Alternate Selection at the Destination
A flight is planned to destination D with two possible alternates. Alternate A is 40 min away; its only instrument approach has a decision height of 340 ft and requires 1200 m visibility. Forecast at the expected arrival time: cloud base 500 ft, visibility 3000 m, wind 240/18 kt. Alternate B is 70 min away; its approach has a decision height of 550 ft and requires 1500 m visibility. Forecast: cloud base 400 ft, visibility 2000 m, wind 200/25 kt gusting 35 kt. The operator uses the standard planning minima rule: ceiling at or above the approach decision height plus 200 ft, visibility at or above the approach minimum plus 800 m, unless a lower published planning minimum applies. Destination weather is forecast at the limit and may require a diversion.
1. Apply the planning minima rule to both alternates and determine which, if either, is usable.
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Alternate A: required ceiling = 340 + 200 = 540 ft; required visibility = 1200 + 800 = 2000 m. Forecast ceiling 500 ft is below 540 ft, so A fails the ceiling test despite visibility 3000 m being adequate. A is not usable as planned.
Alternate B: required ceiling = 550 + 200 = 750 ft; required visibility = 1500 + 800 = 2300 m. Forecast ceiling 400 ft is below 750 ft and visibility 2000 m is below 2300 m. B fails both tests and is not usable.
Neither alternate meets planning minima. The flight cannot be dispatched to D on this plan; the crew must find a third alternate, delay, add fuel for a different destination, or cancel. The gust spread at B (25G35) also warrants attention for crosswind and turbulence, but the minima failure alone is decisive.
2. If the operator holds a specific approval allowing one alternate to be used with a lower published planning minimum, what must be verified before relying on it?
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The approval must be route- and aerodrome-specific and must state the actual planning minima for that approach. The crew must confirm that the forecast (not just the current observation) meets the approved minima at the expected arrival time, including any time window. The approach must be serviceable and the required ground aids available. If the approval is not published for that alternate, the standard rule applies and the alternate remains unusable. Relying on a general company approval without a published lower minimum is not acceptable; the examiner is testing whether you treat approvals as specific documents, not as blanket permissions.