When is an alternate landing airport required?

An alternate landing airport is required when certain conditions exist that render the primary destination unsuitable for landing. This will often include scenarios where the destination airport does not have an Instrument Approach Procedure (IAP), or when the destination is outside the 1-2-3 rule, which refers to the minimum weather conditions for flight.

The 1-2-3 rule essentially states that if a destination airport's weather is below the minimums for landing, or if the flight arrives at the destination airport with less than the required weather conditions for an IAP, an alternate airport is necessary.

In addition to these regulations, if there is inclement weather at the destination, meaning that prevailing conditions could impair safe landing, it further necessitates the need for an alternate landing site. Therefore, when both the absence of an IAP at the primary airport and the breach of the 1-2-3 rule occur, such a circumstance mandates the use of an alternate landing airport, reinforcing the requirement for safety and regulatory compliance in aviation operations. This context underlines why the correct response encompasses the condition where both prior situations warrant the need for an alternate landing airport.