Alternate airports: the art of the second choice

When you plan a flight, your mind builds a story: depart here, fly there, land. The problem with stories is that the world doesn't always cooperate with the ending. The destination might be closed when you arrive, the weather might collapse, the runway might be unusable, the fuel pump might be broken. Pilots who plan for these scenarios in advance — and identify a real alternate airport before they take off — fly long careers. Pilots who don't are the ones who occasionally end up in a field. This is the practical guide.

When alternates are legally required

The legal requirements split by flight rules:

• Day VFR: alternate not legally required in most jurisdictions. Recommended but optional.
• Night VFR: alternate often required (varies by country)
• IFR: alternate required when destination weather is below specific minimums

For IFR specifically, the 1-2-3 rule (FAA) or its EASA equivalent says: file an alternate if the destination weather forecast within 1 hour of ETA shows a ceiling less than 2,000 ft AGL or visibility less than 3 sm.

For UL/LSA day-VFR pilots, the legal answer is "no alternate required". But that's the legal floor — not the smart practice.

The cautious VFR pilot's rule

Even when an alternate isn't legally required, plan one anyway if any of these are true:

• Destination is a remote airfield with no fuel
• Weather forecast (TAF) shows borderline conditions in your arrival window
• The route is over water, mountains, or terrain with limited divert options
• You haven't been to the destination before
• The flight is more than 2 hours long
• You're flying near sunset and the destination is non-towered

For UL pilots flying a 200 km cross-country to an unfamiliar grass strip, having a paved alternative 30 km away is the difference between "easy diversion" and "uncomfortable improvisation".

How to choose an alternate

Five criteria, in order of priority:

1. Reachable: within fuel range from your destination decision point with reserves intact
2. Weather: forecast better than destination, ideally with one full ceiling/visibility step margin
3. Runway: long enough, surface-suitable for your aircraft, available 24/7 (or during your ETA window)
4. Services: fuel, lighting (if night), pilot facilities (if you'll be stuck overnight)
5. Familiar: you've been there before, or you've studied the chart and procedures

For VFR alternates, a small towered airfield 15–30 nm from the destination usually fits all five criteria. For IFR alternates, you need a full instrument approach and weather above the alternate's published IFR minimums.

The "what kills me if X fails" mental model

A useful pre-flight exercise: walk through the failure modes of your destination, in order:

1. Weather closes: ceiling drops below VFR minimums or vis below your personal minimums
2. Runway closes: NOTAM, accident, snow, work in progress
3. Fuel unavailable: pump broken, fueler not at field, wrong fuel type
4. Field closed: night, holiday, lock-out
5. Mechanical: gear/brake issue revealed only on arrival

For each, ask: "if this happened 30 min before I land, can I divert to an alternate I've already identified?". If the answer is no for any of these, you don't have a real alternate plan. Pick a different destination, or pick a real alternate.

For a Tecnam P92 Echo MkII on a 200 km cross-country to a small grass strip, the answer to "what if my destination is closed" should be "I have a paved alternate 25 km away with fuel and a tower" — not "I'll figure it out".

Distance from primary to alternate

Two competing considerations:

• Closer is better (less fuel needed to divert)
• Different weather is better (independent weather systems means less correlation between primary and alternate failures)

A small alternate 5 km from the primary is great for fuel but useless if both have the same weather. A weather-distinct alternate 50 km away requires more fuel reserve.

The compromise for VFR: 20–30 km away, ideally on a different terrain side (e.g., primary is in a valley, alternate is on the coast). For IFR: minimum distance to ensure independent weather is roughly 50 nm in stable air masses, more in unstable ones.

Fuel reserves with alternate planning

When you're carrying fuel for an alternate, the math:

• Fuel to destination: time × cruise burn
• Fuel to alternate from destination: time × cruise burn (assume worst-case heading)
• Final reserve: 30 min VFR / 45 min IFR at cruise burn

A typical IFR cross-country fuel calculation:

• 150 nm to destination: 50 min × 17 l/h = ~14 l
• 30 nm to alternate from destination: 10 min × 17 l/h = ~3 l
• 45 min IFR reserve: ~13 l
• Total fuel needed: ~30 l + taxi + climb (~5 l) = 35 l minimum

If your tank holds 90 l, you have a comfortable 55 l buffer. If your tank holds 50 l, you're tight. Plan accordingly — or pick a closer alternate.

Voliqo's alternates surface

In the planner, the "Alternates" section near the destination shows airports within a configurable distance ring of your final waypoint. The distance defaults to 30 km but can be adjusted. The planner picks alternates by:

1. Finding all airports within the distance ring
2. Filtering to airports with sufficient runway length for your aircraft
3. Sorting by distance from destination
4. Showing the top 4

The alternates don't auto-include weather data — the pilot's job is to pull METAR/TAF for the candidates and pick one with weather margin. Voliqo's /blog post on TAFs covers how to read those.

For a 200 km flight to a small strip, the planner might surface 4 alternates within 30 km. You pick the one with:

• Best runway (paved > grass for emergency diversions)
• Forecast better than destination
• Services available during your ETA

When to declare a diversion

The decision to divert isn't a panic event — it's a planned response to changing conditions. The mental triggers:

• Weather: actual conditions at destination drop below your personal minimums (NOT the legal minimums — your personal ones)
• Fuel: you arrive at your "decision fuel" (typically destination fuel + alternate fuel + 30 min) before reaching the destination
• Mechanical: any indication that lands you with reduced safety margin
• Daylight: the latest landing time is approaching and conditions don't support arrival before sunset

When any of these triggers, declare the diversion early — to ATC if you're under flight following, in your own mind if you're not. Re-plan to the alternate immediately. Don't keep flying toward a deteriorating destination "to see if it improves" — that's how pilots end up landing with 5 minutes of fuel.

What an alternate is NOT

A common pilot trap: treating the closest airfield to your destination as the "alternate". If both are inside the same weather system, both will fail at the same time. The alternate must be weather-distinct to be a real alternate.

Similarly: if your alternate has the same operational hours, the same NOTAMs, or the same fuel pump as the primary, it's not really an alternate. Independence is what makes an alternate work.

Bottom line

Alternates aren't a paperwork exercise — they're the cheapest insurance policy in aviation. For VFR pilots, planning an alternate is optional but strongly recommended for any flight outside of local pattern work. For IFR pilots, the legal requirement is just the floor — real alternates have margin in distance, weather, runway, and services.

The discipline: before every cross-country, name your alternate out loud. Check its weather, its runway, its fuel availability. Have it on paper or on your tablet. If you can't name one with confidence, you're not ready to fly the trip.