What is the exemption for alternate airport?

Exemption 3585 allows the dispatcher to release a flight to an airport OR alternate when the TAF indicates, through the conditional statements (TEMPO, PROB, BECMG) that the weather could be below authorized landing minimums at the estimated time of arrival.



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If either the ceiling or visibility is forecast to be less than 2,000 feet or 3 statute miles during that arrival window, you are required to file an alternate. Also, the destination airport must have at least one instrument approach procedure or an alternate airport will always be required.

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An airport may not be qualified for alternate use if the airport NAVAID is unmonitored, is Global Positioning System (GPS) based, or if it does not have weather reporting capabilities.

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For example, the main body of the forecast may have the visibility at one-half mile, but a TEMPO phrase may show a possible drop to on-quarter mile at the ETA. When this happens, the airlines that have been granted approval to do so can use what is called Exemption 3585.

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Not more than one hour from the departure airport at normal cruising speed in still air with one engine inoperative. (2) Aircraft having three or more engines. Not more than two hours from the departure airport at normal cruising speed in still air with one engine inoperative.

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This is the ol' 1-2-3 rule. There are a couple of gotchas. If your destination has no approach, an alternate is always required. Curiously, though, the alternate need not have an instrument approach if the weather allows a descent from MEA under basic VFR, assuming the airport isn't otherwise disqualified.

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2. Choose an alternative flight. If you still want to travel, your airline must find you an alternative flight. It's up to you whether to fly as soon as possible after the cancelled flight, or at a later date that suits you.

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It's called the 3-2-1 rule, and it's the easiest way to remember the regulation. To recap, if the weather at your destination isn't at least 3 SM of visibility and 2000' AGL ceilings from 1 hour before to 1 hour after your ETA, you need to file an alternate.

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