Comparable Values: RVR 2,400 Starts the Approach, Flight Visibility Decides the Landing

A Category I instrument landing system minimum arrives at the crew as a four-digit number. RVR 2,400. Open Title 14 of the Code of Federal Regulations and a second number sits beside it in a printed table: one half of a statute mile. The paragraph that holds the table is headed "Comparable values of RVR and ground visibility." A dispatcher reading a field report, an airline operations centre watching an arrival bank, and a flight crew briefing the approach will all treat 2,400 feet and a half mile as the same weather.

They are not the same measurement. One of them is produced by an instrument aimed down a single strip of pavement. The other is produced at a weather station and describes the greatest distance visible across at least half of the sky around that station. And a third quantity, which no ground instrument reports at all, is the one that decides whether the airplane is permitted to touch down.

Three Quantities With One Name

Title 14 defines each of them separately. Under U.S. Government Publishing Office, 14 CFR 1.1 General definitions, flight visibility is "the average forward horizontal distance, from the cockpit of an aircraft in flight, at which prominent unlighted objects may be seen and identified by day and prominent lighted objects may be seen and identified by night." Ground visibility, in the same section, is "prevailing horizontal visibility near the earth's surface as reported by the United States National Weather Service or an accredited observer."

Runway visual range is not defined in 14 CFR 1.1. It is defined in the FAA Pilot/Controller Glossary, letter V, which calls it "An instrumentally derived value, based on standard calibrations, that represents the horizontal distance a pilot will see down the runway from the approach end." The same entry draws the contrast itself: "RVR, in contrast to prevailing or runway visibility, is based on what a pilot in a moving aircraft should see looking down the runway." It then states where the number comes from. "It is based on the measurement of a transmissometer made near the touchdown point of the instrument runway and is reported in hundreds of feet." And it states the priority: "RVR, where available, is used in lieu of prevailing visibility in determining minimums for a particular runway."

The glossary also names three readouts, not one. Touchdown RVR is "The RVR visibility readout values obtained from RVR equipment serving the runway touchdown zone." Mid RVR is "The RVR readout values obtained from RVR equipment located midfield of the runway." Rollout RVR is "The RVR readout values obtained from RVR equipment located nearest the rollout end of the runway." Three sensors, three distances, one runway.

The quantity the glossary contrasts RVR against is defined with a clause worth reading twice. Prevailing visibility is "The greatest horizontal visibility equaled or exceeded throughout at least half the horizon circle which need not necessarily be continuous." The National Weather Service Glossary entry for Prevailing Visibility sets the same half-circle test and adds what the number is for: "The visibility that is considered representative of conditions at the station; the greatest distance that can be seen throughout at least half the horizon circle, not necessarily continuous." Representative of the station. Not of the runway, and not of the approach path.

What Paragraph (h) Actually Converts

The table lives in U.S. Government Publishing Office, 14 CFR 91.175 Takeoff and landing under IFR. Paragraph (h) has two subparagraphs and both matter. The first sets the condition and the direction of travel: "Except for Category II or Category III minimums, if RVR minimums for takeoff or landing are prescribed in an instrument approach procedure, but RVR is not reported for the runway of intended operation, the RVR minimum shall be converted to ground visibility in accordance with the table in paragraph (h)(2) of this section and shall be the visibility minimum for takeoff or landing on that runway."

Read what that sentence does and does not do. It converts a published RVR minimum into a ground visibility minimum. It runs in one direction. It operates only when RVR is not reported for the runway of intended operation. And it carries a carve-out at the front for Category II and Category III minimums, which 14 CFR 1.1 defines by reference to Category II and Category III ILS instrument approach procedures "issued by the Administrator or other appropriate authority." The table is a substitution rule for a missing instrument, not a statement that the two columns measure one thing.

The columns themselves say so. The left column header is "RVR (feet)". The right column header is "Visibility (statute miles)". Seven rows: 1,600 feet beside 1/4 mile, 2,400 beside 1/2, 3,200 beside 5/8, 4,000 beside 3/4, 4,500 beside 7/8, 5,000 beside 1, and 6,000 beside 1 1/4. Convert the right column at 5,280 feet to the mile and the pairs stop looking like a unit conversion. A quarter mile is 1,320 feet, which is 280 feet less than the 1,600 it replaces. Half a mile is 2,640 feet, which is 240 feet more than 2,400. The remaining five rows are set out in the table below, and they do not all point the same way.

Paragraph (h)(2): the two printed columns, and what the right column is worth in feet Right-hand values converted at 5,280 feet to the statute mile. Difference is paired visibility minus the RVR figure. RVR (feet) Visibility (statute miles) Same, in feet Difference (feet) 1,600 1/4 1,320 less by 280 2,400 1/2 2,640 more by 240 3,200 5/8 3,300 more by 100 4,000 3/4 3,960 less by 40 4,500 7/8 4,620 more by 120 5,000 1 5,280 more by 280 6,000 1 1/4 6,600 more by 600 Two rows credit less distance than the RVR figure each replaces: 1,600 by 280 feet and 4,000 by 40. The other five credit more. The sign first turns positive between rows one and two, where the Category I minimum of 2,400 feet lives. Source: 14 CFR 91.175(h)(2), columns as printed. Conversion is arithmetic at 5,280 feet to the mile.

Five of the seven rows credit an operator with more distance than the RVR figure the row replaces. Two credit less: the 1,600-foot row, by 280 feet, and the 4,000-foot row, by 40. The sign first turns positive between row one and row two, which is exactly where the standard Category I minimum of 2,400 feet lives.

The Chain

Now follow one arrival. The procedure publishes RVR 2,400. The runway has RVR equipment, so under the Pilot/Controller Glossary the reported RVR is used in lieu of prevailing visibility for that runway, and paragraph (h)(1) has no work to do.

Under U.S. Government Publishing Office, 14 CFR 121.651 Takeoff and landing weather minimums, the first gate is a report, not a sighting. Paragraph (b) opens "Except as provided in paragraphs (d) and (e) of this section, no pilot may continue an approach past the final approach fix, or where a final approach fix is not used, begin the final approach segment of an instrument approach procedure" and then splits into two limbs. The second limb is the one with the number in it: a pilot may not continue "unless the latest weather report for that airport issued by the U.S. National Weather Service, a source approved by that Service, or a source approved by the Administrator, reports the visibility to be equal to or more than the visibility minimums prescribed for that procedure."

So a reported 2,400 opens the door. What happens next is the part that makes this a failure mode rather than a quirk. Paragraph (c) of the same section provides that a pilot who has begun the final approach segment in accordance with paragraph (b), "and after that receives a later weather report indicating below-minimum conditions, may continue the approach to DA/DH or MDA." The weather can be reported as worse than minimums while the airplane is inside the final approach segment, and the approach continues.

At decision altitude the governing quantity changes. Paragraph (c) of 91.175 forbids descent below the authorized MDA or DA/DH "unless" a list of conditions is met, and the second condition is that "The flight visibility is not less than the visibility prescribed in the standard instrument approach being used; and". Flight visibility is the cockpit quantity from 14 CFR 1.1. The third condition requires at least one listed visual reference to be "distinctly visible and identifiable to the pilot", and the first item on that list carries its own limit: "The approach light system, except that the pilot may not descend below 100 feet above the touchdown zone elevation using the approach lights as a reference unless the red terminating bars or the red side row bars are also distinctly visible and identifiable."

The landing gate repeats the same quantity. Paragraph (d)(2) of 91.175 forbids a landing when, "For all other operations under this part and parts 121, 125, 129, and 135, the flight visibility is less than the visibility prescribed in the standard instrument approach procedure being used." And paragraph (e) makes the exit mandatory rather than discretionary: each pilot "shall immediately execute an appropriate missed approach procedure when either of the following conditions exist", the first being that the paragraph (c) requirements are not met while the aircraft is being operated below MDA, or upon arrival at the missed approach point "and at any time after that until touchdown."

Three geometries, three definitions, one published threshold 1. Runway visual range runway, approach end at left Instrumentally derived. Transmissometer near the touchdown point. Horizontal range down one runway, in hundreds of feet. Used in lieu of prevailing visibility for that runway. 2. Ground visibility half the horizon circle, need not be continuous Prevailing horizontal visibility near the surface, reported by the National Weather Service or an accredited observer, and representative of the station. 3. Flight visibility Average forward horizontal distance from the cockpit of an aircraft in flight. Judged at decision altitude. Where each one is decisive 1 and 2 govern whether the approach may be begun or continued past the final approach fix. 3 governs descent below decision altitude and the landing itself, and no ground instrument reports it. A published 2,400 appears in column one of the table. A reported 1/2 can arrive in its place. Approach lights alone stop the descent at 100 feet above touchdown zone elevation. Sources: 14 CFR 1.1 definitions; 14 CFR 91.175(c), (d), (h); FAA Pilot/Controller Glossary; NWS Glossary.

The chain, end to end: a transmissometer near one touchdown point produces a number; a report carries that number; the report opens the final approach segment; a worse report cannot close it; and then the number that governs the last few hundred feet is one that no sensor on the field measures. The go-around that follows is not a judgment call. It is the regulation operating as written.

Three Doors Into the Final Approach Segment

Counting the branches changes the conclusion, so count them. Section 121.651 contains three separate permissions to be in the final approach segment, and the reported number means something different at each.

The first is paragraph (b), already quoted, where the latest report must be at or above the prescribed minimums. The second is paragraph (d): "A pilot may begin the final approach segment of an instrument approach procedure other than a Category II or Category III procedure at an airport when the visibility is less than the visibility minimums prescribed for that procedure if the airport is served by an operative ILS and an operative PAR, and both are used by the pilot." That paragraph then carries its own limiting sentence: "However, no pilot may continue an approach below the authorized DA/DH unless the requirements of § 91.176 of this chapter, or the following requirements are met". The requirements that follow are the same ones, flight visibility among them.

The third is paragraph (e): "A pilot may begin the final approach segment of an instrument approach procedure, or continue that approach procedure, at an airport when the visibility is reported to be less than the visibility minimums prescribed for that procedure if the pilot uses an operable EFVS in accordance with § 91.176 of this chapter and the certificate holder's operations specifications for EFVS operations."

Two of the three doors open while the reported visibility is below the published minimum. The reported number is therefore not a single switch. It is a switch whose position depends on which door the operator is authorized to use, and the first limb of paragraph (b) is a reminder of how thin the floor is: at any airport, the approach requires only that a weather report be issued at all. The requirement that the report be at or above minimums attaches, by the text of (b)(2), only at airports within the United States and its territories or at U.S. military airports, a term the same subparagraph defines as "airports in foreign countries where flight operations are under the control of U.S. military authority." Paragraph (g) sends foreign-airport operations to the weather minimums prescribed by the authority having jurisdiction over the airport, and does so only "Unless otherwise authorized in the certificate holder's operations specifications."

Which number is being read at each gate Gate A, at the final approach fix: a reported number Door 1: latest weather report at or above the published minimum general gate Door 2: operative ILS and PAR both used, reported value may be below the minimum Door 3: operable EFVS reported value may be below the minimum, under the EFVS rule Between the fix and decision altitude: a later report below minimums does not close the door The approach may continue to decision altitude or minimum descent altitude Gate B, at decision altitude, and Gate C, at the landing: a number nobody reported Flight visibility at or above the prescribed value, plus one listed visual reference Otherwise: missed approach, at once Sources: 14 CFR 121.651(b), (c), (d), (e); 14 CFR 91.175(c), (d), (e). Doors 2 and 3 are the carve-outs named in 121.651(b). The EFVS rule itself was not retrieved for this article and is named only as it appears in the two sections above.

Where This Doesn't Apply

Paragraph (h)(1) excludes Category II and Category III minimums from the conversion by its own first words, so the table has nothing to say about a runway operating to those minimums. The conversion also does nothing when RVR is reported for the runway of intended operation, which on an ILS runway with working equipment is the normal case.

The flight visibility gate is not the only route below decision altitude. Paragraph (c) of 91.175 opens "Except as provided in § 91.176 of this chapter", paragraph (d)(1) routes operations conducted under that section to its own paragraphs (a)(3)(iii) or (b)(3)(iii), and both 121.651(c) and 121.651(d) offer the § 91.176 requirements as an alternative to the flight visibility list. An operator flying with an operable enhanced flight vision system is on a different branch of the same tree, and the text of that branch was not retrieved for this article.

Military aircraft of the United States are carved out of 91.175(c), (d) and (e) by name. Takeoff is a separate regime in 91.175(f), a paragraph that by its own first sentence "applies to persons operating an aircraft under part 121, 125, 129, or 135 of this chapter"; where part 97 prescribes no takeoff minimums, the fallback values are stated directly as 1 statute mile for aircraft other than helicopters having two engines or less and 1/2 statute mile for aircraft with more than two engines and for helicopters, rather than as RVR values.

What Would Invalidate This

Three retrievals that would sharpen or overturn parts of this reading were not completed. First, 14 CFR 91.176 was not opened. If its visual-reference requirements are stricter than the flight visibility list, the EFVS door is narrower than described here. Second, the averaging interval and sampling geometry behind the National Weather Service visibility observation were not confirmed from a primary document; a request for the relevant sentence of the ASOS User's Guide returned no verbatim text, so no claim about how the reported value is computed appears above. Third, paragraphs (a), (b) and (g) of 91.175 were not retrieved, so the article cites none of them.

One more limit is worth stating plainly. The Pilot/Controller Glossary names touchdown, mid and rollout readouts, while 91.175(h)(1) speaks of RVR "reported for the runway of intended operation" in the singular. Which readout governs a particular minimum is set outside the two sections quoted here, in the approach procedure and in an operator's authorizations, and this article makes no claim about it. The pattern that survives these gaps is the one the earlier pieces on a measured gage height and a derived streamflow, on which cause gets recorded when a ground hold delays the next departure, and on what happens to an airport after the weather clears ran into: the number that triggers an action and the number that justifies it are often produced by different instruments, and the gap between them only becomes visible when somebody has to explain a go-around.

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