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Can MOS be used to determine if an IFR alternate airport is needed?

Updated: Jul 15, 2022

Unfortunately, the answer is not as cut and dried as you might have hoped.

The regulations state in 14 CFR § 91.169 (b)(2)(i) that an alternate is not required to be included if,

For at least 1 hour before and for 1 hour after the estimated time of arrival, the ceiling will be at least 2,000 feet above the airport elevation and the visibility will be at least 3 statute miles.

Pilots affectionally know this as the 1-2-3 rule. Essentially this says that an airport must be designated in an IFR flight plan to provide a suitable destination if a landing at the intended destination becomes inadvisable. However, waters are muddied because the regulations do not stipulate what weather reports or forecasts to use to make this determination. On a short duration flight filed right before takeoff, surface observations (METARs) should be used to determine the weather "one hour before" arrival. However on a longer flight planned a couple hours in advance, it is the forecast that comes into play most of the time. Even though it's not stated in the regulations, in reading the Instrument Flying Handbook (FAA-H-8083-15B), there's no question that if a Terminal Aerodrome Forecast (TAF) is issued for the destination airport, it must be used to evaluate the 1-2-3 rule. But what if a TAF isn't issued for the proposed destination airport?

How about using the nearest airport where a TAF is issued? Perhaps. As long as the destination airport is within 5 statute miles of that nearby airport, that may be acceptable (probably has never been tested in court). That's because a TAF is only valid 5 statute miles from the center of the airport's runway complex according to the Aviation Weather Services advisory circular, AC 00-45H Change 2. There are not that many airports that fall into this category, however. If the destination is greater than 5 statute miles from the other airport, then how does a pilot evaluate the 1-2-3<