Just been reading the US AOPA mag. They collected data from 3500 Cessna 182 (G1000) flights and this is what they found.
The reason is that even the “average wind” (an exact crosswind) requires you to fly a longer distance because you have to crab into it.
The other part of the question is why forecast headwinds are usually stronger than forecast tailwinds, which seems massively obvious.
This has also been covered here:
Superfluous IMHO, as it is totally obvious that crosswinds work out as partial headwinds and thus reduce the average groundspeed.
I would have guessed that this effect is equalled out by pilots adapting to the winds, by flying higher if there is a tailwind and lower if there is a headwind. Also by choosing the lateral routing that has less wind on a given flight. And even by delaying a flight or going earlier if you see that the winds are more favorable at another time.
But another significant effect when calculating an “average”, apart from the need to crab in a crosswind, is of course that for a given A → B → A flight with a perfect headwind from A to B and perfect tailwind from B to A, you will spend more time on the way out than on the way back.
Even on a round trip you can never win with any wind.
The quickest round trip is the one without wind. The stronger the wind, the longer the round trip will take.
You can win with a round trip if the wind changes strength or direction between out and return legs. I had that happen on a trip from Texas to Florida and back – 40-50 knot tailwind on the outbound leg, <10kt headwind on the return!
Of course I’ve lost out that way before. Once went on a day trip in a C172, light winds on the way out, 50kt headwind on the way home which is painful in a C172.
alioth wrote:
You can win with a round trip if the wind changes strength or direction between out and return legs.
Unfortunately, that’s a once in a lifetime event in the life of most pilots. I am still looking forward to that experience…
alioth wrote:
You can win with a round trip if the wind changes strength or direction between out and return legs.
I seem to find that most of that sentence is true for my trips. However, the change of strength or direction seems to result in a headwind on both legs!
what_next wrote:
Unfortunately, that’s a once in a lifetime event in the life of most pilots. I am still looking forward to that experience…
MedEwok wrote:
I had the opposite happen…
I guess I did not make it clear enough 
Of couse the “once in a lifetime” event is when you have a tailwind both ways. Never happens in an aviator’s life more than once I am told. Don’t know because it has not yet happened even once.
I’ve had it happen, close to where @MedEwok flies. In the morning, flew out from Wangerooge EDWG to Baltrum EDWZ, with around 20 kts tailwind. Then during the day, the wind turned as forecast, and on the way back we had a good 30-40 kts tailwind. Unfortunately it’s a very short flight, so I reduced the engine setting to 2000 RPM in order to profit from the 140 kts GS in a C172 for a bit longer; despite renting wet so it wasn’t to my advantage.
