Nontransport category airplanes: Landing limitations: Alternate airport

When selecting an alternate airport for a nontransport category airplane under Part 121 operations, you must ensure the aircraft can land within 70 percent of the runway's effective length. This calculation uses the airplane's anticipated weight at arrival time and follows the same assumptions specified in § 121.203 for landing distance calculations.

This is a more conservative requirement than for the destination airport itself, providing an extra safety margin since alternates are used when conditions deteriorate unexpectedly. The 70 percent rule means you need significantly more runway available than the calculated landing distance—essentially building in a 30 percent buffer.

For example, if a runway is 5,000 feet long, your airplane must be able to stop within 3,500 feet (70% of 5,000) at its expected arrival weight. If it cannot meet this requirement, you cannot list that airport as your alternate, regardless of how suitable it might otherwise be.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*