In the world of scheduled air carrier service, the requirement for alternates gets fairly complex. Not just destination alternates, but takeoff alternates too.
A takeoff alternate isn’t explicitly required by Part 91 operations. Not so in Part 121 operations. If the weather at the time of departure is below authorized landing minimums for the appropriate runways available, based on weather and equipment, a takeoff alternate must be specified on the dispatch release. This takeoff alternate must also be within one hour of flight at normal cruising speed, in still air, with one engine inoperative.
For destination alternates, the one-two-three rule applies. If at least one hour prior, and one hour past the estimated time of arrival at the destination airport, the weather reports and/or forecasts—or any combination of them—indicate a ceiling less than 2,000 feet and a visibility less than three miles, an alternate airport is required. There are also other criteria that can sometimes require an alternate, such as crosswinds, braking action reports, or runway contamination accumulation rates.
In determination of the alternate airport minimums, my company utilizes a one-navaid, two-navaid method. Should the destination only be served by one operational navigation facility that provides a straight-in, nonprecision or Category I precision approach procedure or even a circling maneuver, derive the alternate minimums by adding 400 feet to the minimum descent altitude (MDA) or descent altitude (DA) and one mile visibility to the landing minimums. Should the destination airport be served by two operational navigation facilities, each providing a straight-in approach procedure to two different suitable runways, derive the alternate minimums by adding 200 feet to the higher MDA or DA and one-half mile to the higher authorized landing minimums.
Should the destination airport weather and the first alternate airport weather be considered marginal, an additional alternate must be listed on the dispatch release. The destination airport weather is considered to be marginal when it is forecast to be less than a 1,000-foot ceiling or less than three miles visibility from the time beginning one hour prior to, until one hour after, the estimated time of arrival. The first alternate weather is considered to be marginal if it is forecast to have less than a 600-foot ceiling or less than two miles visibility at the estimated time of arrival.
Even though these alternate requirements are more complex than the Part 91 regulations, they’re clearly enhanced to provide an additional layer of safety in inclement weather conditions.
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