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]
Figure 2-14. IFR Alternate Minimums.
ALTERNATE MINIMUMS FOR COMMERCIAL OPERATORS
IFR alternate minimums for Part 121 and 135 operators are very specific and have more stringent requirements than Part 91 operators.
Part 121 operators are required by their OpsSpecs and Parts 121.617 and 121.625 to have a takeoff alternate airport for their departure airport in addition to their airport of intended landing if the weather at the departure airport is below the landing minimums in the certificate holder’s OpsSpecs for that airport. The alternate must be within two hours flying time for an aircraft with three or more engines with an engine out in normal cruise in still air. For two engine aircraft, the alternate must be within one hour. The airport of intended landing may be used in lieu of an alternate providing it meets all the requirements. Part 121 operators must also file for alternate airports when the weather at their destination airport, from one hour before to one hour after their ETA, is forecast to be below a 2,000-foot ceiling and/or less than 3 miles visibility.
For airports with at least one operational navigational facility that provides a straight-in non-precision approach, a straight-in precision approach, or a circling maneuver from an instrument approach procedure determine the ceiling and visibility by:
- Adding 400 feet to the authorized CAT I HAA/HAT for ceiling.
- Adding one mile to the authorized CAT I visibility for visibility minimums.
This is but one example of the criteria required for Part 121 operators when calculating minimums. Part 135 operators are also subject to their own specific rules regarding the selection and use of alternate minimums as outlined in their OpsSpecs and Part 135.219 through Part 135.225, and they differ widely from those used by Part 121 operators.
Typically, dispatchers who plan flights for these operators are responsible for planning alternate airports. The dispatcher considers aircraft performance, aircraft equipment and its condition, and route of flight when choosing alternates. In the event changes need to be made to the flight plan en route due to deteriorating weather, the dispatcher will maintain contact with the flight crew and will reroute their flight as necessary. Therefore, it is the pilot’s responsibility to execute the flight as planned by the dispatcher; this is especially true for Part 121 pilots. To aid in the planning of alternates, dispatchers have a list of airports that are approved as alternates so they can quickly determine which airports should be used for a particular flight. Dispatchers also use flight-planning software that plans routes including alternates for the flight. This type of software is tailored for individual operators and includes their normal flight paths and approved airports. Flight planning software and services are provided through private sources.
Though the pilot is the final authority for the flight and ultimately has full responsibility, the dispatcher is responsible for creating flight plans that are accurate and comply with the CFRs. Alternate minimum criteria are only used as planning tools to ensure the pilot-in-command and dispatcher are thinking ahead to the approach phase of flight. In the event the flight would actually need to divert to an alternate, the published approach minimums or lower-than-standard minimums must be used as addressed in OpsSpecs documents.