An alternate aerodrome is the airfield you have planned and fuelled to divert to if landing at your destination becomes inadvisable. Choosing a good one is part weather, part regulation, and part knowing what the field can actually offer when you arrive.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
Why an alternate exists
You file an alternate so that a problem at the destination, weather below minima, a blocked runway, a failed approach aid, does not leave you with nowhere to go and no fuel to get there. The alternate has to be somewhere the weather is expected to be better, with an approach you can fly and a runway you can use, and you must carry the fuel to reach it. That last point is the link between this and fuel planning: the alternate is only real if the fuel is in the tanks. (See our separate guide on fuel planning and reserves.)
The FAA approach: the 1-2-3 rule
In the United States, 14 CFR 91.169 governs whether you need an alternate on an IFR flight plan. The exemption in 91.169(b) has two limbs, and you may leave the alternate off the flight plan only if both are satisfied:
- the destination has a published instrument approach procedure; and
- for the period from 1 hour before to 1 hour after your estimated time of arrival, the forecast ceiling is at least 2000 feet above the airport elevation and the visibility at least 3 statute miles.
The second limb is the one pilots know, and it is what they mean by the 1-2-3 rule. The first limb is the one that gets forgotten, and it bites in exactly the place you would not expect: a destination with no instrument approach takes an alternate however good the forecast is, because there is no published way to get in if you arrive and cannot see the field. A cloudless day at a strip with no IAP is not a no-alternate flight, and reading 91.169 as a weather test alone is how pilots miss that.
If either limb fails, you must file an alternate, and that alternate has to meet its own weather minimums under 91.169(c): the alternate minimums published on its instrument approach, or, where none are specified, 600 feet and 2 miles for a precision approach and 800 feet and 2 miles for a non-precision approach. If the alternate has no instrument approach at all, the forecast must let you descend from the minimum en-route altitude, approach, and land under basic VFR. Fuel for the diversion is set separately by 14 CFR 91.167: enough to reach the destination, then the alternate, then fly 45 minutes at normal cruise.
The EASA approach: planning minima
EASA commercial air transport rules take a different shape. CAT.OP.MPA.182 sets the aerodrome selection policy, covering destination and take-off alternates, and the IFR planning minima for each alternate sit in AMC6 CAT.OP.MPA.182. A destination alternate is generally required unless specific conditions are met, such as an isolated destination or a destination with reliable weather and suitable runways, and the planning minima for each alternate are built up from that aerodrome's approach minima plus defined increments rather than from a single fixed figure. There is also a take-off alternate requirement when the weather at the departure aerodrome is below landing minima. The numbers and the conditions differ from the FAA scheme, and non-commercial operations under the EASA rules for non-commercial flying have their own, so use the regulation that binds your operation rather than carrying one authority's figures across to another.
The practical checks
The rules tell you whether a candidate is legal. Airmanship tells you whether it is sensible. Before you commit to an alternate, confirm:
- Weather over the window. The forecast across your possible arrival times, not just a single point estimate, should meet the planning minima with margin.
- A suitable approach. An instrument approach you are qualified, current, and equipped to fly, with the aids in service.
- Runway and surface. Length, width, and surface adequate for your aircraft, allowing for a wet or contaminated runway and the wind.
- NOTAMs. That the aerodrome, runway, and approach are open and the navigation aids serviceable. A perfect-weather alternate with a closed runway is no alternate.
- Services and access. Fuel, opening hours, fire and rescue category, and any customs, immigration, or handling you will need on a diversion.
- Terrain and distance. A field you can reach comfortably on the fuel you carry, clear of terrain that drives the minimum safe altitude up.
Take-off alternates and isolated aerodromes
A destination alternate is the one pilots think of first, but two other kinds of alternate come up, and both follow from the same logic of always having somewhere to go.
A take-off alternate is required when the weather at the departure aerodrome is at or below the applicable landing minima, so that if you had to return immediately after take-off, you could not get back in. In that case the rules require an alternate within a defined distance or flying time of the departure point. The EASA requirement sits in CAT.OP.MPA.182, the aerodrome selection policy, with the maximum distance to a take-off alternate set in AMC1 CAT.OP.MPA.182 by aircraft type and condition. The principle is simple: do not depart into a situation where an immediate problem leaves you with nowhere to land.
An isolated aerodrome is the opposite problem: a destination so remote that no suitable alternate exists within range. Here the rules replace the alternate with extra fuel and a decision point. Instead of carrying fuel to divert, you carry additional fuel to hold and absorb delay, and you fly to a point of no return, a position beyond which you are committed to the destination because you no longer have the fuel to turn back. Past that point, the destination must be usable, so the weather requirement at the destination is correspondingly stricter.
Longer over-water and remote operations add a further layer, the adequate and suitable aerodrome distinction used in extended operations planning, where en-route alternates must be available within a specified diversion time. The detail of that belongs to the operator's approval, but it rests on the same idea threaded through all of this: every phase of the flight should have a reachable, usable place to land.
For most general aviation IFR flying, the take-off alternate and isolated-aerodrome cases are rare, but knowing they exist explains why the rules treat departure weather and remote destinations specially, rather than only asking about the destination alternate.
A worked alternate decision
Put the rules and the practical checks together with a worked example. You are planning an IFR flight to a destination that does have a published ILS, so the first limb of 91.169(b) is satisfied and the decision turns on the weather. The forecast around your arrival shows a ceiling of 1500 feet and visibility 4 km, with a chance of lower. Four kilometres is about 2.5 statute miles, so the ceiling is below 2000 feet and the visibility below 3 statute miles: the 1-2-3 test fails and an alternate is required.
You have two candidates:
- Airport A, 20 minutes away, forecast ceiling 2500 feet and visibility 8 km, with an ILS, a long runway, and no relevant NOTAMs.
- Airport B, 10 minutes away, forecast ceiling 900 feet and visibility 3 km, with only a non-precision approach, and a NOTAM that its approach aid is out of service this week.
Airport B is closer, but its weather is marginal for its non-precision approach, and the NOTAM removes the approach you would rely on, so it fails on both weather and serviceability. Airport A is slightly further but has the better weather, a precision approach, an adequate runway, and no NOTAM problems, and its forecast comfortably meets the alternate planning minima. Airport A is the sound choice, and you confirm you carry the fuel to reach the destination, then divert to A, then hold your required reserve.
The example shows the order of reasoning: confirm an alternate is needed, then judge each candidate on weather across the arrival window, the available approach, the runway, the NOTAMs, and the fuel, rather than on distance alone.
Common pitfalls
- Treating the 1-2-3 rule as the whole test. It is the weather limb of 91.169(b). The other limb is that the destination must have a published instrument approach, so a field without one takes an alternate whatever the forecast says.
- Picking on weather alone. A clear sky over a closed runway helps no one; read the NOTAMs.
- Forgetting the time window. The forecast must hold across your whole arrival window, not just your planned minute of arrival.
- Carrying one authority's numbers to another. The FAA 600-2 and 800-2 figures are not the EASA planning minima, and neither is universal.
- Choosing a field you cannot use on the day. Check fuel, hours, fire category, and approach currency, not just the runway.
- Underfuelling the diversion. The alternate only counts if you have the fuel to reach it with your required reserve.
In Pilot EFB
Pilot EFB lets you build a flight that includes your destination and your alternates, pull the weather and NOTAMs for each, and save them into the flight's folder as timestamped snapshots, with an integrity check so the briefing you saved is the briefing you keep. Its airport reference gives you the runway data, field NOTAMs, and sunrise and sunset to weigh a candidate quickly. It is a planning and organising aid: it does not decide whether an aerodrome is a legal alternate for your operation or compute your diversion fuel, so apply the regulation for your flight and your own judgement. Snapshots you have saved stay available offline; pulling fresh data needs a connection. Pilot EFB is offline-first and is not a certified Electronic Flight Bag.