Weather Minimums: What VFR and IFR Actually Require
Visual flight rules require you to see. Instrument flight rules require you to be equipped, qualified and cleared not to. That is the whole distinction, and every minimum that follows from it is an attempt to write down how much seeing is enough.
The numbers vary by country, by airspace class and by altitude, which frustrates people looking for one answer. What does not vary is the structure, and once the structure is clear the local table is easy to read.
What the flight rules actually mean#
Under VFR the pilot is responsible for seeing and avoiding other aircraft and terrain, so the rules specify how much visibility and how much separation from cloud is required to make that possible. Under IFR the aircraft is separated by air traffic control and navigated by instruments, so the limiting numbers move to the approach: how low the cloud base and how poor the visibility can be while the runway environment can still be acquired in time to land.
- VFR minimums protect see-and-avoid, so they are about flight visibility and distance from cloud.
- IFR minimums protect the approach, so they are about ceiling and runway visual range at the decision point.
- Special VFR is a limited clearance to operate below normal VFR minimums in a control zone.
- Marginal VFR is not a legal category; it is an informal warning that conditions are close to the limit.
The most common serious accident in general aviation involves a VFR flight continuing into instrument conditions. The minimums exist to make that boundary visible before it is crossed.
The flight categories used in reports#
| Category | Ceiling | Visibility |
|---|---|---|
| VFR | Above 3,000 ft | More than 5 statute miles |
| MVFR — marginal | 1,000 to 3,000 ft | 3 to 5 statute miles |
| IFR | 500 to below 1,000 ft | 1 to less than 3 statute miles |
| LIFR — low IFR | Below 500 ft | Less than 1 statute mile |
These four categories come from the United States National Weather Service and are used worldwide as informal shorthand. They summarise conditions; they are not the legal minimum for any specific flight, which depends on the rules, the airspace, the aircraft and the crew.
VFR minimums vary, and here is the pattern#
Exact figures are set nationally, but the shape is consistent almost everywhere. Requirements increase with altitude, because closing speeds are higher. They are more demanding in controlled airspace than outside it. And they relax at low level in uncontrolled airspace, where traffic is slower and less dense.
| Situation | Typical requirement |
|---|---|
| Controlled airspace, at or above 3,000 ft | 8 km visibility, 1,500 m horizontally and 300 m vertically from cloud |
| Controlled airspace, below 3,000 ft | 5 km visibility, same cloud separation |
| Uncontrolled, low level, slow aircraft | Reduced visibility permitted, clear of cloud and in sight of the surface |
| Control zone, special VFR | By clearance only, with a ground visibility floor |
| Night VFR | More restrictive or not permitted at all, depending on the state |
Always read your own authority’s table. This is the shape of the rule, not a substitute for the regulation that applies to you.
IFR approach minimums#
An instrument approach has a published minimum, and it is not one number but a pair: a height at which the approach must be discontinued if the required visual reference is not in sight, and a visibility or runway visual range below which the approach may not be started or continued. Precision approaches with vertical guidance go lower than non-precision ones, and better ground equipment plus crew qualification goes lower still.
| Approach type | Typical decision height | Typical visibility |
|---|---|---|
| Non-precision (VOR, NDB) | Around 400–600 ft above the aerodrome | 1,500 m or more |
| RNP with vertical guidance | Around 250–300 ft | 1,000–1,200 m |
| ILS CAT I | 200 ft | 550 m RVR |
| ILS CAT II | 100 ft | 300 m RVR |
| ILS CAT IIIb | Below 50 ft or no decision height | 75–125 m RVR |
CAT II and III need certified ground equipment, certified aircraft, a qualified crew and low-visibility procedures in force at the aerodrome. All four have to be present, which is why a CAT III capable aircraft still diverts sometimes.
Alternates, and the rule that catches people out#
Planning does not stop at the destination. If the forecast at the destination is below a defined threshold for a window around the estimated arrival, an alternate must be nominated, and fuel carried to reach it after an approach. Some rules require two alternates when the weather is genuinely marginal, and an alternate whose own forecast is poor does not qualify.
- Read the destination TAF for the window around your arrival, not just the arrival minute.
- Apply the applicable alternate requirement — the thresholds differ by state and operator.
- Choose an alternate far enough away that it is not under the same weather system.
- Check the alternate’s own TAF against the higher alternate planning minima.
- Carry the fuel: to the destination, an approach, to the alternate, an approach there, plus reserves.
The mistake that recurs is choosing a close alternate. Close is convenient and useless — the point of an alternate is to be somewhere the weather is different.
Reading the weather against the minimum#
- Compare the forecast ceiling against the decision height for the approach you expect to fly.
- Compare the forecast visibility or RVR against the approach minimum, using the worst TEMPO or PROB value in the window.
- Check whether low-visibility procedures would be in force, which changes the arrival rate as well as the minimum.
- Look at the trend group — a NOSIG at the limit is very different from a BECMG improving.
- Check the wind against the runway that the approach serves; a legal ceiling on an unusable runway is not a plan.
Everything on this page is educational. The applicable minimums for a given flight come from the state’s regulations, the operations manual and the approach chart, and the commander applies them.
Frequently asked questions
What is the difference between VFR and IFR minimums?
VFR minimums are about being able to see: they specify flight visibility and distance from cloud so that see-and-avoid works, and they vary by airspace class and altitude. IFR minimums are about the approach: they specify a decision height and a visibility or runway visual range at which the approach must be discontinued if the runway environment is not in sight. VFR minimums protect the whole flight; IFR minimums bite at the very end of it.
What do VFR, MVFR, IFR and LIFR mean on a weather map?
They are summary flight categories defined by ceiling and visibility: VFR is a ceiling above 3,000 feet with more than 5 statute miles visibility; MVFR is 1,000 to 3,000 feet or 3 to 5 miles; IFR is 500 to below 1,000 feet or 1 to under 3 miles; LIFR is below 500 feet or under 1 mile. They come from the United States National Weather Service and are used worldwide as shorthand. They summarise conditions rather than defining what any particular flight is permitted to do.
Can a flight land in fog?
Yes, within limits and with the right combination of equipment. A CAT I ILS typically needs 550 m runway visual range and a 200 ft decision height. CAT II goes to 300 m and 100 ft, and CAT IIIb can operate at 75 m with little or no decision height. But all of it requires certified ground equipment, a certified aircraft, a qualified crew and low-visibility procedures in force at the aerodrome — remove any one and the aircraft diverts even though it is technically capable.
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