I'm working my way through a textbook on trigonometry, a subject I've never studied, but find fascinating due to all of it's applications. In the text it shows two ways of expressing bearings. One method is when a single angle is given, it is understood that bearing is measured in a clockwise direction from north. In the other method, it states that you start with a north-south line and use an acute angle to show the direction, either east or west, from this line.
So, how do aircraft navigate? Is it using a north-south line oriented to the longitude of Earth, or is it referenced off of the nose of the aircraft? Say, for example, when air traffic control says "turn to 240", how is that computed on board the aircraft?
Gregory

Always measured off of north. Very rarely youll hear “callsign, turn 10 deg left” usually for traffic separation, but it happens. But 99% of the time they tell you what heading to turn to, and if critical, which way to turn

Imagine the CF it would be if every single turn command was in incremental vs absolute

r6russian wrote:

Always measured off of north. Very rarely youll hear “callsign, turn 10 deg left” usually for traffic separation, but it happens. But 99% of the time they tell you what heading to turn to, and if critical, which way to turn

Imagine the CF it would be if every single turn command was in incremental vs absolute

I hear "turn 10/20 degrees left" all the time, especially being vectored to final on an approach. It gets you right where the controller wants, instead of them having to turn you again quickly because of wind shifts.

360 is North, 270 West, 180 South, 090 East. Everything in between. Usually you get a 0 at the end. Sometimes you get a 5. It is supposed to be the magnetic heading, but most ATC are primarily looking at ground track.

All ATC vectors or heading issued in the U.S., will be magnetic. The phrase used above of "turn 10 degrees left" can be used for traffic separation as mentioned however, it is used mostly when the controller does not know what your exact heading is and at times will be followed with "say new heading" so then they know exactly what your heading will be going forward. Terminal controllers have a fairly good idea of how the wind will impact the heading given so we correct if if need by, some by using the "turn 10 degrees right/left". Last thing ATC wants is to tell the pilot to turn right heading 240 and they are already heading 250!! Most professional pilots will question that instruction to verify the turn direction as a 350 degree turn in a congested airspace the wrong way will create a mess for all involved.

When I was controlling and didn't know the exact heading, but knew what the heading was I wanted the pilot to fly I'd simply say "fly heading 240" and the pilots make the shortest turn either left or right to get to that heading. Again, professional pilots as I've seen some low time private and student pilots make the big turn in the wrong direction so as a controller you have to know your audience so to speak.

With vectoring on downwind legs descending from say 10,000' to 3,000' the wind might have an impact as the aircraft descends and might not being on the desired track you want. With FMC's and GPS you might tell the pilot to "track 085" which keeps them on the same path until you're ready to turn them on a base leg, though not too many controller do that, but it has been heard and is a very effective tool rather than changing headings when you're very busy. For another conversation another time.

N1120A wrote:

I hear "turn 10/20 degrees left" all the time, especially being vectored to final on an approach. It gets you right where the controller wants, instead of them having to turn you again quickly because of wind shifts.

360 is North, 270 West, 180 South, 090 East. Everything in between. Usually you get a 0 at the end. Sometimes you get a 5. It is supposed to be the magnetic heading, but most ATC are primarily looking at ground track.

Ahhh man, you beat me to that stuff!!

As mentioned, navigation is referenced to magnetic north (true north in some particular areas near the magnetic north pole). Relative Bearings are used for traffic awareness sometimes, but expressed in "clock" terms. E.g. "traffic at your two o'clock, same level".

r6russian wrote:

Always measured off of north. Very rarely youll hear “callsign, turn 10 deg left” usually for traffic separation, but it happens. But 99% of the time they tell you what heading to turn to, and if critical, which way to turn

Imagine the CF it would be if every single turn command was in incremental vs absolute

Headings are almost always magnetic, not true. Only in areas near the magnetic north pole will true headings be used.

"Turn 10 degrees left" and such is quite common, for traffic, or as clearance for requested weather avoidance.

Starlionblue wrote:

Headings are almost always magnetic, not true. Only in areas near the magnetic north pole will true headings be used.
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The stupid way I remembered this in private pilot ground school eons ago was "the heading ATC is telling me isn't true". Worked like a charm to remember if they used true or magnetic heading come exam time!

Snuffaluffagus wrote:

Starlionblue wrote:

Headings are almost always magnetic, not true. Only in areas near the magnetic north pole will true headings be used.
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Hopefully that is about to change. NavCanada has been at the forefront of an initiative to get ICAO to change all heading/bearing references to degrees True as that is what all ancient INS's and modern day GNSS's do. Software then converts the information to degrees magnetic relative to the area being flown.

Magnetic compasses would be relegated to the history of the astrolabe, the sextant and VLF/Omega/LORAN.

IAHFLYR wrote:

All ATC vectors or heading issued in the U.S., will be magnetic. The phrase used above of "turn 10 degrees left" can be used for traffic separation as mentioned however, it is used mostly when the controller does not know what your exact heading is and at times will be followed with "say new heading" so then they know exactly what your heading will be going forward. Terminal controllers have a fairly good idea of how the wind will impact the heading given so we correct if if need by, some by using the "turn 10 degrees right/left". Last thing ATC wants is to tell the pilot to turn right heading 240 and they are already heading 250!! Most professional pilots will question that instruction to verify the turn direction as a 350 degree turn in a congested airspace the wrong way will create a mess for all involved.

When I was controlling and didn't know the exact heading, but knew what the heading was I wanted the pilot to fly I'd simply say "fly heading 240" and the pilots make the shortest turn either left or right to get to that heading. Again, professional pilots as I've seen some low time private and student pilots make the big turn in the wrong direction so as a controller you have to know your audience so to speak.

With vectoring on downwind legs descending from say 10,000' to 3,000' the wind might have an impact as the aircraft descends and might not being on the desired track you want. With FMC's and GPS you might tell the pilot to "track 085" which keeps them on the same path until you're ready to turn them on a base leg, though not too many controller do that, but it has been heard and is a very effective tool rather than changing headings when you're very busy. For another conversation another time.

May I add that it is not uncommon (at least on ATC recordings I’ve listen to) to hear “turn left/right heading 280” (sometimes followed by “for spacing) when already on that heading: a 360 degree turn for spacing, altitude reduction and so on.

“Turn right heading 240” when on 250 would sound weird though (I wonder how I’d react, as a private pilot, in that instance) : heading to same heading is pretty obvious it’s a complete 360 turn, that may either throw me or I’d just do “as told”! But we normally get told “left/right orbit above XYZ”.

Houston is like that departing 15s. They emphasize the right turn to 020 and if there’s any confusion they’ll say turn right the long way around to heading 020.

So, if you were flying a due north and were instructed to turn to 270 degrees, would you go right three quarters of the way around the circle , or turn left 90 degrees? As you can tell, I'm not a pilot, but it seems to me that in situations where the flight is told to make numerous course changes, it would get somewhat confusing.
Gregory

Unless otherwise directed you turn the short way so when flying 360 you turn left to 270.

Woodreau wrote:

Unless otherwise directed you turn the short way so when flying 360 you turn left to 270.

Correct. If ATC wants you to turn opposite to the shortest, they will stipulate TURN LEFT HEADING 270 (say, as an example, you were on a heading of 180).

gregorygoodwin wrote:

So, if you were flying a due north and were instructed to turn to 270 degrees, would you go right three quarters of the way around the circle , or turn left 90 degrees? As you can tell, I'm not a pilot, but it seems to me that in situations where the flight is told to make numerous course changes, it would get somewhat confusing.
Gregory

Departing the London TMA and you’ll get a dozen vectors plus a number of level changes and a “joining ckearance” if leaving EGLF, best not be confused. Is EGLF still Class G?

Unless instructed otherwise, always turn in the shortest direction to the heading.

gregorygoodwin wrote:

So, if you were flying a due north and were instructed to turn to 270 degrees, would you go right three quarters of the way around the circle , or turn left 90 degrees? As you can tell, I'm not a pilot, but it seems to me that in situations where the flight is told to make numerous course changes, it would get somewhat confusing.
Gregory

Typically they'd tell you left or right, but as mentioned if they didn't you'd turn the short way.

Numerous course changes are uncommon except in the approach and departure environments. At that point you're expecting vectors anyway and you have an idea what will happen anyway.

Woodreau wrote:

Houston is like that departing 15s. They emphasize the right turn to 020 and if there’s any confusion they’ll say turn right the long way around to heading 020.

Back in the day we never did that right turn to 020, it's just something they started when the new kids got into the tower and were afraid to cross overtop of the 26's & 27 finals as they didn't think they could get 1,000' above them. We did it with DC9's, MD80's and B727's, these days when I see the tower give the left turn 020 the planes are at least out of 1,800-2,000'.

But to your point, yep "long way around" is what you'll hear on LiveATC when someone asks to "verify right turn".

I often hear "turn 10 left" in my area when we fly from one approach sector to the next. There are a few departures where they want us to turn the long way from, say, a 130 heading to 250, so we usually get a repetition, "Left turn, left turn heading 250."

I recently had a "Turn right heading 070" when we were at 080 due to winds, so I replied "123 is presently 080, verify you want right or left 070" and they figured it out and gave us 090. If something seems strange, it's better to ask and verify than to just do something.

Thanks for the answers. I assume that when you say you would turn the short direction, it means you, the pilot turn the yoke. If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

Yes, we bank to start the turn the plane in the shortest direction. The autopilot will, too, but we slew the heading bug.

gregorygoodwin wrote:

If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

It will turn in the direction you tell it to turn. So if you turn the heading select knob to the left, it will turn left. If you turn the heading select knob to the right, it will turn right.

AirKevin wrote:

gregorygoodwin wrote:

If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

It will turn in the direction you tell it to turn. So if you turn the heading select knob to the left, it will turn left. If you turn the heading select knob to the right, it will turn right.

That only if you push the HDG SEL switch first and the turn the selector left or right. If you turn the selector first, then push it to engage the HDG SEL mode, it will command the turn in the shortest direction to the new heading regardless of which way you spun the selector.

If HDG SEL is already engaged and you turn the selector it will do as you said.

AirKevin wrote:

gregorygoodwin wrote:

If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

It will turn in the direction you tell it to turn. So if you turn the heading select knob to the left, it will turn left. If you turn the heading select knob to the right, it will turn right.

That really depends. Many older autopilots may start to turn that way, but will then turn the other way as soon as you pass 180 degrees from where you started. If you get a long way around turn, you are better off turning the direction you want and then continuing to spin the heading bug until you are where you want to be.

In the end, it’s second nature.

gregorygoodwin wrote:

Thanks for the answers. I assume that when you say you would turn the short direction, it means you, the pilot turn the yoke. If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

As others have said, it depends on the airplane and how the AP was designed. The ones I fly let you twist the heading bug the long way past 180 and it will follow that turn, but only if you're already in heading mode. If you're on a DP in nav mode and you twist the left 250 from my earlier example and then engage it, it'll go right. Seems like many other systems use that logic, but not all. During such maneuvers it's always good to watch it like a hawk and verify that it's doing what you want, and then be ready to twist the knob or hit the disconnect button.

On the 'bus, if you want to make the autopilot turn "the long way around" is to turn the heading knob until your commanded heading 150-160 degrees in the direction of the turn. Once the aircraft is 20-30 degrees from the commanded heading, turn more until you get to the heading you want. If you just turn it to the desired heading first, it will turn the short way.

I suppose you could program an orbit in the FMS and then select "immediate exit" once it has come around to the right heading. Doing that sort of crap for tactical maneuvering is a good way of giving the captain an aneurysm.

gregorygoodwin wrote:

Thanks for the answers. I assume that when you say you would turn the short direction, it means you, the pilot turn the yoke. If you are on autopilot, flying due north and are told to turn to 270, you dial this in, does the flight computers also turn the short direction? Can you direct it to do otherwise?

Even if we fly manually, the heading indicator/bug should always be set correctly. In this case the PF would hand fly in the desired direction, while the PM sets the indicator/bug.

In context, I think on the 777 you have to set the heading bug even in LNAV?

Wow! This seems to be an interesting subject. I thank each and every one of you for responding, As a non-pilot member of the forum, it is still somewhat confusing to me, but I have learned a lot from this post.
Another question. If headings and navigation are centered along magnetic lines, and if the magnetic north is slightly off from true north, are flight management systems set up to account for this? Is there any system being considered, or in reality, that would use, say GPS systems to identify true north/south along the longitudinal lines of Earth's sphere?
Gregory

Inertial reference systems already do everything in True oriented to the geographic North Pole, the FMS has algorithms to apply magnetic corrections to true headings so the pilot sees magnetic. Old INS systems worked and displayed in True, which is easier to work with in some ways. GPS only senses position from the satellite constellation, it takes an FMS to use raw position and change in position to make all the navigation solutions. Most planes use both, inertial and GPS.

An INS alignment is interesting. Once started, the accelerometers sense eastward movement, fast at low altitudes, slower at high. Once it has enough movement sensed, 90 degrees to the left is the North Pole. There’s a lot more in the details, but essentially that’s it.

Even light airplanes now frequently have AHRS assisted magnetometers that determine magnetic headings...and prove that ATC are not all that good at doing corrections, especially in areas of higher magnetic interference lol.

GalaxyFlyer wrote:

An INS alignment is interesting. Once started, the accelerometers sense eastward movement, fast at low altitudes, slower at high. Once it has enough movement sensed, 90 degrees to the left is the North Pole. There’s a lot more in the details, but essentially that’s it.

I didn't follow this - did you mean "fast at low latitdues, slow at high" perhaps?

Accelerometers on the aircraft align based on earth movement. At high latitudes, the angular rotation rotation of the earth is slower whereas as at the equator the angular rotation is faster. Same reason why they launch rockets near the equator to get into space, they use the faster rotation at the equator to gI’ve the rockets more “speed”/assist to get into space.

When the aircraft is closer to the equator, the aircraft INS aligns faster.

Auto correct, yes latitudes, not altitudes.

GalaxyFlyer wrote:

Auto correct, yes latitudes, not altitudes.

Well, according to Jimmy Buffett, both change together