The requirement for an alternate depends on the aircraft category, equipment installed, approach NAVAID and forecast weather. For example, airports with only a global positioning system (GPS) approach procedure cannot be used as an alternate by TSO-C129/129A users even though the N/A has been removed from the approach chart. For select RNAV (GPS) and GPS approach procedures the N/A is being removed so they may be used as an alternate by aircraft equipped with an approach approved WAAS receiver. Because GPS is not authorized as a substitute means of navigation guidance when conducting a conventional approach at an alternate airport, if the approach procedure requires either DME or ADF, the aircraft must be equipped with the appropriate DME or ADF avionics in order to use the approach as an alternate.

For airplane Part 91 requirements, an alternate airport must be listed on IFR flight plans if the forecast weather at the destination airport, from a time period of plus or minus one hour from the estimated time of arrival (ETA), includes ceilings lower than 2,000 feet and/or visibility less than 3 SM. A simple way to remember the rules for determining the necessity of filing an alternate for airplanes is the 1, 2, 3 Rule. For helicopter Part 91, similar alternate filing requirements apply. An alternate must be listed on an IFR flight plan if the forecast weather at the destination airport or heliport, from the ETA and for one hour after the ETA, includes ceilings lower than 1,000 feet, or less than 400 feet above the lowest applicable approach minima, whichever is higher, and the visibility less than 2 SM.

Not all airports can be used as alternate airports. An airport may not be qualified for alternate use if the airport NAVAID is unmonitored, or if it does not have weather reporting capabilities. For an airport to be used as an alternate, the forecast weather at that airport must meet certain qualifications at the estimated time of arrival. Standard alternate minimums for a precision approach are a 600-foot ceiling and a 2 SM visibility. For a non-precision approach, the minimums are an 800-foot ceiling and a 2 SM visibility. Standard alternate minimums apply unless higher alternate minimums are listed for an airport.

On NACO charts, standard alternate minimums are not published. If the airport has other than standard alternate minimums, they are listed in the front of the approach chart booklet. The presence of a triangle with an on the approach chart indicates the listing of alternate minimums should be consulted. Airports that do not qualify for use as an alternate airport are designated with an N/A. [Figure 2-14]