Thursday, 20 October 2005 07:43
Hi Capt Lim,
I have a bet with my boss about travel speed by the Great Circle versus catching a push from the jet stream. We both travel a lot from LAX (Los Angeles) to China, Taiwan, Japan, etc; I told him that he would come back slightly faster (Eastbound) due to the jet stream. He disagrees and believes that the speed of jet stream versus the Great Circle is over rated - that you save more time by going the Great Circle route rather than the jet stream. Then I ask him: why an airline leaving LAX for TYO (Tokyo) via the Great Circle still take a longer flight time than TYO back to LAX via the jet stream?
Can you please settle this argument once and for all? Let say, two airplanes leaving from TYO for LAX at the same time; one takes the Great Circle route over the Poles, the other hitches a ride on the jet stream (I*m not even sure this is a valid situation, but lets say the tail wind is at a constant 200 mph), which one will reach LAX first? And does this prove anything?
Thanks in advance for your time and effort in settling a dispute.
First, you have to understand that the direction of a jet stream may not necessary be along a Great Circle track of your intended route. In a real situation, it is never possible to get a constant 200 mph tail wind all the way. Hence you may enjoy the push for only about half the time or even less unless you are very lucky to find one on a particular day as its direction changes from time to time. But overall, you will get an average tail wind of between 60 to 80 mph, depending on the time of the year and direction of your flight.
Your scenario: two planes leaving Tokyo for Los Angeles - one along the Great Circle and the other one following a jet stream. Which one will arrive first?
It is difficult to say off hand now but it is possible to know the result (without even flying) when all the data for a particular day are fed into the computer. Remember, most flights are generally planned along Great Circle routes anyway as it is the shortest distance between two points on the Earth.
From the plane*s computer, I have extracted the distance from Tokyo to Los Angeles as 4,736 nautical miles and any plane flying along this route is already enjoying the tail wind effect. On the other hand, the other plane trying to take advantage of the 200 mph jet stream may end up somewhere else or probably have to fly more than 5000 miles!
What is certain is that, a flight from Tokyo to Los Angeles is always faster than the return journey as the computerized flight planning will make sure that it takes advantage of both - the Great Circle track as well as the jet stream. In this sense, your boss may be right because, he is also enjoying the push as he flies along the great circle track. You are also right because any Eastbound flight have the benefit of the Westerly flow jet stream pushing the plane along - not necessary at 200 mph though!
Wish you a strong tailwind on your next flight!