Aviation

[happy_face]

How come when you take a return flight somewhere that not both journeys take the same amount of time? Take for example when I go to Denmark it takes about 1hr and 30min to get there but coming back it's always about 1hr and 50.

Does the wind affect it? and if so how come the flight time doesn't change as the wind changes ?

Also does plane fly in different altitudes. What I mean is would my plane going to Denmark be flying higher/lower than the one coming back ? Or is it a different flight plan that makes the journey longer coming home ?

Thanks

[kempie]

Long distance eastbound flights are normally quicker than westbound due to the effect of the jet stream.

[mikeyp]

Hmm, does anyone know whether the movement of the earth has any effect? I mean, if you went up in a straight line, then straight down again, has the earth moved under you?

[pjm007]

A strong head-wind could make a difference.

[brachiopod]

There are a number of factors that affect flight times.

As you mention, winds will affect aircraft flight.

A tail wind assists the aircraft's flight, and may shorten the flight time, or, where an aircraft has a definite landing slot (usually on shorter haul flights), it allows the pilot to throttle back, arriving at the same time, but burning less fuel in the process.

Conversely, a head wind will 'slow' an aircraft down, (relative to the ground), and the pilot will use more fuel by throttling up to increase the air speed in order to arrive on time.

It can be very noticeable on trans-Atlantic flights where the prevailing winds are roughly West to East.

(I believe East-bound flights are often able to fly with the assistance of the jet streams and that the air corridors ('air lanes') are adjusted to enable this. Can anyone confirm this?)

The other thing that can cause increased flight times is if one of your legs (outbound or return) is flying into a busy airport - when you could be in a circling stack for 45 minutes !!

[brachiopod]

Whoops, some cross posting there! There were no answers when I started replying!

mikeyp, the Earth movement has no effect on an aircraft, as the atmosphere is rotating with the Earth at effectively the same rate. 

[Clanad]

As kempie states, winds aloft are usually moving northwest to southeast in the northern hemisphere.  Keep in mind that most jet aircraft operate at 30,000 to 35,000 feet above sea level.  Many newer generation aircraft operate up to 45,000 feet ASL.  At those altitudes the winds can easily be in excess of 100 knots per hour.  I've experienced  nearly 200 kph. Picture a boat on a river whose flow of water is 10 miles per hour in speed.  Two boats with the same engine set at the same power will have different speeds across the river bed; the one going down stream adds the 10 mph flow and the one going upstream subtracts the 10 mph flow. The resultant in aircraft is known as ground speed.  And, yes, aircraft going eastbound are normally assigned odd altitudes (25,000, 27,000, etc.) while those west bound are set on even altitudes 26,000, 28,000, etc.) Until very recently the aircraft had to be separated by 2,000 feet vertically for traffic purposes, but now appropriately equipped aircraft are separted by only 1,000 feet.

Finally, the routes can vary between the same to origin/destination cities.  It may be slightly different going than coming, resulting in different distances...

[Clanad]

Wow, in the 10 minutes it took to construct my reply several more answers arrived... sorry for the duplication... for brachiopod, power setting are generally the same regardless of the direction of flight.  The engine manufacturer, at the time of the certification of the aircraft determines the most fuel/power efficient settings for given altitude/temperature combinations.  The pilots (and the passengers) just have to suffer the degraded ground speed...

[Clanad]

Finally, in answer to mikeyp's question... yes the earth would move under you.  Keep in mind the earth's rotation at the equator is near 1,000 mph, while near the poles it is a few tens of mph.  This is part and parcel of the ability of certain synchronous orbit satellites to remain in one place relative to a person on the ground.  they orbit at the same speed as the earth.  While non-synchronous satellites move either slower or faster than the earth.  This is a greatly simplified explanation however.  The rotation of the earth is partly responsible for the winds, either aloft or near the surface and is termed corilois affect...

[Clanad]

Bad keyboard... that's Coriolis Affect...

[cheyenne]

The plane's loaded with CARLSBERG.And they dont WANT IT TO LEAVE!!

[dgrey83]

Here is a manual that will tell you all you want to know about why aircraft do what they do.  It is the Air Traffic Control Handbook.  This manual is for Air Traffic Controllers.  You can find out almost anything.  If you ever listen to the pilots and controllers talking when you fly, and wondered what they are talking about, this book will tell you. 

http://www.faa.gov/ATpubs/ATC/INDEX.HTM

If you want more information about aircraft and/or air traffic controllers go to:

www.faa.gov

[SteveD]

Clanad, "knots per hour"!!?? Tut, tut!

[silly moo]

FLIPPANT ANSWER

Look at a map and see if you are flying up or down hill.

[Clanad]

Steve D, I'm scratching my head and wondering about the intent of your entry. Sometime in the early 1970's in an attempt to standardize speed references, airpseed indicators were changed from miles per hour to knots per hour.  Winds aloft have always been measured in nautical miles per hour. I suppose the absolutely correct term would be nautical miles per hour, however, the standard of the aviation industry for decades has been knots per hour as the reference... sometimes shortened to just knots.  1 nautical mile is equal to 1.15 statute miles by the way...

[SteveD]

Clanad, the definition of "knot" in my Oxford dictionary is "(naut).  Unit of speed of one nautical mile per hour."

Consequently, it is surely incorrect to say "knots per hour" as this would be the equivalent of saying "nautical miles per hour per hour".

I will bow to your superior knowledge as I believe you are a pilot.

[brachiopod]

"knots per hour" would be an acceleration;

ie; nautical miles per hour�

;o)

[Clanad]

For what it's worth...

Using the definition of a nautical mile for distance at sea, the challenge was to measure speed -- i.e. what is the ship's speed in nautical miles per hour?  Since [speed] = [distance] divided by [time], if we measure a small distance (or length) in a small time we can do the math and figure our speed.

The device that sailors used to make their speed measurement was called the "chip log". Chip as in chip of wood, and log as in to record in a log. The chip was a wedge of wood about 18" in size; it was tied to one end of a rope on a large spool. The rope had knots tied into it about every 47'3".

The wooden chip was thrown overboard at the ship's stearn (back end). Because of its wedge shape, it would "grab" the water and start pulling out rope as the ship moved forward at some yet unknown speed. One man would hold the spool of rope as it played out; another man would start a sandglass filled with 30 seconds of sand; and a third man would count the knots as they passed over the stearn board. When the 30 seconds of sand expired, the time keeper would call out and the counting of knots would stop.

The faster the ship was sailing, more knots and a longer length of rope were played out. The number of knots in the rope that were counted in 30 seconds, then, was equal to the speed of the ship in nautical miles per hour. A "knot", therefore, is not a nautical mile, it is a nautical mile per hour. Thus 1 knot was equivalent to 1 nautical mile per hour; 5 knots were equivalent to 5 nautical miles per hour; etc. The similar sound of "knot" and "naut" is entirely coincidental...  As SteveD indicated, the use knots per hour is somewhat redundant but I felt appropriate for the audience... but brachiopod's usually correct assesment is in error this time...

[brachiopod]

It was actually meant as a joke, clanad, but since you come to mention it, would you care to point out my error?

Your phrase �Knots per hour�
= nautical miles per hour per hour
= nautical miles per hour squared = an acceleration.

By the same token, the SI unit for acceleration is the metre per second squared (ms -� ), or �metre per second per second�.

[Clanad]

Brachiopod, As usual, you are precisely correct.  Forgive my lack of recognition of the  humor aspect.  No excuse, just a focus that should have been broader... Thanx for your response!