"Larry Dighera" wrote in message
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So I appears that my recollection was faulty. But it seems counter
intuitive, that a 90-degree crosswind contributes half its velocity to
a tailwind component.

That's because you need to take into account the application of that
particular resource. Applying that sort of thinking to cruise flight IS
counter-intuitive, because it's not correct in that context.

It's not even literally correct in the context of the article you quoted,
but nevertheless the article you quoted has useful information in it.
First, it's a discussion of landing, not cruising. Second, it's a
collection of rules of thumb, not a precise analysis of reality.

It is easy to show that mathematically, a 90 degree crosswind results in no
tailwind component. Without a correction, it results in no headwind
component as well.

But when dealing with mountain flying, and in particular landing on a short
runway, assuming a tailwind component for a 90 degree crosswind is
conservative approach. That is, a 90 degree crosswind clearly doesn't add
half the wind speed to your groundspeed, but the crosswind does create other
effects that could result in a lengthening of the room required to land,
roughly equivalent to a similar increase in groundspeed.

Note that while a tailwind is estimated at full strength, when coming from
within a 30 degree angle, a headwind is estimated only a 3/4 strength, even
when coming from the same angle (in the other direction, of course).

I believe that is the true nature of the article you've quoted: to provide
rules of thumb that offer safe guidance to pilots landing in constrained
areas, especially when the landing area is defined not by prevailing winds
but by terrain restrictions, preventing the pilot from taking best advantage
of the current winds. Where the winds increase the landing distance, they
are assumed to be greater than actual, and where the winds might shorten the
landing distance, they are assumed to be lesser than actual. In neither
case do the estimates provide any assistance in judging the effects of winds
aloft during cruise flight.

Hope that helps.

Pete