What additional requirement applies when filing an airport with only one dedicated runway as an alternate?

When filing an airport that has only one dedicated runway as an alternate, the requirement to add 400 feet to the Decision Height (DH) or Minimum Descent Height (MDH) is mandated for safety reasons. This rule is established to account for the increased risk associated with having only one runway available, which could affect a pilot's ability to land or take off, especially under adverse weather conditions.

The additional altitude provides a buffer to ensure that the aircraft can safely maneuver in the event of an approach miss or other abortive maneuvers. This precaution helps enhance safety by ensuring that if an approach is not successful, there is more altitude to work with during a go-around or alternative maneuvers. Thus, adding 400 feet to the DH/MDH when using an airport with only one dedicated runway enhances the margin of safety for operations under Part 135 regulations.