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Intercontinental Navigation  
User currently offlineturk0167 From United States of America, joined Jul 2006, 12 posts, RR: 0
Posted (2 years 2 months 2 days 16 hours ago) and read 4222 times:

I just returned from Europe on a LHR-SFO flight. Of course, like most northern hemisphere flights of that length, the flight path initially heads northernly towards the Arctic circle and then goes southernly towards the destination.

The thing I'm curious about is since the flight path looks like a big arc, are they just making a slow 10 hour left turn? Does the pilot have to change the heading by 1 or 2 degrees every 10 minutes, or is the path pre-programmed in the computer for the auto-pilot to follow? Also, since the heading is changing so often, do they have to talk to atc about every little deviation?

I'm very curious about this. Any insight would be appreciated. Thank you.

14 replies: All unread, jump to last
 
User currently offlinenomadd22 From United States of America, joined Feb 2008, 1896 posts, RR: 0
Reply 1, posted (2 years 2 months 2 days 16 hours ago) and read 4204 times:

Just picture the extreme case in your head. If you flew half way around the world in the northern hemisphere the shortest route would be over the north pole, so that route on a flat map would look extremely curved. Flights further south won't look as extreme on a flat map, but the only place where the line would look perfectly straight would be if you flew along the equator.
You can see it by looking at your route on a globe. If you fly eat to west along the shortest line, the middle of the route will always be further north than the end points, so the line on a flat map will look curved.

[Edited 2012-09-25 09:02:54]


Andy Goetsch
User currently offlineTheCol From Canada, joined Jan 2007, 2039 posts, RR: 6
Reply 2, posted (2 years 2 months 2 days 14 hours ago) and read 4060 times:

Quoting turk0167 (Thread starter):
Does the pilot have to change the heading by 1 or 2 degrees every 10 minutes, or is the path pre-programmed in the computer for the auto-pilot to follow?

No, modern aircraft are equipped with at least one FMS, though most have dual FMS or a number of additional back-ups, which have company routes pre-programmed into the database. The flight crew can also enter and modify routes into the database as agreed on the flight release or for diversionary purposes. Here's an article which covers the FMS in a little more detail:

http://en.wikipedia.org/wiki/Flight_management_system

Quoting turk0167 (Thread starter):
Also, since the heading is changing so often, do they have to talk to atc about every little deviation?

Generally no, since aircraft status and position reports are automatically sent to the company and ACC.



No matter how random things may appear, there's always a plan.
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3308 posts, RR: 13
Reply 3, posted (2 years 2 months 2 days 14 hours ago) and read 4040 times:
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They are flying a [basically] straight route. Take a string and connect London and San Francisco on a globe with the shortest amount of string possible. Your string is straight, but laying it flat on the globe you'll see that it adopts what would be a curve on a flattened map. It's called the "Great Circle" distance and is just a product of two-dimensional representations and printing.

TIS



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User currently offlineCOEWR787 From United States of America, joined Mar 2005, 338 posts, RR: 3
Reply 4, posted (2 years 2 months 2 days 14 hours ago) and read 4012 times:

They may or may not fly the great circle route depending on wind conditions and geopolitical considerations.

For example the Newark - Delhi flight tends to fly south of the great circle route to Afghanistan and then follows a path with several turns to cross Afghanistan and Pakistan into India. On the way back same somewhat more curvy route to Afghanistan and and then typically fly a little north of the great circle route to avoid flying straight into the jet stream. Sometimes they go pretty far north by the Svalbards, whereas the great circle route is further south crosing into/out of Europe somewhere near Bodo Norway.

The most extreme case that I have come across is the Newark Singapore non-stop, which on occasions flies the southern route through India, and at other times flies the great circle route, or closer to it over the top. All depends on how the wind blows.


User currently offlineaviatorcraig From United Kingdom, joined Mar 2010, 245 posts, RR: 0
Reply 5, posted (2 years 2 months 2 days 13 hours ago) and read 3928 times:

The aircraft is flying a straight path (other than minor deviations for oceanic track to get the best winds etc). It is the map that is distorted.

The earth is a globe. in order to illustrate the world in two dimensions, i.e. in a book (atlas) you have to make some big distortions (projections in map makers speak). This is why you think you are flying in an arc, but you are not. Also, it is not to stay closer to land (I've heard that one before as well!). It is because it is the shortest route.

There is a great website that covers all of this - Great Circle Mapper. Have a look at the two following URLs, the first one shows the earth as an atlas type projection and the second one as a globe with the North Pole facing you - this should answer it for you:

http://www.gcmap.com/mapui?P=lhr-sfo&MS=wls&DU=mi

http://www.gcmap.com/mapui?P=lhr-sfo&MS=wls&MP=p&DU=mi



707 727 Caravelle Comet Concorde Dash-7 DC-9 DC-10 One-Eleven Trident Tristar Tu-134 VC-10 Viscount plus boring stuff!
User currently offlineturk0167 From United States of America, joined Jul 2006, 12 posts, RR: 0
Reply 6, posted (2 years 2 months 2 days 12 hours ago) and read 3907 times:

I understand what you guys are saying about the fligh path generally being a straight line, and a I appreciate your responses. However, according to flightaware, when the plane got established on it's course (UA955), it was heading @ 334, and as it approached SFO it's heading was @ 196. The FMS will thinks it's making a big left turn. So, to maintain the "straight line" the plane would need to constantly change it's indicated heading, right? To go to SFO without changing it's indicated heading, it seems it would need to fly @ 255.

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 7, posted (2 years 2 months 2 days 12 hours ago) and read 3891 times:

As you noticed, to fly a great circle route the heading will need to change. The FMS calculates this and steers the aircraft automatically. The aircraft won't continually turn left, it makes frequent course adjustments to stay on the route.

A route flown on a constant heading would resemble a flat map straight line route.

Google Earth is a good way to visualise this. Ensure the grid is enabled in the view menu and use the ruler to set the start and end points of the route. You can rotate the earth and look at the ruler line (which is a great circle) from any angle.

[Edited 2012-09-25 13:56:56]


The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFlighty From United States of America, joined Apr 2007, 8707 posts, RR: 3
Reply 8, posted (2 years 2 months 2 days 12 hours ago) and read 3891 times:

Quoting turk0167 (Reply 6):
it was heading @ 334, and as it approached SFO it's heading was @ 196. The FMS will thinks it's making a big left turn. So, to maintain the "straight line" the plane would need to constantly change it's indicated heading, right? To go to SFO without changing it's indicated heading, it seems it would need to fly @ 255.

Absolutely. The thing we can remember is the Earth is 24,000 miles around the Equator but only a arbitrary small point at the North Pole. A "straight heading" of 90 while located above the 89th parallel is not a straight path at all. The compass says you are going straight heading 90, but actually you are banking in a tight circle.

This suggests why an arc such as JFK-NRT is perfectly straight, but according to the compass, you are making a left turn. The compass is wrong; so it says you are turning left while you're actually not.

I'm thinking there are only four possible "straight" compass headings: 90, 180, 270 and 360. The other headings are curved. Edit: Now I am not sure anymore  Sad

[Edited 2012-09-25 13:43:19]

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2572 posts, RR: 25
Reply 9, posted (2 years 2 months 2 days 11 hours ago) and read 3855 times:

Quoting Flighty (Reply 8):
I'm thinking there are only four possible "straight" compass headings: 90, 180, 270 and 360. The other headings are curved. Edit: Now I am not sure anymore

Only if you use true heading to steer, so they aren't compass headings, strictly. Also, 90 and 270 can only be straight great circle headings on the equator.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineFlighty From United States of America, joined Apr 2007, 8707 posts, RR: 3
Reply 10, posted (2 years 2 months 2 days 11 hours ago) and read 3842 times:

Quoting Jetlagged (Reply 9):
Only if you use true heading to steer, so they aren't compass headings, strictly. Also, 90 and 270 can only be straight great circle headings on the equator.

Yeah, well done. That was the missing part.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 11, posted (2 years 2 months 2 days 5 hours ago) and read 3738 times:

Quoting ANITIX87 (Reply 3):
They are flying a [basically] straight route. Take a string and connect London and San Francisco
on a globe with the shortest amount of string possible.

I think the thing that people are missing is that the flight path is straight but the heading is constantly changing because the direction to the north pole at each point is changing, not because the aircraft is turning.

As you fly over the north atlantic (assuming you're on a great circle route, which you aren't really but it's usually close) you basically don't turn. But the heading is constantly changing "underneath you" because the relationship between your inertial vector in space and the direction to the north pole is changing.

Think about driving by some big landmark on a straight highway...you never turn, but your bearing to the landmark is constantly changing. The "straight highway" is your flight path. The "landmark" is the north pole. Since heading is, by definition, your bearing relative to the north pole, as you fly past the pole your heading changes even though you haven't turned at all.

The airplane knows which way it's pointed in inertial space and where it is on the globe...from that information, it knows what the heading is. That's the part that constantly updates...nothing to do with turning the airplane.

Tom.


User currently offlinesaafnav From South Africa, joined Mar 2010, 285 posts, RR: 1
Reply 12, posted (2 years 2 months 2 days 4 hours ago) and read 3721 times:

Quoting turk0167 (Reply 6):


I understand what you guys are saying about the fligh path generally being a straight line, and a I appreciate your responses. However, according to flightaware, when the plane got established on it's course (UA955), it was heading @ 334, and as it approached SFO it's heading was @ 196. The FMS will thinks it's making a big left turn. So, to maintain the "straight line" the plane would need to constantly change it's indicated heading, right? To go to SFO without changing it's indicated heading, it seems it would need to fly @ 255.

That would probably be Magnetic Headings. Due to local variations, this will constantly change.

Also, on a Lambert's Conical Conformal Map, a straight line drawn on the map will be a Great Circle Route. But, since the Meridians Curve, the heading from True North will change as you fly East or West.
On a Mercator Projection, a straight line drawn on the map will give you a Rhumb Line Track, but the line you have drawn will have constant declination from North.

As said before, you pretty much fly straight. Gauss proved in ca. 1825 that is impossible to project a sphere on a flat surface without distortions.
It is only different projections that show a Great Circle Track in different ways.
http://en.wikipedia.org/wiki/Map_projection

Erich



On-board Direction Consultant
User currently offlinemandala499 From Indonesia, joined Aug 2001, 6956 posts, RR: 76
Reply 13, posted (2 years 2 months 2 days 1 hour ago) and read 3669 times:

It's straight... here's a projection that'll show it's straight...



On this straight line, the heading changes...but the line is straight so you don't make turns. Let the compass swing around instead. Automation using FMCs will in LNAV (or Lateral Managed) steer the plane to a point, not follow a heading. The heading to waypoint and current heading, gets adjusted accordingly. ie: It calculates the current heading to that point to make that line... and then make adjustments for deviations from that line.



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlinePihero From France, joined Jan 2005, 4630 posts, RR: 77
Reply 14, posted (2 years 2 months 1 day 18 hours ago) and read 3559 times:
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This thread is, as a matter of fact only about maps, projections and, of course, representation of different trajectories.
The first remark I could make is that we are used to Mercator p^rojections on which parallels and meridians are straight lines : These maps are good for short -ish distances and they present as straight lines - intersecting the meridians and the parallels at the same constant angle - what we call "rhumb lines or loxodromes"; but these lines are not the shortest distances between two points on the surface of the earth... those are the "orthodromes or great circles"

Now, great circles are - please bear with me - the intersection of the terrestrial sphere with a plane which include the centre of the earth... therefore, if we use a projection that has the earth centre as its projecting point, all great circles will show as straight lines, and the most famous type of maps that are derived is called the "stereo polar" projection, in which one of the poles is the center of the representation : That's what Mandala499 has shown above.
So the best way to identify a stereographic projection is see that [b]all[/b) meridians are straight lines and when parallels are represented as circles, we are seeing a stereo-polar map.
That projection is very handy when flying long distances : one can make a direct measurement of the direct track at every point of the route.
But, as navigation becomes quite complicated in terms of variation in the value of the heading / track parameter. The solution is to "paste" over the map a grid aligned with one meridian... We have then our orthodrome cutting the elements of the grid at a constant angle... on which we align the directional gyro, taking advantage of its "stability" in inertial space.
We canh then fly a great circle route with a constant "grid" track angle.



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