How to make the weather decision for a UK IFR flight
By Rory Bennett (ATPL, FI: CPL, IR, ME, UPRT) · Published 16 July 2026
Direct answer
The UK IFR weather decision is a sequence. Confirm you can take off against the departure minima; nominate a take-off alternate if the departure weather is below landing minima; check the destination against its operating minima; decide how many destination alternates you need; and confirm each alternate meets its planning minima. Commercial flights apply Part-CAT (CAT.OP.MPA.182); non-commercial flights in other-than-complex aircraft apply Part-NCO (NCO.OP.140 and NCO.OP.143). Then weigh the en-route threats[1][2].
Flight planning gives you a route, levels and fuel. The weather decision is the separate judgement of whether the forecast actually lets you fly that plan, and what you have to carry, nominate or avoid in order to. The rules for it are not in one place: they are spread across SERA and the Air Operations regulation, and since the 2025 All-Weather Operations changes a good deal of the detail now sits inside the fuel-planning AMCs. Taken as an ordered set of questions, though, it is manageable. This article works through them in the order they arise, answering each for both Part-CAT (commercial air transport) and Part-NCO (non-commercial, other-than-complex aircraft), by sole reference to the UK CAA Air Operations regulation. For the rules behind the route itself, the minimum levels, cruising levels and fuel, see UK CAA IFR flight planning.
Can you take off?
The first question is whether the departure weather meets your take-off minima, expressed as a runway visual range (RVR) or visibility. Part-CAT gives you a table of values to work to; Part-NCO leaves the figure to the pilot-in-command.
Regulation: NCO.OP.110, NCO.OP.175
Regulation: CAT.OP.MPA.110, SPA.LVO.100
Do you need a take-off alternate, and what weather must it have?
A take-off alternate is somewhere to divert to if you get airborne but cannot get back into the departure aerodrome, usually because the weather there has fallen below its landing minima. It is worth knowing from the outset that this is a Part-CAT requirement only; Part-NCO has no equivalent rule, so for most private IFR flying the question does not formally arise.
Regulation: Part-NCO (Annex VII): no take-off alternate provision
Regulation: CAT.OP.MPA.182(b); AMC1 and AMC5 CAT.OP.MPA.182
What weather do you need at the destination?
This is more straightforward than it is often made to sound. Plan the destination to be at or above the operating minima of the approach you intend to fly; there is no planning increment added to the destination itself, as those apply only to the alternates. What the destination forecast does decide is how many alternates you have to carry, which is the next question.
Regulation: NCO.OP.140
Regulation: CAT.OP.MPA.182
How many destination alternates do you need?
Here the two regimes take different approaches. Part-CAT is built around a single idea, that you arrive with two options for a safe landing; Part-NCO instead applies one straightforward test.
Regulation: NCO.OP.140
Regulation: CAT.OP.MPA.182(d); AMC2 CAT.OP.MPA.182
What weather must the destination alternates be forecasting?
A destination alternate is planned against raised minima: its forecast has to be better than the approach minima by a set margin, so that a diversion arrives with something in hand. Part-CAT does this by adding an increment to the approach minima, set out in AMC6 CAT.OP.MPA.182 Table 2, which since the All-Weather Operations changes sits among the fuel-planning material rather than with the minima. Part-NCO gives you fixed figures instead.
Regulation: NCO.OP.143, NCO.OP.142
Regulation: AMC6 CAT.OP.MPA.182 Table 2
| Approach at the alternate | Ceiling | RVR / visibility |
|---|---|---|
| Type B (DH below 250 ft) | DA/H + 200 ft | RVR or VIS + 800 m |
| Type A (DH or MDH 250 ft or more) | DA/H or MDA/H + 400 ft | RVR or VIS + 1500 m |
| Circling | MDA/H + 400 ft | VIS + 1500 m |
| Approach at the alternate | Ceiling | Visibility |
|---|---|---|
| DH below 250 ft | At least DH/MDH + 200 ft | At least 1500 m |
| DH or MDH 250 ft or more | At least DH/MDH + 400 ft | At least 3000 m |
| No instrument approach | At least the higher of 2000 ft and the minimum safe IFR height | At least 5000 m |
Worked example
Flight: IFR, non-complex aeroplane, to an aerodrome of 100 ft elevation whose only approach is an RNP to LNAV with an MDH of 520 ft. Arrival forecast BKN008 (800 ft) and 6000 m.
Do you need an alternate? (NCO.OP.140). No alternate is required only if the forecast ceiling is at least 1000 ft above the MDH, that is at or above 1520 ft, and visibility at least 5000 m. The forecast ceiling is 800 ft, well below 1520 ft, so a destination alternate must be nominated.
Choosing it (NCO.OP.143). An alternate with an ILS to a DH of 200 ft sits in the "below 250 ft" band, so its forecast for the window must show a ceiling at least DH + 200 ft (at or above 400 ft) and visibility at least 1500 m. It must also offer a non-GNSS approach unless the SBAS conditions are met (NCO.OP.142). Under Part-CAT the same flight would instead need the AMC6 Type B increment of + 200 ft and + 800 m on the alternate's approach minima.
How do you assess the en-route weather and threats?
The decision does not end once the aerodromes check out. You still have to study the weather along the route itself and plan around its threats, and that is a regulatory duty as much as airmanship. Under NCO.OP.135(b) the pilot-in-command must study the available weather reports and forecasts and plan an alternative course of action for the eventuality that the flight cannot be completed as planned because of the weather. Part-CAT reaches the same end through CAT.OP.MPA.175, the commander satisfying the operations manual provisions on fuel, minima and alternate availability[1][2]. The three threats that most often drive that alternative course of action are icing, cumulonimbus and embedded cumulonimbus.
Airframe icing
Airframe ice forms when supercooled water droplets, liquid water existing below 0 °C, freeze on impact with the aircraft. It comes in three forms:
- Clear ice is the most hazardous. Large supercooled drops, typically in freezing rain, spread back over the surface before they finish freezing, forming a sheet of solid glazed ice that badly disrupts the aerofoil and adds weight. The risk of clear ice is severe from about 0 °C to −15 °C.
- Rime ice forms from small droplets that freeze on contact, trapping air to give a rough, opaque, brittle deposit on the leading edges. It accretes slowly with little weight penalty and is most common from about −10 °C to −40 °C.
- Mixed ice, a combination of the two, is what most icing encounters in cloud actually produce.
Icing is generally most severe just below 0 °C, most frequent above about −10 °C, and diminishes rapidly below −20 °C. Freezing rain lies beneath warm fronts and occlusions, where warmer air aloft drops rain into a sub-zero layer below; it produces clear ice fast.
Cumulonimbus and thunderstorms
A cumulonimbus (Cb) is the most dangerous cloud for an aircraft. Because it is driven by strong vertical currents, an aircraft entering one can meet updraughts and downdraughts exceeding 3000 fpm. It also produces:
- Hail, which can be thrown out into clear air several miles from the storm core and should be anticipated beneath the anvil. Large hail causes severe airframe damage, which is why the standing advice is to avoid Cb.
- Lightning, most likely within about 5000 ft of the freezing level, which can damage the structure and disrupt electrical, navigation and communication equipment.
- Severe icing and turbulence, and visibility close to zero within the cloud.
Embedded cumulonimbus
An embedded cumulonimbus is a Cb obscured within surrounding layers of other cloud, so you cannot see it to avoid it visually. They are associated with unstable air behind fronts and occlusions. Because the eyeball is no help, the avoidance has to be planned from the forecast and flown using airborne or ground radar, giving any cell a wide berth.
How does the weather brief come together?
Put in order, the go/no-go weather decision reads as a short sequence:
- Is the departure at or above take-off minima? (Part-CAT CAT.OP.MPA.110; Part-NCO NCO.OP.110/175.)
- Is a take-off alternate required, and does it meet its actual landing minima? (Part-CAT only, CAT.OP.MPA.182.)
- Is the destination forecast at or above its operating minima?
- How many destination alternates does that forecast require? (CAT.OP.MPA.182 or NCO.OP.140.)
- Does each alternate meet its planning minima? (AMC6 CAT.OP.MPA.182 Table 2 or NCO.OP.143, plus NCO.OP.142.)
- What are the en-route threats, and what is the alternative course of action? (NCO.OP.135(b) / CAT.OP.MPA.175.)
Related
For the rules behind the route, levels and fuel, see UK CAA IFR flight planning; for the approach classifications the planning minima depend on, see 2D, 3D, Type A and Type B approaches explained. Both sit in the IFR flight planning cluster.
Common mistakes
- Adding the alternate planning increment to the destination itself. The raised increments (AMC6 CAT.OP.MPA.182 Table 2, or the fixed NCO.OP.143 values) apply to the destination alternate, the fuel en-route alternate and isolated aerodromes. The destination is planned to its own operating minima; the increment is not added to it.
- Carrying the Part-CAT increments across to a Part-NCO alternate. NCO.OP.143 gives fixed values (for example 3000 m for a Type A approach, 1500 m for Type B), not an additive on the approach figure. Read them straight off the NCO.OP.143 table rather than applying the AMC6 Type A/B increments.
- Treating RVR 550 m as the take-off floor. RVR 550 m is only the definition of a low-visibility take-off. The lowest standard take-off value without an SPA.LVO approval is 400 m (500 m with nil facilities by day). Below 400 m needs the approval.
- Planning to avoid an embedded cumulonimbus by eye. An embedded Cb is hidden inside surrounding cloud, so it cannot be seen and avoided visually. Plan the avoidance from the forecast and fly it on radar. If embedded Cb is forecast on route, decide the diversion or delay before departure.
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Frequently asked questions
What are the instrument flight rules?
The instrument flight rules (IFR) are the rules for flying by reference to instruments rather than by visual reference to the surface. In UK airspace they form part of the Standardised European Rules of the Air (SERA), retained in UK law. You plan IFR whenever the conditions are below the SERA.5001 VMC minima, or the airspace is IFR-only (such as Class A). The separate requirement to hold an instrument rating comes from the UK Aircrew regulations.
What is planning minima?
Planning minima are the raised weather figures a destination alternate must be forecasting before you may plan to use it, set higher than the approach minima so that a diversion arrives with margin in hand. For Part-CAT they are an increment added to the approach minima (AMC6 CAT.OP.MPA.182 Table 2); for Part-NCO they are fixed values (NCO.OP.143). They are distinct from operating minima, which are the actual minima you may descend to on the approach itself.
What is a Type A or Type B approach?
The classification turns on the decision height. A Type A instrument approach has a decision height or minimum descent height at or above 250 ft (most non-precision and RNP approaches). A Type B approach has a decision height below 250 ft (the CAT I, II and III precision approaches). The 250 ft split sets which row of the planning-minima table applies to a destination alternate.
What is a take-off alternate, and when do you need one?
A take-off alternate is a nearby aerodrome you could divert to shortly after departure if you could not return to the departure aerodrome. Under Part-CAT (CAT.OP.MPA.182) one is required when the departure weather is below the applicable landing minima; it must be within the aircraft distance limit (for a twin, one hour at one-engine-inoperative cruise) and is planned to the actual landing minima. Part-NCO has no take-off alternate requirement.
How many destination alternates does a UK IFR flight need?
Part-CAT (CAT.OP.MPA.182) plans for two safe-landing options: usually one destination alternate, none if the destination is isolated or the flight is under 6 hours with two usable runways and a forecast of at least 2000 ft and 5 km, and two if the forecast is below planning minima or no met information is available. Part-NCO (NCO.OP.140) needs one alternate, unless the forecast gives a ceiling at least 1000 ft above the approach DH/MDH and visibility at least 5000 m.
Do I always need a destination alternate under Part-NCO?
No. Under NCO.OP.140 you may dispense with a destination alternate if the forecast for the hour either side of ETA (or from departure to an hour after ETA, whichever is shorter) shows a ceiling at least 1000 ft above the DH or MDH of an available instrument approach and visibility at least 5000 m. Otherwise at least one alternate must be filed; there is no two-alternate case in Part-NCO.
Sources
- UK Regulation (EU) No 965/2012 (Air Operations), Annex IV (Part-CAT): CAT.OP.MPA.110, CAT.OP.MPA.175, CAT.OP.MPA.181 to 182 and SPA.LVO.100, via the UK CAA Regulatory Library. The fuel/energy scheme and All-Weather Operations changes came into UK force on 30 October 2025 (CAA ORS9 Decision 47). regulatorylibrary.caa.co.uk/965-2012
- UK Regulation (EU) No 965/2012 (Air Operations), Annex VII (Part-NCO): NCO.OP.110, 135, 140, 142, 143 and 175, via the UK CAA Regulatory Library (All-Weather Operations and fuel-planning amendments in UK force 30 October 2025). regulatorylibrary.caa.co.uk/965-2012
- UK Regulation (EU) No 923/2012, Standardised European Rules of the Air (SERA): SERA.5001 (visual meteorological conditions minima), via the UK CAA Regulatory Library. regulatorylibrary.caa.co.uk/923-2012
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This article does not constitute flight instruction. Always defer to the guidance of your qualified flight instructor and to current charts and procedures or regulations published by your country's aviation authority.
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