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Flight Times + World Turning

Sat Jan 17, 2004 9:31 pm

Hello, I was wondering why on nearly all international flights (eg London - Singapore) on the way over might be 30-40 minutes shorter than on the way back? I was thinking that because of the world turning, and the plane flying opposite to that direction, that the time is shorter (the world is turning while the aircraft is flying, thus "bringing" the destination closer.)

Am I right, or slightly mad?

It has intrigued me for a long time, and I really need to know!

Thanks, Ash
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RE: Flight Times + World Turning

Sat Jan 17, 2004 10:00 pm

i think it might have something to do with the winds, but you might be right awell, since I once read that, e.g, throwing a ball westwards with the same force, will cause the ball to cover more ground distance compared to throwing it eastwards.

Now am I right or am I too slightly mad?

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RE: High Wing Vs. Low Wing Aircraft

Sun Jan 18, 2004 4:51 am

Pretty sure that it's all the winds. For example, flying from Boston to Los Angeles (~2000 nm) takes around 6 hours, while flying from Los Angeles to Boston takes a little over 5 hours, due to the high altitude jetstream. We're frequently battling close to 100 mile per hour winds heading west, and heading east, we're riding them. The same thing happens on the way to Europe, and from there to India, in my experience.
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RE: Flight Times + World Turning

Sun Jan 18, 2004 4:54 am

The reason is ONLY because the dominant winds are generally coming from the West, and this "pushes" aircraft when they fly towards the East, and slows them down when they fly towards the West... Average some 50 kph.
These winds are dominant in mid latitude areas. In regions closer to Polar areas, you encounter the "jet streams" which are even stronger, at 100 or even 200 kph, sometimes.
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RE: Flight Times + World Turning

Sun Jan 18, 2004 10:48 pm

the world is turning while the aircraft is flying, thus "bringing" the destination closer

This would be true only if your aircraft was out of the atmosphere, but like any other bird, your aircraft is "turning" with the world. Wind is the answer to your question.

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RE: Flight Times + World Turning

Mon Jan 19, 2004 9:31 pm

Well, what you have observed is actually an artefact of the Earth's rotation. This is combined with water and solar energy to create the weather patterns that we experience.

Think of the main weather driver as a series of giant convection systems that operate in belts around the Earth (six in total - three in the northern and three in the southern hemispheres). The inner most belts extending into the north and south of the tropics have the greatest influence, as they receive the highest solar energy.

As you know, hot air rises. This is what happens constantly around the tropics, whilst drawing air in from north and south of the tropics at surface level to replace the air that has risen. The air can only go so far up before it runs out of room in the atmosphere and has to deflect to the north and south of the tropic zone, eventually cooling and falling back to the surface of the Earth in the lower temperate zone to continue the convection cycle.

How does this create the prevailing winds I hear you ask?

Remember that the Earth is a sphere that rotates on an axis. Air rises at the equator at relatively the same speed as the Earth's surface. So if you were on the equator, the air would generally seem fairly still because it is mostly swirling and moving upwards towards the sky (unless you were in a tropical storm of course).

When this air deflects north and south after rising, it maintains a lot of the surface speed generated at the equator, making it relatively faster than the Earth’s surface the further north or south you go. Remember, the circumference of the Earth at the equator is larger than at any other longitude, so it has further to travel each revolution and thus is why it is faster relative to the rest of the Earth’s surface speed.

So, the further north or south the air from the tropics travels, the more it seemingly accelerates towards the East because the surface is moving slower than where the air was generated.

I hope this makes sense, because it is hard to explain without being able to draw diagrams. You may want to look up topics like “convection winds” and “coriolis force” to get a better understanding of why there is a previling westerly in the world's temperate zones.

Some pilots will use these winds to their advantage. For example, winter in Australia brings the very fast jetstream westerly winds further north. Airlines will pick up on these winds on PER-MEL and SYD/MEL-NZ flights (PER-MEL can be over an hour quicker than MEL-PER during these winds).

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RE: Flight Times + World Turning

Mon Jan 19, 2004 11:46 pm

These guys are right.
It is the prevailing winds.

BTW, the earth rotates toward the east. Therefore, if rotation was a factor eastbound flights would take longer, not less time. The destination would be retreating from you where westbound it would be racing to meet you.

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