C (unless Mode A somehow is disabled).

If we include the military's version of this mode, we might call it mode A+C/3 — but nobody does that. Everybody just says "Mode C."

There are no civilian equivalents for Modes 1, 2, 4, and 5.

Finally, Mode S is applicable to both military and civilian operations. This is an advanced system associated with Traffic Information Service (TIS). (AIM 4-5-6)

TIS improves the safety of flight through an automatic display that informs the pilot of nearby traffic and potential conflict situations. It employs an enhanced capability of the terminal Mode S radar system, which contains the surveillance data, as well as the data link required to uplink this information to suitably-equipped aircraft. As such, it would be very unusual to find Mode S capability on a small aircraft dedicated to flight training or personal use.

Pilots not in communication with ATC are expected to set their transponders to 1200, which indicates that the flight is VFR. Pilots in contact with ATC will be asked to "squawk" a specific transponder code.

Pilots should be aware of three codes that are reserved for specific purposes:

• 7500: Hijack (i.e., air piracy)
• 7600: Radio failure
• 7700: Mayday

Pilots are cautioned not to accidentally enter one of these three codes into a transponder while setting any new code. Even a momentary entry will alert ATC of a possible emergency.

Also note that it's not necessary to squawk 7700 for the duration of an emergency, such as an off-airport landing. Once contact has been made with ATC on the emergency frequency (121.5), the transponder can be re-set to the previous code. Setting 7700 in the transponder sets off an alarm. When ATC is aware of your location and situation, they probably will ask you to turn it off.

Aircraft Speed Limits

Per 14 CFR 91.117 and 91.817, there are three posted speed limits in the National Airspace System, as stated for the following areas:

• Sub-Mach 1: No person may operate a civil aircraft in the United States at a true flight Mach number greater than 1 without a special flight authorization.
• 250 knots (288 mph): This includes all airspace below 10,000 feet, with some exceptions.
• 200 knots (230 mph): Airspace at or below 2,500 AGL within four (4) nautical miles of an airport at the center of Class C or D Airspace.
• 200 knots (230 mph): Airspace underlying Class B airspace.
• 200 knots (230 mph): In a VFR corridor through Class B airspace.

It might be easier to state the rule-set in a different manner:

• There is no subsonic speed limit at or above 10,000 MSL;
• Below 10,000 MSL, the speed limit is 250 knots;
• Aircraft are limited to 200 knots if they are operating in:
  • Class D airspace;
  • The parcel of Class C airspace that would be occupied by a Class D cylinder;
  • Under, or in a tunnel through, Class B airspace.

For test prep, the best recall method is to ask "Where are both slow and fast traffic likely to conflict?" Faster and slower airplanes are concentrated…

• within Class B VFR Flyways and VFR Corridors;
• at Class C and D airports.

These are the areas where the 200-knot IAS limit is applied.

It's also helpful to bear in mind the probable rationales for these limits:

• There isn't any subsonic speed restriction above 10,000 MSL because slower aircraft typically do not operate above that altitude.
• Below 10,000 MSL, IAS is restricted to 250 knots because of the mix of fast and slow traffic.
• Near Class C and D airports, and in Class B flyways and corridors, IAS is restricted to 200 knots because of the close concentration of fast and slow traffic.
• And while there may be a mix of fast and slow aircraft at Class E and G airports, the limit there is 250 knots instead 200 knots because of the reduced traffic density at most of these airports — but certainly not all of them.

Indicated, True, Ground

Note that speed restrictions are based on indicated airspeed. True airspeed and groundspeed don't matter.

One or Two Exceptions…

1. If the minimum safe airspeed for any particular operation is greater than the maximum speed, the regulations state that aircraft may be operated at that minimum speed.
2. Military aircraft on Military Training Routes (MTRs) below 10,000 MSL operate at airspeeds in excess of 250 KIAS. It's possible that the above statement is the legal basis for this exception.

What's the Class B Speed Limit?

Most knowledgable pilots will state that the speed limit within Class B is 250 knots, but this is not accurate.

Per § 91.117(b), Class B indicated airspeed limits comply with § 91.117(a), which is to say at or below 250 knots IAS when operating below 10,000 MSL.

Since most Bravo airspaces terminate at 10,000, it's tempting to arrive at something like a rule — e.g. "Class B is limited to 250 knots IAS." However, four Bravo airspaces terminate above 10,000 MSL (see below). When operating in those airspaces at or above 10,000 MSL, the 250 knots IAS restriction does not apply.

Simply put, the regulations do not stipulate one speed limit for Class B airspace. However, nearly everyone infers a 250 knots IAS limit.

And if details matter, here's another: Most Class B airspaces extend up to and including 10,000 MSL, while the 250 knots IAS limit terminates at 9,999 MSL. Therefore, it's possible for an aircraft to operate at exactly 10,000 MSL and obtain a Bravo clearance while operating at any subsonic airspeed.

"Underlying Class B"

§ 91.117 includes a bit of ambiguous language: Indicated airspeeds are limited to 200 knots "in the airspace underlying a Class B airspace area designated for an airport."

This might mean one of two things:

• All aircraft operating in airspace underlying Class B are restricted to 200 knots IAS, provided that the Bravo airspace has been designated for an airport, i.e. a central terminal.
• All aircraft operating in airspace underlying Class B are restricted to 200 knots IAS, provided that they also are operating within airspace that has been designated for an airport that underlies that airspace.

Short version: If you are preparing for a knowledge test and are asked about airspeeds under a Bravo shelf, the answer is 200 knots IAS. Ignore the nuances of 91.117. If you are in a testing center, everything under the shelf has been dropped to 200 kts.

Moreover, § 71.41 states "Each Class B airspace area designated for an airport… contains at least one primary airport around which the airspace is designated." Thus, the phrase "designated for an airport" appears in Part 71, where airspace is defined.

However, this doesn't explain why 91.117 has superfluous language, or why 71.41 is oddly self-referential. To describe Class B as an "area designated for an airport" is redundant. It's like saying "Class B airspace area designated for an airport where both nitrogen and oxygen are present."

If we were to read 91.117 in the alternate manner, what then would constitute "airspace designated for an airport"?

• If Class C airspace underlies Class B airspace, then the whole of that Class C airspace would be limited to 200 knots IAS, not just the cylinder with a four-mile radius up to 2,500 AGL. Oakland's Charlie airspace, which underlies San Francisco's Bravo, is an example.
• Class D airspace seems obvious. However, note that Class D can vary from four to five miles, depending on local requirements. The above graphic visualizes the speed-limit rule with a Class D cylinder that has a four-mile radius, which is common. However, if Class D airspace underlies Class B airspace, then the whole of the airspace would be limited to 200 knots, even if the radius is more than four nautical miles.
• All Class E airspace that's centered on an airport would be restricted to 200 knots. This includes Echo that's been dropped to 700 AGL and to the surface, but not Echo at 1,200 AGL, since Echo that begins at that altitude is not "designated for an airport."

Published VFR Routes

The Aeronautical Information Manual (AIM), Chapter 3.5, provides details on Published VFR Routes. This general term refers to several kinds of airspace, and the AIM (sort-of) acknowledges that this has led to some confusion. Published VFR Routes include:

• VFR Flyways
• VFR Transition Routes
• VFR Corridors

"Terminal Area VFR Routes" also is in use. More on that in a bit.

VFR Flyways

VFR Flyways are designed solely to assist pilots in planning flights under and around busy Class B airspace without entering Class B airspace. An ATC clearance is not required to utilize a VFR flyway, although the AIM states that they "are not intended to discourage requests for VFR operations within Class B airspace."

VFR Flyways are shown on the reverse side of Terminal Area Charts (which is in fact the "VFR Flyway Planning Chart"). They are depicted with blue arrows and altitude restrictions. The AIM emphasizes that the "entire Class B airspace, and the airspace underneath it, may be heavily congested with many different types of aircraft."

Because a VFR Flyway typically is found under a Bravo shelf, airspeed is restricted to 200 knots IAS.

VFR Transition Routes

VFR Transition Routes provide an accommodation for VFR traffic transiting through positively controlled airspace. In many cases, it's to get aircraft from one side of the airspace to the other directly over the central terminal. This keeps transiting aircraft clear of the airport's arrival and departure legs.

Because Class B VFR Transition Routes typically transit the airspace, they require a clearance to enter the Class B airspace and ongoing communication with ATC.

VFR Transition Routes are presented on Terminal Area Charts and VFR Flyway Planning Charts. On the chart below, they are depicted with magenta arrows. The chart notes each route's name and additional information.

Because VFR Transition Routes are charted within Bravo airspace, airspeed is restricted to 250 knots IAS.

VFR Corridors

VFR Corridors are similar to VFR Transition Routes, but they do not enter Class B airspace. Instead, a tunnel of Class E airspace is feature of the Class B airspace design.

Aircraft may operate in VFR Corridors without an ATC clearance or communication with air traffic control. However, it's still within the Class B veil, so Mode C and ADS-B Out are required.

VFR Corridors have defined vertical and lateral boundaries. The AIM notes that "extreme caution and vigilance must be exercised" because of the volume of VFR traffic — not unlike many cars trying to get somewhere using a freeway tunnel.

Airspeed is restricted to 200 knots IAS within a Class B VFR Corridor.

San Diego (KSAN) has an established VFR Corridor with the following details on the Terminal Area Chart (TAC):

• An inset box points to the terminal and notes two vertical dimensions for the associated sector of Class B airspace: Surface to 3,300 and 4,700 to 10,000 MSL. This indicates that aircraft in the corridor must remain between 3,301 and 4,699 MSL to avoid a vertical incursion into Bravo.
• The Bravo segment that overlies the terminal has additional values to the south: 2,800 to 3,300 and 4,700 to 10,000 MSL. The higher floor here is because the NZY Class D airspace terminates at 2,799 MSL.
• The segment of Class B airspace directly over the terminal has two magenta arrows, which are placed to either side of a dashed magenta meridian.
• A magenta information box includes an arrow that points to the meridian. This box sets out guidance for the corridor to prevent horizontal incursions into Bravo.
• The corridor guidance notes that no "communications or clearances with ATC are required while transiting," but it also provides a radio frequency (126.050) for pilots within the corridor.

There is a VFR Corridor through the New Orleans (MSY) Bravo airspace. However, a note on the TAC describes it as a "flyway." Per the AIM, the tunnel through the MSY airspace corresponds to a VFR Corridor, not a VFR Flyway.

Class C: Published VFR Routes

Published VFR Routes may be associated with C airspace to route traffic around, above, or through the airspace. These probably will be designated as VFR Transition Routes on most charts.

They also might be considered Terminal Area VFR Routes, but this is unclear. While the AIM does not define a "Terminal Area VFR Route," the term is mentioned in the PHAK as currently in use.

The AIM suggests that all Class B VFR Transition Routes will enter the airspace, and thus require a clearance. However, it doesn't offer any guidance on VFR Transition Routes associated with Class C or D airspace.

The Class C Airspace at Portland (PDX) is a good example. There are three Published VFR Routes, two of which — Timbers and Thorns — remain outside the airspace. Meanwhile, Blazers crosses the Class C Terminal with a restriction "at or above 3,500," which means that any flight at or below 4,000 MSL would require ATC radio contact before entering the airspace, while any flight above the airspace would not require communication with ATC.

The Seattle/Portland TAC includes a supplemental note on the PDX VFR Transition Routes. When operating on one of the routes outside of Class C airspace, pilots are encouraged to use VFR Flight Following.

Class Bravo: The Finer Points

While all of the definitions and operating rules regarding airspace, including Class B, are found in Parts 71-77 and 91, the best resource is the Aeronautical Information Manual (AIM) (Chapters 3.2 and 3.5).

Dimensions

The dimensions of Class B are described in the AIM as "Generally, that airspace from the surface to 10,000 feet MSL surrounding the nation's busiest airports." Note the word "generally." There are several Class B airspaces that terminate at altitudes other than 10,000 MSL:

• Atlanta (12,500 MSL)
• Boston (7,000 MSL)
• Cleveland (8,000 MSL)
• Dallas/Fort Worth (11,000 MSL)
• Denver (12,000 MSL)
• Honolulu (9,000 MSL)
• JFK / La Guardia / Newark (7,000 MSL)
• Kansas City (8,000 MSL)
• Miami (7,000 MSL)
• New Orleans (7,000 MSL)
• Philadelphia (7,000 MSL)
• Phoenix (9,000 MSL)
• Pittsburgh (8,000 MSL)
• St. Louis (8,000 MSL)
• Salt Lake City (12,000 MSL)

Certification

FAA knowledge tests emphasize that a student pilot certification is sufficient to enter Class B airspace, and to land at an airport within Class B, provided that the student's logbook has the required endorsements. However, there are twelve Class B airports that require at least a Private Pilot certificate for purposes of landing. Students, Recreational Pilots, and Sport Pilots are excluded.

Specifically, "No person may take off or land a civil aircraft at the following primary airports … unless the pilot-in-command holds at least a private pilot certificate."

• Andrews AFB
• Atlanta Hartsfield
• Boston Logan
• Chicago O'Hare
• Dallas/Fort Worth
• Los Angeles
• Miami
• Newark
• New York Kennedy
• New York La Guardia
• Ronald Reagan Washington National Airport, DC
• San Francisco

It appears that Recreational, Sport, and Student pilots are permitted to enter these airspaces, provided that they meet all other legal requirements.

Arrivals and Departures

If you are flying a "large, turbine powered airplane" and operating to or from the central terminal of Class B airspace, you "must operate at or above the designated floors" of the airspace. This means that smaller aircraft under the shelf of Class B airspace are very unlikely to encounter large passenger and cargo aircraft, even if they are several miles away from the central terminal. Thus, operating under Bravo provides an additional margin of safety.

Veil Exception

Aircraft not certified with an "engine-driven electrical system" can operate within the Mode C/ADS-B veil without the required equipment, provided that they:

• do not enter Class A, B, or C airspace;
• remain below 10,000 feet;
• remain below the ceiling of an associated Class C airspace, if one is present.

Part 93: Special Areas & Specific Rules

Special Air Traffic Rules (SATR) and Special Flight Rules Areas (SFRA) are documented in 14 CFR 93, while details can be found in the Aeronautical Information Manual, Chapter 3.5.7. Special Flight Rules Areas are noted on aviation charts.

Pearson Field (VUO) is one of the oldest airports in the United States, while Portland International (PDX), just across the Columbia River, was built decades later. Because of its historic significance, there are no plans to close Pearson Field. However, departing and arriving traffic can conflict with much larger aircraft at PDX, which is why it's designated as a "Special Rule Area" on the chart.

The Seattle sectional has an information inset, to the west of Astoria, that provides basic operational information for Pearson Field.

Several locations in the United States with specific aviation requirements have been designated as Special Flight Rules Areas, and their associated Special Air Traffic Rules are set forth in Part 93. They include:

• Anchorage, Alaska, Terminal Area
• Vicinity of Niagara Falls, New York
• Valparaiso, Florida, Terminal Area
• Vicinity of Los Angeles International Airport
• New York North Shore Helicopter Route
• Lorain County Regional Airport
• High Density Traffic Airports (La Guardia, Newark, JFK, O'Hare, Reagan National)
• Ketchikan International Airport
• Pearson Field (Vancouver, WA)
• Vicinity of Luke AFB, AZ
• Allocation of Commuter and Air Carrier IFR Operations at High Density Traffic Airports
• Ronald Reagan Washington National Airport
• Vicinity of Grand Canyon National Park
• Washington, DC Metropolitan Area
• College Park Airport, Potomac Airfield & Washington Executive/Hyde Field Airport
• New York Class B Airspace Hudson River and East River Exclusion

While FAA knowledge tests don't cover this topic in detail, it may come up during the oral portion of a practical test if a Special Flight Rules Area is near your training airport.

Additionally, pilots should become familiar with all Part 93 requirements if they will be operating in any of these areas.

Otherwise, pilots should generally be aware that Part 93 exists to establish flight rules for areas that require more operating restrictions than those provided by Part 91.