When is an alternate airport required according to the 1-2-3 rule?

The requirement for an alternate airport is a fundamental consideration in flight planning, ensuring that pilots have a backup landing location should the primary destination become unapproachable. According to the 1-2-3 rule dictated by FAR 91.169, an alternate airport is required when the destination airport does not have an Instrument Approach Procedure (IAP). This is because without an IAP, the likelihood of being able to land safely, especially in adverse weather conditions, is significantly reduced.

To elaborate, even though it is important for pilots to consider factors such as distance and weather conditions, the most immediate action dictated by the 1-2-3 rule is the lack of an IAP. If there is no established procedure for instrument landings at the destination, the pilot must ensure there is an alternate available to provide a higher safety margin.

In practical terms, this means that if a pilot is planning a flight to a destination that doesn't have the infrastructure for instrument landing, they must identify and plan for an alternative airport that does have the necessary navigational support to assist in a safe landing, regardless of the distance or altitude at which they are flying.