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Flight Durations - A Question  
User currently offlineCxsjr From , joined Dec 1969, posts, RR:
Posted (8 years 4 months 2 days 1 hour ago) and read 4723 times:

Hi folks,

Later this year, I'm flying from DXB - KIX with Emirates (A340-500) which, according to Great Circle Mapper is 4717 miles. The flight duration is shown as 8h50m.

The return journey is shown as 11h40m.

Whilst I know there is generally a differential between East to West and vice versa (flying West generally takes longer due to gulfstreams/jetstreams?) this seems to be a much bigger difference than any I've known.

Does anyone have any explanations as to why this is the case? (BTW, both are shown as non-stop flights).

25 replies: All unread, showing first 25:
 
User currently offlineSkyslave From United States of America, joined Oct 2005, 44 posts, RR: 0
Reply 1, posted (8 years 4 months 1 day 19 hours ago) and read 4668 times:

Quoting Cxsjr (Thread starter):
Whilst I know there is generally a differential between East to West and vice versa (flying West generally takes longer due to gulfstreams/jetstreams?) this seems to be a much bigger difference than any I've known.

Does anyone have any explanations as to why this is the case? (BTW, both are shown as non-stop flights).

Heh, its the jetstream (gulfstream is a business jet). To answer your question, I think fighting the wind is the only reason. My dad pilots a 747 from detroit direct to Narita. Going there takes significantly longer than comming back.


User currently offlineCxsjr From , joined Dec 1969, posts, RR:
Reply 2, posted (8 years 4 months 1 day 19 hours ago) and read 4663 times:

Quoting Skyslave (Reply 1):
Going there takes significantly longer than comming back.

I guessed this was the case but the differential from UK to West Coast US and vice-versa is only about an hour so nearly three hours seems excessive?

Thanks for your post!


User currently offlineJamesbuk From United Kingdom, joined May 2005, 3968 posts, RR: 4
Reply 3, posted (8 years 4 months 1 day 18 hours ago) and read 4644 times:

Also one way you are going the opposite way of rotation of the earth so this would be shorter as the destination is moving closer as are you and one way you are almost chasing your destination.

Rgds --James--

P.S. Hope that made sense



You cant have your cake and eat it... What the hells the point in having it then!!!
User currently offlineJoness0154 From United States of America, joined Nov 2005, 667 posts, RR: 0
Reply 4, posted (8 years 4 months 1 day 9 hours ago) and read 4599 times:

Quoting Jamesbuk (Reply 3):
Also one way you are going the opposite way of rotation of the earth so this would be shorter as the destination is moving closer as are you and one way you are almost chasing your destination.

I don't believe this is true, as the atmosphere moves in relation to the earth also. The earth has a rotational speed of a little more than 1000mph at the surface, so that theory doesn't work at all.

Thats like saying if you jump straight up in the air, you'll land in a different spot than where you jumped.



I don't have an attitude problem. You have a perception problem
User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 5, posted (8 years 4 months 1 day 8 hours ago) and read 4590 times:

Quoting Joness0154 (Reply 4):
I don't believe this is true, as the atmosphere moves in relation to the earth also. The earth has a rotational speed of a little more than 1000mph at the surface, so that theory doesn't work at all.

Thats like saying if you jump straight up in the air, you'll land in a different spot than where you jumped.

http://en.wikipedia.org/wiki/Coriolis_force
Its very real.


User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 6, posted (8 years 4 months 1 day 8 hours ago) and read 4588 times:

Quoting Jamesbuk (Reply 3):
Also one way you are going the opposite way of rotation of the earth so this would be shorter as the destination is moving closer as are you and one way you are almost chasing your destination.

Rgds --James--

P.S. Hope that made sense

Thats true, but your explaination is off. You're fighting the prevailling winds, that is why the journey takes longer. The winds are a result of the uneven heating of the earth and its rotation.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17015 posts, RR: 67
Reply 7, posted (8 years 4 months 1 day 1 hour ago) and read 4540 times:

Remember that average hold and taxi times are included in the timetables.


"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAndz From South Africa, joined Feb 2004, 8450 posts, RR: 10
Reply 8, posted (8 years 4 months 22 hours ago) and read 4500 times:
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I went to Australia a few years ago and the route had similar differences. From the SAA timetable:

JNB-SYD: 10h45
SYD-JNB: 14h05



After Monday and Tuesday even the calendar says WTF...
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (8 years 4 months 21 hours ago) and read 4477 times:

Quoting Seanp11 (Reply 5):
http://en.wikipedia.org/wiki/Coriolis_force
Its very real.

Could you steer us to the confirmation of this statement on that site. The closest thing I could find on there was a mere mention of coriolis effects in ballistics. (specifically regarding the "Paris gun")

I'm curious because if it is true then all pilots, aircraft dispatchers and flight planners for the past 103 years have been wrong as there is absolutely no factor for this in the specific steps relating to flight planning.

edit: I'm also a little curious why this doesn't produce a 689-knot wind where I live.

[Edited 2006-04-23 19:49:04]


Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineMatt72033 From United Kingdom, joined May 2005, 1617 posts, RR: 4
Reply 10, posted (8 years 4 months 20 hours ago) and read 4462 times:

Quoting SlamClick (Reply 9):
I'm curious because if it is true then all pilots, aircraft dispatchers and flight planners for the past 103 years have been wrong as there is absolutely no factor for this in the specific steps relating to flight planning.

a helicopter in a hover would also be interesting  Wink


User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 11, posted (8 years 4 months 20 hours ago) and read 4462 times:

Quoting SlamClick (Reply 9):

Could you steer us to the confirmation of this statement on that site. The closest thing I could find on there was a mere mention of coriolis effects in ballistics. (specifically regarding the "Paris gun")

I'm curious because if it is true then all pilots, aircraft dispatchers and flight planners for the past 103 years have been wrong as there is absolutely no factor for this in the specific steps relating to flight planning.

edit: I'm also a little curious why this doesn't produce a 689-knot wind where I live.

Well, the absolute result of the coriolis effect is the prevailing winds and the jetstreams, so it has been taken into effect for as long as there has been aviation.

I do think, if you are flying (for example) from the equator to the polar circle, the corilois effect will come into play, however, if you are flying a polar route from say, EWR to HKG, the overall difference between rotational velocities at the origin and destination isn't too great, so I am not sure if the corilois effect does come into play or not.


User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
Reply 12, posted (8 years 4 months 20 hours ago) and read 4458 times:

Quoting Cxsjr (Thread starter):
Later this year, I'm flying from DXB - KIX with Emirates (A340-500) which, according to Great Circle Mapper is 4717 miles. The flight duration is shown as 8h50m.

DXB-KIX is 4536 nm, return is 4511 nm along the prescribed airways.

The reason for the time difference is that generally there is a westerly component which help the flight out, and will hinder the flight back.

The reason for the westerly winds are predominately due to the sub tropical jetstream (STJ), this jetstream varies depending on the time of year.

Early in the year, January the STJ will lie around 25 deg N over Africa with a strength of 100 kt and crossing Japan around 40 deg N at 240 kt, as the sun is in the southern hemisphere (around 20 deg south) at that time the strong temperature gradient leads to high winds. About 6 months later the sun is back up in the north hemisphere around 20 deg north, the STJ move slightly further north and wind strength is much lower as the temperature gradient is lower with the sun in the northern hemisphere, around 30-40 kt from Africa to Japan.

So the flight time should not only vary with the direction of travel, also with the time of year.

Quoting Jamesbuk (Reply 3):
Also one way you are going the opposite way of rotation of the earth so this would be shorter as the destination is moving closer as are you and one way you are almost chasing your destination.

This is incorrect in the internal frame of reference.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 13, posted (8 years 4 months 20 hours ago) and read 4451 times:

Quoting Seanp11 (Reply 11):
so it has been taken into effect for as long as there has been aviation.

No, this is completely different:

Quoting Jamesbuk (Reply 3):
you are going the opposite way of rotation of the earth so this would be shorter as the destination is moving closer as are you and one way you are almost chasing your destination.

...and seconded by this post:

Quoting Seanp11 (Reply 5):
Quoting Joness0154 (Reply 4):
I don't believe this is true, as the atmosphere moves in relation to the earth also. The earth has a rotational speed of a little more than 1000mph at the surface, so that theory doesn't work at all.

Thats like saying if you jump straight up in the air, you'll land in a different spot than where you jumped.

http://en.wikipedia.org/wiki/Coriolis_force
Its very real.

This is a statement that if you jump off the surface of the earth you will land in a different place because of its rotation.

I don't believe it and I am asking to be proven wrong.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
Reply 14, posted (8 years 4 months 20 hours ago) and read 4447 times:

Quoting SlamClick (Reply 13):
This is a statement that if you jump off the surface of the earth you will land in a different place because of its rotation.

I don't believe it and I am asking to be proven wrong.

You are correct, earths rotation is irrelevant, otherwise you would have a ground speed of close to 1500 kts eastbound, and -500 kts westbound.

Technically a very small time dilation occurs over long flights due to relativity, the time change is so small that it is also irrelevant.



We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 15, posted (8 years 4 months 19 hours ago) and read 4426 times:

Quoting SlamClick (Reply 13):

This is a statement that if you jump off the surface of the earth you will land in a different place because of its rotation.

I don't believe it and I am asking to be proven wrong.

Yeah, you are entirely correct. If you jump into the air, you will land in the same place, because you are still moving at the same speed relative to the earth.


User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 16, posted (8 years 4 months 19 hours ago) and read 4426 times:

Quoting Zeke (Reply 14):
You are correct, earths rotation is irrelevant, otherwise you would have a ground speed of close to 1500 kts eastbound, and -500 kts westbound.

Mind you the earth's rotation is irrelevant in a simple east-west direction, but if you head north, the earth's angular velocity will drop relative to you.


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 17, posted (8 years 4 months 17 hours ago) and read 4393 times:

One poster got it right: Inertial frame of reference.

Quoting Seanp11 (Reply 16):
if you head north, the earth's angular velocity will drop relative to you.

Not angular velocity. If any two points (besides the true poles) on the Earth moved at a different angular velocity, they wouldn't make a revolution every 24 hours, and things would be very different.

Before you take off, you're already moving with the Earth. To simplify things, assume you take off to the true North, and land to the true North, and your destination is due North of your departure point, and there are no winds.

When you take off, you're moving with a certain angular velocity; it happens to be one revolution in 24 hours. The destination runway also goes around at one revolution every 24 hours. Mr. Newton tells us that if we're already moving, and we don't do anything to change that movement, we're going to keep on moving. So we land with the same angular velocity we had when we took off; namely, one revolution every 24 hours. This requires the basic assumptions we made earlier, but it is possible to extend more realistic conditions to the problem and see it would remain the same.

The tangential velocity does depend on your latitude. In fact, it's the product of angular velocity and radius from rotational axis. This is why it takes less fuel to launch a spacecraft closer to the equator. It only matters to spacecraft and not airliners though because even low-altitude spacecraft which enter an unpowered stationary orbit depart the inertial frame of reference of the Earth's surface, but airliners never do.



Position and hold
User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 18, posted (8 years 4 months 17 hours ago) and read 4386 times:

Quoting Bri2k1 (Reply 17):

Not angular velocity. If any two points (besides the true poles) on the Earth moved at a different angular velocity, they wouldn't make a revolution every 24 hours, and things would be very different.

Before you take off, you're already moving with the Earth. To simplify things, assume you take off to the true North, and land to the true North, and your destination is due North of your departure point, and there are no winds.

When you take off, you're moving with a certain angular velocity; it happens to be one revolution in 24 hours. The destination runway also goes around at one revolution every 24 hours. Mr. Newton tells us that if we're already moving, and we don't do anything to change that movement, we're going to keep on moving. So we land with the same angular velocity we had when we took off; namely, one revolution every 24 hours. This requires the basic assumptions we made earlier, but it is possible to extend more realistic conditions to the problem and see it would remain the same.

Ok, yeah, I got the tangential and angular velocity mixed up, its been a little while since I've done rotational mechanics.

Quoting Bri2k1 (Reply 17):
The tangential velocity does depend on your latitude. In fact, it's the product of angular velocity and radius from rotational axis. This is why it takes less fuel to launch a spacecraft closer to the equator. It only matters to spacecraft and not airliners though because even low-altitude spacecraft which enter an unpowered stationary orbit depart the inertial frame of reference of the Earth's surface, but airliners never do.

The atmosphere is not in the same frame of reference as the earth's surface. If it was, then naval gunners would not have to take the corilois effect into account, and we would not have the prevailing winds.


User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 19, posted (8 years 4 months 16 hours ago) and read 4358 times:

Quoting Seanp11 (Reply 18):
The atmosphere is not in the same frame of reference as the earth's surface.

After thinking this over, I think I should rephrase this to the atmosphere is not entirely in the same frame of reference as the surface of the earth. The earth's rotation is not 'enforced' on objects moving north in the atmosphere as objects firmly on the land.


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 20, posted (8 years 4 months 14 hours ago) and read 4329 times:

Inertial frames of reference do not inherently have fixed dimensions in space. They can be defined at will to discuss movements of objects with respect to other objects. Remember the formula for calculating gravitational force between two objects depends only on their mass and the distance between them. In broad terms, every atom in existence acts on every other atom, subject to the inverse square of their distance and product of their masses.

If you could look at a plane flying due North above the Earth's surface with respect to the Sun instead of the Earth, it would indeed have a curved path, due to its inertia when it took off. From the Earth, the same plane would appear to fly in a straight line. This is how inertial frames work, and why it's often much more convenient to solve a problem in one frame versus another. The plane's motion with respect to the Earth is much simpler than with respect to the Sun.

Given all that, the point I'm trying to make is that the Earth's atmosphere can or can not be in the same intertial frame as the Earth. When designing airfoils or jet engines, the relative wind is considered to be in the same intertial frame as the airplane, thus allowing us to calculate the flow through or around the plane. When planning time enroute, though, we define the frame with respect to the Earth but not the relative wind. This is so the forces due to wind will have an effect on the plane's position about the Earth. Otherwise, our arrival times would be accurate with respect to an air mass, but not with respect to fixed places on the Earth.

In meteorological terms, the Coriolis Effect is due to uneven heating of the Earth's surface and varying friction as warm air rises. I don't think it has anything to do with varying tangential velocities at varying latitudes.

Speaking in relative terms, cooler air is nearer the surface and is accelerated more due to friction with the surface. Air rises when it is warmed. Once it cools, it descends. Air at the same altitude is warmed more nearer the equator. This fully explains the Coriolis Acceleration on air masses. While it may be true that the acceleration due to warming is higher nearer the equator due to the higher tangential velocity, it is a negligibly small difference compared to the difference in heating between the equator and the poles. This is why there are two distinct bands of Coriolis Circles between the equator and each pole, and no more.



Position and hold
User currently offlineBond007 From United States of America, joined Mar 2005, 5408 posts, RR: 8
Reply 21, posted (8 years 4 months 14 hours ago) and read 4321 times:

This is great  Smile

...and I'm serious.

He only asked why the flight times were different Big grin

Quoting Skyslave (Reply 1):
Heh, its the jetstream (gulfstream is a business jet).

Well, he was closer than 'a business jet' !

The gulfstream is a (or some) warm ocean currents, created by similar processes to the jetstream.


Jimbo



I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 22, posted (8 years 4 months 13 hours ago) and read 4310 times:

Quoting Bri2k1 (Reply 20):
Given all that, the point I'm trying to make is that the Earth's atmosphere can or can not be in the same intertial frame as the Earth.

Well, glad to see we can agree on that.

Quoting Bri2k1 (Reply 20):
When planning time enroute, though, we define the frame with respect to the Earth but not the relative wind. This is so the forces due to wind will have an effect on the plane's position about the Earth. Otherwise, our arrival times would be accurate with respect to an air mass, but not with respect to fixed places on the Earth.

Yeah, what I am saying though is there is deviation from the coriolis effect that is probably unwittingly accounted for by the pilot when heading north or south.

Quoting Bri2k1 (Reply 20):

In meteorological terms, the Coriolis Effect is due to uneven heating of the Earth's surface and varying friction as warm air rises. I don't think it has anything to do with varying tangential velocities at varying latitudes.

I think you should re-read the link I posted http://en.wikipedia.org/wiki/Coriolis_force
It has everything to do with the rotation of the earth. If it were not for the corilois effect, there would be two convection cells on the earth, one for the northern hemisphere and one for the southern hemisphere.

Quoting Bri2k1 (Reply 20):
This is why there are two distinct bands of Coriolis Circles between the equator and each pole, and no more.

There's three: the hadley, the ferrel and the polar cells.


User currently offlineSeanp11 From United States of America, joined Jan 2006, 290 posts, RR: 0
Reply 23, posted (8 years 4 months 13 hours ago) and read 4309 times:

Quoting Bond007 (Reply 21):
This is great Smile

...and I'm serious.

He only asked why the flight times were different Big grin

And it all happened because I misread what somebody said.  Wink

Yay finals.  Wink


User currently offlineZeke From Hong Kong, joined Dec 2006, 8996 posts, RR: 75
Reply 24, posted (8 years 4 months 11 hours ago) and read 4291 times:

Quoting Seanp11 (Reply 22):
Yeah, what I am saying though is there is deviation from the coriolis effect that is probably unwittingly accounted for by the pilot when heading north or south.

No not at all.

Quoting Seanp11 (Reply 22):
I think you should re-read the link I posted http://en.wikipedia.org/wiki/Coriolis_force
It has everything to do with the rotation of the earth. If it were not for the corilois effect, there would be two convection cells on the earth, one for the northern hemisphere and one for the southern hemisphere.

That is incorrect, the reason for the three cells (polar, ferrel, and hadley) is in the northern and southern hemisphere is to do with temperature gradients, the movement of them is to with the angle of rotation of earth (which give sus seasons). The rotation of earth is a factor, however not in the way you think, it has more to do with the geostrophic winds, pressure gradients, and surface friction.

Unfortunately, this depth of meteorology is often not taught well to people in industry, and instructors often rely on "Coriolis force" to explain the concept. The FAA required level of knowledge in this area is minimal, JAR its a lot more extensive. I fond FAA trained people walk out with the correct concept, however they are also walking away with the incorrect technical reasons and knowledge gaps. In depth knowedge of this is not important for pilots, the general concepts and general expectations is all that is required.

The movement of the cells are seasonal, in particular the ITCZ (where the two hadley cells meet) will move to the northern and southern hemisphere following the sun, and extending normally further north or south over land masses.




We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
User currently offlineScarletHarlot From Canada, joined Jul 2003, 4673 posts, RR: 56
Reply 25, posted (8 years 4 months 9 hours ago) and read 4275 times:

Hm.

Username: seanp11
Age: 16-20
Occupation: Geology and Planetary Science student


or...

Username: SlamClick
Age: 56-65
Occupation: Retired airline pilot
(It no longer says that on his profile, but we all know this)

Username: Zeke
Age: 36-45
Occupation: Airline Pilot


I wonder who I should believe on this issue?  scratchchin 



But that was when I ruled the world
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