What is a requirement for using an alternate airport indicated in green on Enroute high altitude charts?

The correct answer is the requirement of a minimum 5,000 feet hard-surfaced runway for using an alternate airport indicated in green on Enroute high altitude charts. This specification is crucial because the purpose of an alternate airport is to provide a safe landing option in case the primary destination becomes unapproachable due to weather, operational, or other issues.

The minimum runway length ensures that aircraft can safely land and take off, especially for those that might have performance limitations in adverse conditions or with a specific configuration. The 5,000-foot minimum allows for adequate space for various aircraft operations and increases the likelihood that the alternate can accommodate a range of aircraft in case of an emergency.

Maintaining a high standard in runway length is a safety measure grounded in aviation regulations, which dictate that alternate airports must meet specific criteria to ensure flight safety and operational effectiveness in emergencies.