According to the 123 rule, when is an alternate destination required?

The 123 rule specifies that an alternate airport is required unless a published Instrument Approach Procedure (IAP) is available for the destination airport. This means that if the destination has an IAP that is properly published and accessible, then the flight may proceed without an alternate. The rationale behind this rule is to ensure that, in the event of deteriorating weather conditions or other unforeseen circumstances at the destination, pilots would have the ability to safely land at the intended airport due to the available procedures.

In contrast, the other options do not accurately reflect the requirements set forth by the 123 rule. For instance, claiming that an alternate is always required ignores the exceptions specified by the rule, while stating that an alternate is needed for any flight over 1 hour does not take into account whether an IAP exists. The stipulation regarding flight visibility being less than 5 miles is also not directly relevant to the necessity of an alternate destination as established by the 123 rule. The rule fundamentally focuses on the availability of published IAPs at the destination as a key factor in determining if an alternate is necessary.