Head wind TailWind
 

Hi Folks ,
Very quick question , a friend of mine has asked me about prevailing winds . he stated that when flying to Thailand that on the out bound leg that the flight takes 9 hours and on the way back that it ALWAYS takes 12 hours , surely the wind direction can change and tail wind can be a head wind at some other time ... would any one be able to help with this .
S&F

While local and surface winds can change direction, prevailing winds on a global scale do not do so to the same extent; the jetstream moves about a bit, but is *always* a westerly wind, so flying east will be with a tailwind and flying west with a headwind. Therefore Europe-SEAsia is faster than SEAsia -Europe.

The jetstream is predominantly easterly over the equator but is generally sporadic seldom reaching 70kts. Then there are two bands of predominantly westerly jet streams over about 20N-30N and 50N-60N. It is all quite approximate. These northen jetstreams are much stronger at 60kts-200kts (300kts sometimes over the west atlantic).

Hi Folks ,
Cheers for that much appericated .
S&F

Interesting info there from the Cabotage Kid, my own observation supports his argument.

Last year on a flight from Zurich to Bangkok in a SWISS MD11 the groundspeed was 1111 kph for over 2 hours after reaching cruising altitude out of Zurich.

This week, returning from Jo'burg to Kuala Lumpur in a Malaysian 747 the groundspeed was 1076 kph after reaching cruising altitude of 33000 feet, that lasted about 2 hours, from Maputo (Mozambique) to far side of Madagascar.

Later in the flight the groundspeed was 858 kph at 37000 feet, for the last 2 hours into KL, this was in a band approx 3 degree either side of the equator.

Please excuse the anorak response, I am a lifetime aviation nerd including 35 years as a ppl.