The Operational Flight Plan, or OFP, will specify a take-off alternate — Page 35, Lesson 57

I want to walk you through the rules for selecting take-off alternates and destination alternates. These are critical planning decisions you'll make before every flight, and the regulations are very specific about the weather conditions and distances involved. Let's start with the take-off alternate. The Operational Flight Plan, or OFP, will specify a take-off alternate. This is an aerodrome you can use if, after take-off, you cannot return to the departure aerodrome. That might be because the weather at departure has deteriorated, or because of a performance limitation—for example, you're too heavy to land back immediately. Now, what weather does a take-off alternate need to have? For an aerodrome to be selected as a take-off alternate, the weather reports and forecasts must show that during the period from 1 hour before until 1 hour after your estimated time of arrival at that alternate, the meteorological conditions will be at or above the applicable aerodrome operating minima. Those minima are defined in Figure 4.1, which we'll look at shortly. There's an important detail here: you must take the ceiling into account when the only available instrument approach options are non-precision or circling approaches. Ceiling means the height of the lowest cloud base that is reported as broken or overcast. Additionally, you must consider any limitations related to one engine inoperative—because if you lose an engine right after take-off, you need to be able to reach that alternate safely. That brings us to the distance limitation. For a two-engine aeroplane, the take-off alternate must be located within either one hour flight time at the one-engine-inoperative cruise speed, or, where approved, the operator's ETOPS diversion time, subject to any MEL restriction. MEL stands for Minimum Equipment List—that's the list of equipment that may be inoperative and still allow dispatch. The maximum is 2 hours at the one-engine-inoperative cruise speed. For aeroplanes with three or more engines, the limit is two hours flight time at the one-engine-inoperative cruise speed specified in the AFM, which is the Aeroplane Flight Manual. That calculation is based on still air and standard conditions, using the actual take-off mass. The note clarifies that if the AFM does not specify a one-engine-inoperative cruise speed, you use the speed that can be achieved with the remaining engines set at maximum continuous power. Now let's move to destination alternates. For an aerodrome to be selected as a destination—except for isolated destination aerodromes—the weather reports and forecasts must indicate that during the period from 1 hour before until 1 hour after the ETA at that aerodrome, the meteorological conditions will be at or above the following: the RVR or visibility must be as required for the aerodrome operating minima. RVR stands for Runway Visual Range—it's a more precise measure of visibility along the runway. For a non-precision or circling approach, the ceiling must be at or above the MDH, which is the Minimum Descent Height. If those conditions are not met, then you must select two destination alternates. Let's look at the planning minima table for destination alternates, isolated destinations, and the 3% ERA—that's the En-route Alternate, which we'll cover later. The table lists the type of approach and the required planning minima. For CAT II and CAT III approaches, the planning minimum is CAT I RVR. CAT II and CAT III are precision approaches with very low visibility minima; CAT I is the least demanding of the precision categories. For CAT I approaches, the planning minimum is Non-precision RVR and the ceiling must be above MDH. There's a note here: "Non-precision" means the next highest minimum that is available at the aerodrome. So for a CAT I ILS, this could be an ILS without glidepath, or a VOR/DME approach. For Non-precision approaches, the planning minimum is Non-precision RVR and the ceiling must be above MDH plus 200 feet and 1000 metres. For Circling approaches, the planning minimum is Circling (Vis/RVR/MDH(VM(C)))—that's the visibility, RVR, and MDH for the circling minima. The note repeats that "Non-precision" means the next highest minimum available at the aerodrome. So for a CAT I ILS, that could be an ILS without glidepath, or a VOR/DME. The same applies to non-precision approaches. That figure shows the table we've just discussed. The key takeaway is that you must plan your alternates with very specific weather and distance criteria, and the type of approach available at the alternate determines the exact minima you need to meet.