What is the difference between RVR and prevailing visibility, and which one controls for an ILS approach?

Prevailing visibility is a human or automated estimate of the greatest distance an observer can see over at least half the horizon, measured in statute miles. RVR is an instrument-derived measurement in feet of how far down the runway centerline a pilot can see high-intensity lights or markings, taken right at runway level. When an approach procedure publishes RVR minimums and the RVR equipment is operating for that runway, RVR controls and you cannot legally substitute the reported statute-mile visibility.

What does an RVR report of M600 or P6000 mean?

An RVR of M600 means the transmissometer is reading below its lower measurement threshold of 600 feet—visibility is at or below 600 feet, but the instrument cannot determine exactly how low. An RVR of P6000 means visibility exceeds the sensor's upper limit of 6,000 feet. In both cases the prefix letter indicates the reading is beyond the sensor's range, not that visibility is exactly at that value.

Can a Part 91 pilot take off when the airport has no published takeoff minimums and visibility is reported as 1/4 mile?

No. Under 14 CFR 91.175(f), when no departure minimums are published for a runway, a Part 91 IFR pilot may not take off when visibility is less than 1 statute mile or RVR 5,000 feet. A reported visibility of 1/4 mile is well below that standard, making departure illegal unless the pilot holds a special authorization or the airport has published lower takeoff minimums with specific equipment requirements.

Runway Visual Range Reporting and Low-Visibility Takeoff/Landing Minima

When visibility drops below the values where a pilot can comfortably assess the runway environment, the aviation system shifts from simple reported prevailing visibility to a far more precise measurement called Runway Visual Range (RVR). RVR is an instrumentally derived value that tells a pilot, in feet, how far down the runway centerline a pilot at the cockpit height of a landing aircraft can expect to see high-intensity runway lights or pavement markings. Because it is measured at runway level rather than at an elevated weather station, RVR is far more representative of what the crew will actually see during the most critical phase of flight. For ATP operations—particularly those involving Category II and Category III approaches—RVR is not merely an advisory number; it is a legal controlling value that determines whether a flight may even begin or continue an approach.

The regulatory framework governing low-visibility operations is spread across 14 CFR Parts 91, 121, and 135, as well as the Aeronautical Information Manual (AIM) Chapter 7, Section 1-25, which provides the definitive technical description of how RVR is measured, what the equipment categories mean, and how reported values must be interpreted and applied. This article assembles those threads into a complete operational picture for ATP-level study.

How RVR Is Measured and Reported

RVR is measured by automated instruments called transmissometers or, on newer installations, forward scatter meters. Both types sample a column of air and quantify how much light is scattered or absorbed over a known baseline distance, converting that optical measurement into an equivalent visibility in feet. The instruments are sited at specific points along the runway: the touchdown zone (TDZ) sensor is located approximately 1,000 feet from the runway threshold; a midpoint (MID) sensor is near the runway centerline midpoint; and on longer runways, a rollout (RO) sensor is placed in the last third of the runway. Each sensor feeds its own RVR readout to air traffic control and the Automated Surface Observing System (ASOS) or Automated Weather Sensor System (AWSS).

The value is computed as a 10-minute harmonic mean that is updated every 60 seconds for human observers or every minute for automated systems, but the reported value the controller reads is actually a one-minute average intended to smooth out momentary fluctuations. Importantly, RVR accounts for both the ambient illumination (day versus night) and the runway light setting in use. The higher the light intensity, the farther those lights can be seen through a given density of obscuration, so the RVR system automatically adjusts its calculation based on the runway light setting reported by ATC. This is why ATC will often advise, and regulations may require, that runway lights be at maximum intensity during low-visibility operations.

Reportable Range and Sensor Limits

Transmissometers have defined upper and lower limits. The standard RVR system reports values from 600 feet to 6,000 feet in 100-foot increments below 3,000 feet and 200-foot increments from 3,000 to 6,000 feet. When conditions exceed the upper limit, the report is given as "P6000" (plus 6,000 feet), meaning visibility is better than the instrument can accurately measure. When visibility falls below the lower limit, the report is "M600" (minus 600 feet). AIM 7-1-25 notes that when RVR is reported as M600, the visibility is below 600 feet RVR, which corresponds to the most restrictive Category III operations. Pilots and dispatchers must understand these ceiling/floor designations because a reading of M600 does not mean zero visibility—it simply means the sensor cannot distinguish how bad conditions are below that threshold.

RVR in Lieu of Prevailing Visibility

14 CFR 91.175 and the equivalent Parts 121 and 135 rules specify that when an approach procedure has RVR minimums listed, RVR controls over any reported prevailing visibility. You cannot substitute statute-mile visibility for an RVR requirement if RVR equipment is operating for that runway. The AIM makes clear that the touchdown zone RVR is always the controlling value for determining whether approach and landing minimums are met. Midpoint and rollout RVR readings are additional pieces of information—they tell the crew what to expect after touchdown during the rollout phase—but they do not override the TDZ reading for the purpose of commencing or continuing an approach to a decision altitude or decision height.

There is a critical operational nuance: if the TDZ RVR equipment is out of service, you may substitute the midpoint RVR, or in some cases prevailing visibility, per specific notes on the approach procedure. However, you cannot substitute a rollout sensor for a TDZ sensor. Always consult the notes section of the instrument approach procedure plate for the specific substitution authority.

Low-Visibility Takeoff Minimums

For Part 91 operators, 14 CFR 91.175(f) establishes that no pilot may take off under IFR when the airport's published takeoff minimums are not met, or when no takeoff minimums are published, when visibility is less than 1 statute mile or RVR 5,000. For Part 121 and 135 operators, takeoff minimums are specified in operations specifications (OpSpecs) and may be significantly more restrictive or, with approved equipment and training, lower than the Part 91 standard.

ATP candidates must understand that low-visibility takeoff (LVTO) operations—sometimes called Special Authorization CAT II/III for takeoff—permit departures at RVR values as low as RVR 600 feet for suitably equipped and approved operators. These operations require specific crew training, aircraft equipment certification (particularly runway centerline lighting and, for the aircraft, head-up display or enhanced vision capability in some cases), and airport infrastructure including runway centerline lights, touchdown zone lights, and high-intensity runway edge lights. The FAA grants this authority through OpSpecs, and crew members must be specifically trained and qualified.

Category I, II, and III Approach Minimums

The three ILS approach categories differ primarily in their decision height (DH) and RVR requirements:

Category I: DH not lower than 200 feet HAT; RVR not less than 1,800 feet (or, with certain aircraft approach category and system qualifications, 1,400 feet). Standard for all IFR-rated pilots.
Category II: DH between 100 and 200 feet HAT; RVR not less than 1,200 feet (or 1,000 feet with special authorization). Requires specific aircraft certification, crew training, and airport equipment including approach lighting, TDZ/centerline lighting.
Category IIIa: DH below 100 feet or no DH; RVR not less than 700 feet. Requires fail-operational autopilot or HUD guidance for rollout.
Category IIIb: DH below 50 feet or no DH; RVR between 150 and 700 feet. Requires fully fail-operational systems and special crew qualification.
Category IIIc: No DH, no RVR minimum (zero-zero). Not currently authorized at any U.S. airport for commercial operations as of the most recent AIM publication.

For CAT II and III operations, the alert height concept is important: this is a height above touchdown, based on aircraft failure analysis, above which a CAT III approach would be continued even after a failure of a redundant system. Below the alert height, a detected failure triggers a missed approach. This is built into the aircraft's Flight Management System logic and is an integral part of CAT III certification.

Why RVR Matters Operationally

The operational stakes of misreading or misapplying RVR cannot be overstated. A crew that commences an approach when the TDZ RVR is below minimums has violated 14 CFR 91.175 (or the applicable Part 121/135 rule), potentially placing the aircraft in a position where the runway environment cannot be positively identified before the decision height. At CAT III minimums, the margin between a safe landing and a runway excursion is measured in feet and seconds. Similarly, a crew that departs with below-minima visibility may find themselves in a rejected takeoff scenario where there is insufficient visual reference to keep the aircraft on the centerline.

Ground controllers, dispatchers, and pilots all share responsibility for confirming RVR is at or above the required value before issuing and accepting takeoff clearances. A critical AIM point: if RVR drops below minimums after a flight has passed the final approach fix (FAF) on a CAT I approach, the approach may be continued to the DH, but the missed approach must be executed if the required visual references are not established at DH. On CAT II/III approaches, the relevant decision point is the DH or alert height as specified in the OpSpecs.

Key Numbers and Rules

RVR reporting range: M600 feet (below sensor floor) to P6000 feet (above sensor ceiling).
Part 91 standard takeoff minimum (no published minimums): 1 SM or RVR 5,000.
CAT I ILS minimum RVR: 1,800 feet (standard); 1,400 feet with special authorization.
CAT II ILS minimum RVR: 1,200 feet (standard); 1,000 feet with special authorization.
CAT IIIa ILS minimum RVR: 700 feet.
CAT IIIb ILS minimum RVR: 150–700 feet.
TDZ RVR is the controlling value; MID and RO are advisory for rollout planning.
RVR in lieu of statute-mile visibility: 1 SM ≈ RVR 5,000 feet; 1/2 SM ≈ RVR 2,400 feet.
RVR cannot be substituted with prevailing visibility when the RVR equipment is operational for that runway.

Common Test Traps

Substituting visibility for RVR: The ATP written frequently offers a scenario where the prevailing visibility is acceptable but the TDZ RVR is below minimums. The correct answer is that the RVR controls—you may not commence the approach.
Midpoint vs. TDZ control: Students confuse which sensor is controlling. Only the TDZ RVR governs approach commencement; midpoint and rollout sensors inform rollout planning, not the approach decision.
M600 means zero visibility: It does not. M600 means the equipment cannot read below 600 feet; actual visibility is somewhere at or below that value.
CAT III always means no DH: CAT IIIa may still have a DH (below 100 feet); only CAT IIIc is truly no DH/no RVR, and it is not authorized in U.S. operations.
Part 91 takeoff minimums apply to all operators equally: They do not. Part 121/135 operators are governed by OpSpecs that can establish both lower and higher minimums than the Part 91 standard.