VFR, IFR and Meteorological Conditions

Two pairs of terms run through air law and are easily confused. Visual flight rules (VFR) and instrument flight rules (IFR) are sets of rules: the pilot chooses one, files it in the flight plan and flies by it. Visual meteorological conditions (VMC) and instrument meteorological conditions (IMC) describe the weather, defined by visibility, distance from cloud and ceiling against minima that depend on the airspace and the altitude.

The pairs meet in one rule: a VFR flight must stay in VMC. An IFR flight may be in VMC or IMC, and an IFR flight in sunshine is still an IFR flight, with all its requirements for rating, equipment, levels and clearances.

VMC and IMC defined

ICAO defines VMC as meteorological conditions, expressed in terms of visibility, distance from cloud and ceiling, equal to or better than specified minima, and IMC as conditions below them. An aircraft is in IMC whenever the VMC criteria for its airspace class and altitude are not met.

Two kinds of visibility are used. Flight visibility is the forward distance at which prominent objects can be seen from the cockpit of an aircraft in flight, unlighted ones by day and lighted ones by night, and is judged by the pilot. Ground visibility is reported by an observer or an automatic station at the aerodrome. The VMC minima use flight visibility; the rules for taking off and landing, and ATC's condition for a special VFR clearance, use ground visibility.

The FAA also speaks of IFR conditions, meaning weather below VFR minima. The VFR, MVFR, IFR and LIFR categories on its weather charts help planning but are not legal minima.

Visual flight rules

Under SERA, a VFR flight other than a special VFR flight must keep at least the VMC minima of the next section, and further limits apply:

• Unless cleared by ATC, it may not take off or land at an aerodrome within a control zone, or enter the aerodrome traffic zone or traffic circuit, when the ceiling is below 450 m (1,500 ft) or the ground visibility below 5 km.
• It may not fly in Class A airspace.
• It may not fly above FL195 except in an airspace reservation allowing VFR or, with ATS authorisation, up to FL285; never above FL285 where 1,000 ft separation is applied above FL290; and not at transonic or supersonic speed unless authorised.
• More than 900 m (3,000 ft) above the ground or water, in level cruise, it flies a VFR cruising level (see cruising levels and minimum heights).

If a VFR flight in controlled airspace cannot stay in VMC, the pilot asks for an amended clearance to continue in VMC or to leave the airspace; failing that, the pilot tells ATC what action is being taken to leave it or to land at the nearest suitable aerodrome. Within a control zone the pilot may instead ask for special VFR, and a suitably qualified pilot for an IFR clearance. The opposite change is made only by the pilot: an IFR flight in VMC may transmit "cancelling my IFR flight", and should do so only if it expects to remain in uninterrupted VMC for a reasonable time. In the United States, Class A, from 18,000 ft MSL to FL600, is IFR only.

VFR weather minima by airspace

The SERA table follows ICAO:

FL100 replaces 10,000 ft where the transition altitude is lower. The Class A values are for guidance only, since VFR is not allowed there. In Classes F and G the competent authority may accept flight visibility down to 1,500 m for flights at 140 kt IAS or less, and lower values for slow helicopters.

The FAA's basic VFR weather minimums in 14 CFR 91.155 are set in statute miles and feet:

At night in Class G an aeroplane may fly an airport traffic pattern within half a mile of the runway in 1 SM, clear of clouds. Within the surface area of Class B, C, D or E airspace, no VFR flight may operate beneath a ceiling below 1,000 ft, or take off, land or enter the pattern with ground visibility below 3 SM, without a special VFR clearance.

Night VFR

SERA defines night as the hours between the end of evening civil twilight and the beginning of morning civil twilight; the ICAO definition also lets the authority set another period between sunset and sunrise. 14 CFR 1.1 uses the civil-twilight boundaries, although other FAA rules count from sunset (see time, sunrise and twilight).

Night VFR is permitted only where the State allows it. SERA then adds conditions: a flight plan if the flight leaves the vicinity of an aerodrome; two-way radio contact where a channel is available; a ceiling of at least 450 m (1,500 ft); none of the reduced visibilities allowed by day; continuous sight of the surface at and below the higher of 900 m (3,000 ft) AMSL and 300 m (1,000 ft) above terrain; and, where the State sets no minimum altitude, a level at least 300 m (1,000 ft) above the highest obstacle within 8 km, or 600 m (2,000 ft) over high or mountainous terrain. France defines the vicinity as the lateral limits of the control zone or, without one, a restricted area protecting the aerodrome or 12 km (6.5 NM) from it, requires promulgated aerodromes, and raises the aeroplane minimum away from published routes to 450 m (1,500 ft).

Under Part-FCL a PPL or LAPL holder needs a night rating: at least 5 hours of night flight, 3 of them dual including 1 hour of cross-country navigation with a dual cross-country flight of at least 50 km, plus 5 solo take-offs and 5 solo full-stop landings. To carry passengers at night, one of the take-offs and landings required in the preceding 90 days must have been at night, unless the pilot holds a valid instrument rating. The FAA instead requires 3 take-offs and 3 full-stop landings within 90 days, made between 1 hour after sunset and 1 hour before sunrise.

The aeroplane and fuel requirements change too. 14 CFR 91.205(c) adds position lights, an anti-collision light system, an adequate source of electrical power, spare fuses and, if operated for hire, a landing light. The FAA's VFR fuel reserve rises from 30 to 45 minutes at night, at normal cruising speed; AMC1 NCO.OP.125(b) sets the same 30 and 45 minutes for non-commercial flights in non-complex aeroplanes, but at holding speed at 1,500 ft above the destination.

Special VFR

Special VFR (SVFR) lets a VFR flight enter, leave or operate within a control zone when the weather is below VMC. Under SERA.5010 it needs an ATC clearance and exists only in a control zone. An aeroplane must stay clear of cloud with the surface in sight, with at least 1,500 m flight visibility, at 140 kt IAS or less so that there is time to see and avoid; a helicopter needs 800 m. ATC grants it by day only unless the authority permits otherwise, and only when the ground visibility is at least 1,500 m (800 m for helicopters) and the ceiling at least 180 m (600 ft). ATC separates SVFR flights from IFR flights and may refuse the clearance when traffic does not allow it. France, for example, allows night SVFR for helicopters only.

In the United States, 14 CFR 91.157 allows special VFR within the surface areas of Class B, C, D and E airspace, with an ATC clearance, 1 SM flight visibility and clear of clouds. Taking off or landing needs a reported ground visibility of at least 1 SM, with flight visibility used only where ground visibility is not reported. Between sunset and sunrise the pilot must meet the Part 61 requirements for instrument flight and the aircraft must be equipped for IFR. Fixed-wing SVFR is prohibited at the busy airports listed in Part 91 Appendix D, marked NO SVFR on sectional charts; helicopters are exempt.

VFR-on-top

VFR-on-top is an FAA term for an IFR clearance, not a kind of VFR flight. A pilot on an IFR flight plan may request it and, if approved, chooses an altitude instead of flying an assigned one. The pilot must fly a VFR cruising altitude under 91.159, keep the VFR visibility and cloud clearances of 91.155 and see and avoid other traffic, while still following the IFR rules: minimum IFR altitudes, position reports, radio communication, course and adherence to the clearance. The pilot should tell ATC before any altitude change. VFR-on-top is not permitted in Class A or in certain restricted areas.

VFR over-the-top is different: a VFR flight, on no IFR flight plan, above a cloud layer. It is legal for any pilot who keeps the VFR minima above the layer, but a pilot without an instrument rating depends on finding a legal gap to descend through, and an engine failure or a closed-in destination leaves no good option. The VFR-on-top clearance is an FAA procedure; under SERA a VFR flight above cloud simply keeps the VMC distances from cloud, and low down in Classes F and G it must keep the surface in sight.

Instrument flight rules

IFR lets a qualified pilot in a suitably equipped aircraft fly without outside visual references. Annex 1 allows a licence holder to act as pilot-in-command or co-pilot under IFR only with a valid instrument rating for the aircraft category, and 14 CFR 61.3(e) requires one to act as pilot-in-command under IFR or in weather below VFR minima. The aircraft needs the IFR equipment of the applicable rules, such as Part-NCO for a private light aeroplane or 91.205(d) in the United States.

An IFR flight must stay at or above the minimum flight altitude set by the State or, where none is set, the SERA.5015 minimum of 300 m (1,000 ft) above the highest obstacle within 8 km, 600 m (2,000 ft) over high or mountainous terrain (see minimum safe altitudes). A flight that will receive an ATC service files a flight plan, under SERA at least 60 minutes before departure, and 14 CFR 91.173 requires an IFR flight plan and clearance before IFR flight in US controlled airspace; US Class G needs neither. Outside controlled airspace an IFR flight flies the semicircular cruising levels and files a flight plan if it enters advisory airspace.

Controlled flight

A controlled flight is any flight subject to an ATC clearance. Which flights are controlled depends on the airspace class (see airspace classification):

Special VFR flights are always controlled, since they need a clearance. A controlled flight files a flight plan, obtains a clearance before entering controlled airspace, keeps two-way radio contact and follows its clearance. It also tells ATC if its sustained speed at cruising level differs from the flight plan by Mach 0.02 or 10 kt TAS or more (5 per cent of TAS until SERA was amended in May 2025), or if an estimate for the next reporting point, FIR boundary or destination is more than 2 minutes in error.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.