Final answer:
Filing an alternate airport is generally required when weather conditions (like low ceilings or poor visibility) at the destination airport are forecasted to be below certain minima, or when the airport lacks specific navigational aids. The '1-2-3 rule' is often used as a guideline for this in the U.S.
Step-by-step explanation:
Conditions Requiring Filing an Alternate Airport
When planning a flight, pilots are often required to file an alternate airport in their flight plan. The conditions that dictate filing an alternate are usually outlined in aviation regulations, such as Federal Aviation Regulations (FARs) for the United States. A pilot must file an alternate if the weather reports or forecasts for the destination airport, at the estimated time of arrival (ETA), indicate that the ceilings will be less than 2,000 feet above ground level or the visibility will be less than 3 statute miles. Additionally, if the destination airport lacks weather reporting capabilities, or if it does not have an operational Instrument Landing System (ILS) and certain other navigational aids that might be necessary due to the weather conditions forecasted, an alternate must be filed.
Furthermore, pilots must consider the '1-2-3 rule', which is a standard guideline in the U.S., indicating that an alternate is required if, within one hour before to one hour after the ETA, the weather conditions are expected to be below certain minima (specifically, less than 2,000 feet ceiling or less than 3 statute miles visibility). It's worth noting that the exact regulations and values can differ between countries and should be verified with the relevant aviation authority.
Thus, the primary reasons for filing an alternate airport usually stem from anticipation of poor weather conditions or insufficient navigational aids at the planned destination airport.