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Ambient vs Forecast Conditions - P Chart

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BR3NT created the topic: Ambient vs Forecast Conditions - P Chart

WIND
"Be careful with wind in P Chart questions. Candidates need to ask themselves is this an actual (or ambient) wind I’m given or is it a forecast wind? It should be remembered that there is NO GUARANTEE that forecast winds will actually be present at the time of operations whereas actual or ambient winds may be used."


I've checked CAO 20.7, do we use both ambient and forecast winds when given? or if a question gives a forecast winds do we use no wind unless its something that would deteriorate landing/takeoff performance?
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  • John.Heddles
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  • ATPL/consulting aero engineer
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John.Heddles replied the topic: Ambient vs Forecast Conditions - P Chart

.. is this an actual (or ambient) wind .. or is it a forecast wind

The distinction is made between takeoff and other situations, especially landing.

In the former, there is no problem running preliminary planning based on forecast as the actual is established prior to takeoff and, should this then require load adjustment, so be it. Refer CAO 20.7.4.4.1(a) - actual wind for takeoff.

However, for the latter, with the aircraft in flight when you discover that the forecast is not quite right, it's a tad late to fix things up.

Hence, looking at the landing situation, it is established protocol to use nil wind on the main runway at the planning stage. Refer CAO 20.7.4.4.1(d) - nil wind for landing at planning. However, when you get to the destination/alternate, you will have the actual wind available (or a reasonable pilot estimate) so you can then use the actual wind - refer CAO 20.7.4.5.1(a) - actual wind for landing (at the destination/alternate).

In years gone by, there was an additional sensible requirement, then, to cover an adverse crosswind situation but this appears to have fallen by the wayside. Being a sensible approach, I suggest that it still be used so that the practical approach becomes, for landing, at the planning stage - ie before takeoff:

(a) establish the RLW for landing on the main runway with nil wind, as the CAO requires.

(b) if the forecast suggests that the crosswind on the main runway might be a problem, then establish the minimum RLW on a suitable runway to accommodate the forecast wind should the main runway not be available due to limit crosswind as the ANO/CAO used to require in days now gone. This should be done for a wind speed which just exceeds the main runway crosswind limit - the aim is to maintain conservatism so that we reasonably minimise the probability of finding ourselves embarrassed at either the destination or alternate, where required. (Although it should not need saying, we also make sure that we don't get caught out with an excessive tailwind along the way).

(c) RLW-limited RTOW then becomes the lesser of (a) and (b).

Is this all still a bit rubbery ? Of course it is .. but it is the reasonable best we can do in the absence of a crystal ball prior to departure. Aircraft risk and safety is not absolute; we can only take reasonable steps to cover reasonable real world probabilities.

Engineering specialist in aircraft performance and weight control.
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