When is an alternate landing airport required?

An alternate landing airport is required under specific conditions that enhance safety and ensure proper planning for flight operations. The correct answer reflects the need for an alternate landing airport when both conditions are satisfied.

An Instrument Approach Procedure (IAP) is critical for landing especially in low visibility conditions. If the destination airport lacks an IAP, a pilot cannot guarantee a safe landing in situations where instrument navigation is required. Furthermore, the 1-2-3 rule is a guideline that suggests pilots always have an alternate airport if the destination is more than one hour’s flying time away, or if the weather is forecasted to be below certain minimums. If a flight is planned to an airport that doesn't meet these criteria or is outside the regulated distance, the necessity for an alternate airport becomes paramount.

In addition, inclement weather at the destination serves as a legitimate reason for needing an alternate airport, since it can hinder safe landings.

Thus, the requirement for an alternate airport arises when both the lack of an IAP and the disregarded 1-2-3 rule conditions exist, making the fourth choice the most comprehensive and correct answer.