Selecting a suitable alternate

The nearest airport isn't always the best one. A diversion is a judgment call — the right field is the one that solves your actual problem, not just the closest dot on the GPS.

What makes an airport suitable

A field you can divert to has to actually work for your situation:

• Runway — long enough for your airplane and a surface you can use. A 2,000-ft grass strip is not an alternate for most pilots in most conditions.
• Services — fuel if you're diverting low on fuel; maintenance if something's wrong; lighting if daylight is running out.
• Surroundings — terrain and obstacles around the field, and whether it's easy to find and enter the pattern.
• Help, if you need it — a towered field puts a controller and often crash/fire/rescue at your disposal; a quiet non-towered field is simpler on traffic. Match the choice to whether this is a precaution or a problem.

And read "long enough" in context: a runway that's plenty on a cool day at sea level can be marginal on a hot, high-density-altitude afternoon (the takeoff-and-landing performance math from I.F). The suitable field is the one suitable today — in these conditions, in this airplane, at this weight.

The factors that rank them

These aren't a flat checklist — they have a hierarchy, and pilots who memorize a flat list miss it:

1. Weather is the primary driver. It decides the diversion the large majority of the time, and it's the first test of any candidate: the field must have ceilings, visibility, and wind you can actually use. A closer airport that's also socked in is no alternate at all.
2. Fuel is the constraint. It sets the range of fields you can even consider — anything outside your fuel radius, reserves included, is off the table before weather even matters.
3. Distance/time and airspace are routing inputs. Among the fields that pass the weather and fuel tests, pick the one that's a sensible, safe route to — not through a ridge you can't outclimb or a Class B floor.

So the decision flows: filter by fuel range → filter by usable weather → among the survivors, take the closest and easiest to reach. Weather drives, fuel constrains, distance and airspace shape the route.

A quick worked example. Three fields sit within fuel range: A is just 12 NM away but reporting an 800-ft ceiling; B is 20 NM out, solid VFR, with fuel and a paved 4,000-ft runway; C is 18 NM, VFR, but only a 2,200-ft strip with no services. Weather eliminates A outright. Between B and C, the runway and fuel at B are worth the eight extra miles — you've traded a little distance for a field you can actually land at and leave again. The closest dot lost; the most suitable field won. That's the whole task in miniature.

The pre-flight homework

The best alternates are chosen before you launch. §91.103 requires you to be familiar with all available information for the flight, including alternatives if it can't be completed as planned. So on a cross-country, note a few candidate fields along the route — with fuel, decent runways, and their frequencies — before you go. In the air you're then recognizing an alternate you already scouted, not researching one cold while the situation deteriorates.