What constitutes an alternate airport, and when should you plan one?

• A. An alternate is a backup maintenance facility; plan one when maintenance is due.
• B. An alternate is a second landing site planned in case the destination is unavailable; plan one when weather or other conditions at the destination may prevent landing there.
• C. An alternate is a backup airfield for parking; plan one when parking is full.
• D. An alternate is an additional crew member; plan one when crew is unavailable.

Answer: (B)

An alternate airport is a backup landing site you plan ahead of time in case you can’t land at your destination. You plan one when conditions at the destination may prevent a safe landing, such as forecast weather that would make the approach or landing impractical, a runway closure, or other factors that could render the destination unusable. The idea is to have a prepared, suitable place to land that has workable approaches and minimums you can meet with your aircraft’s performance and fuel. So this concept fits best: you’re planning a second landing site in case the destination is unavailable due to weather or other conditions. The other options describe maintenance, parking, or an extra crew member, which aren’t about where you would land if the destination can’t be used.