Relative Velocity

Those of us who fly out of NZ reasonably frequently will have noticed how flight times differ significantly depending on whether one is travelling west to east or east to west. Take my recent flight to and from Perth, in Western Australia.  The Auckland to Perth flight was timetabled to last 7 hours 25 minutes, while the return flight was timetabled for just 6 hours 10 minutes – 17% less time than the outward flight. The actual time in the air is (usually) less than this, with the timetabling including all the stuff that happens on the ground when the doors have been closed. (What are they doing in this time, I sometimes wonder…)

Now, for my recent flights, the flight times were very telling. The outward flight time (the time in the air) was actually 7 hours 30 minutes –  longer than the full timetabled travel time, while the return journey just 5 hours 30 minutes. The reason was the winds. My on-seat screen gave me some information about our progress, including actual (ground) speed and wind speed.  For the last part of the outward journey we were having really strong headwinds – up to about 250 km/h.  Now,  a Boeing 787-9 cruises at about 900 km/h, meaning that in a 250 km/h headwind, it is actually covering ground only at 650 km/h. Conversely, with a 250 km/h tailwind (for the first part of the return journey), it covers ground at 1150 km/h. Hence the sizeable difference in flight times.

These are jet stream winds – high altitude – (in)conveniently at about the height that commerical airliners should be, that circle the earth west-to-east-ish. I say ‘ish’ because they do meander about a lot.  There is a lot of variation as to exactly where they are and in which direction they go in, but they are used by airlines to reduce flight times and fuel costs where possible. They are also responsible for taking volcanic ash all the way around the globe, sometimes more than once.

Unfortunately, for my flight out, there was no way of routing around the jet stream. without missing Perth altogether. We just had to tackle it, causing some degree of frustration as I monitored our ever-slowing progress across the Southern Ocean and then Western Australia.




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