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AstroNav
Topic Author
Posts: 28
Joined: Wed Jun 03, 2020 8:51 am

Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 3:52 pm

Hello,


I was reading a trip report from another user re: an QANTAS Antarctica 744 tourist flight.

They said that >60 deg S, magnetic bearings don't work. Announced by the crew.

Similar, I am assuming for >60 deg N.

I'd like to delve in to that for random interest.


Questions:


Does this apply to > 60N/ 60S?
In glass cockpits, do you have backup compasses? That is to say - not an electro-mechanical magnetometer et al. A pure compass.
What do you see at those latitudes on the backup compass? Does the compass needle randomly move/ swing about - over short distances?
How close to to Magnetic or Geographic North/ South do inter-continental pilots get?
Are there any Polar routes via the Antarctic?
Is it more dangerous down in the Antarctic than the Arctic?
What happens if the electric jets fail to produce any electric at those latitudes/ worst case scenario's?


Thank you!
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 4:03 pm

The FMS switches over to true, stops computing magnetic outputs, so courses, tracks hand heading are displayed in True referenced to the North Pole. The old INSs systems often used True all the time regardless of latitude.

Yes, the mag compass wanders around rather uselessly. A “pure” compass is magnetic. Yes, around 60N/S systems switch over to True, but the actual latitude of the change varies, some systems haven’t been approved for high latitudes.
 
AstroNav
Topic Author
Posts: 28
Joined: Wed Jun 03, 2020 8:51 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 4:08 pm

Thank you GF - it leads me to a few other questions:

Does this mean that magnetic is primary, therefore?
I assumed that position fixing and bearings are electronic primarily.
True is primary and Magnetic is backup? Or the other way around?
 
Woodreau
Posts: 1899
Joined: Mon Sep 24, 2001 6:44 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 4:18 pm

Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 4:19 pm

The “compass”, the navigation displays, really, normally show magnetic heading, the course is also in mag. The displays go to True at the designed high latitude and return when leaving the high latitudes.
 
AstroNav
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Posts: 28
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Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 4:37 pm

It is very interesting that aircraft navigate magnetically. I assumed that it was electronic fix/ bearing with GNSS - TRUE bearings.

Thanks Woodreau.
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 6:02 pm

AstroNav wrote:
It is very interesting that aircraft navigate magnetically. I assumed that it was electronic fix/ bearing with GNSS - TRUE bearings.

Thanks Woodreau.


The heading reference isn’t the basic compass, it’s derived from the IRS which always works in true (aligned with the geographic North Pole), the FMS applies an algorithm to compute local magnetic variation which takes true applies the mag variation to display magnetic heading and compute magnetic courses. At high latitudes, the system cannot accurately compute the rapidly changing variation, so just displays the true output from the IRS.

GF
 
thepinkmachine
Posts: 413
Joined: Tue Apr 28, 2015 4:43 pm

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 6:41 pm

It’s worth noting that in the Northern hemisphere, the magnetic works at slightly higher latitudes - 70 deg N, or so, with the exeption of the ‘keyhole’, ie. the area In the vicinity of the magnetic North Pole.
"Tell my wife I am trawling Atlantis - and I still have my hands on the wheel…"
 
CosmicCruiser
Posts: 2415
Joined: Tue Feb 22, 2005 3:01 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 10:34 pm

I was a little disappointed that the mag compass didn't point at the mag N pole. It just wandered a little. Best I remember our FMS went from NAV to True Track to finally Track Hold. Click on my avatar and it shows how close we got.
 
CosmicCruiser
Posts: 2415
Joined: Tue Feb 22, 2005 3:01 am

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 10:37 pm

tried to post a photo but couldn't get it to upload.
 
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Starlionblue
Posts: 19934
Joined: Fri Feb 27, 2004 9:54 pm

Re: Magnetic Navigation >60 deg lat N/S

Mon Jun 29, 2020 11:24 pm

AstroNav wrote:
Hello,


I was reading a trip report from another user re: an QANTAS Antarctica 744 tourist flight.

They said that >60 deg S, magnetic bearings don't work. Announced by the crew.

Similar, I am assuming for >60 deg N.

I'd like to delve in to that for random interest.


Questions:


Does this apply to > 60N/ 60S?
In glass cockpits, do you have backup compasses? That is to say - not an electro-mechanical magnetometer et al. A pure compass.
What do you see at those latitudes on the backup compass? Does the compass needle randomly move/ swing about - over short distances?
How close to to Magnetic or Geographic North/ South do inter-continental pilots get?
Are there any Polar routes via the Antarctic?
Is it more dangerous down in the Antarctic than the Arctic?
What happens if the electric jets fail to produce any electric at those latitudes/ worst case scenario's?


Thank you!


Does this apply to > 60N/ 60S?
It isn't exactly 60N/60S. Taking the A350 numbers as examples, magnetic heading and track are not provided beyond the following limits. You select "TRUE" reference and continue on true headings.
- North of 73N between 90W and 120W, due to the proximity of the magnetic north pole
- North of 82N at other longitudes
- South of 60S

- Do we have backup compasses in glass cockpits? Yes. The magnetic compass is typically mounted at the top of the centre windshield pillar.. (AFAIK some business jets, e.g. the G550, do not have a backup compass.)

- How close to the poles do we get? I've been within 60 miles of the geographic North Pole. This is routine. Aircraft fly near the magnetic north pole all the time as well.

- Are there any polar routes via the Antarctic? Not like over the Arctic. This is purely due to the geography of destinations.

- Is it more dangerous down in the Antarctic than the Arctic? No. There is no difference that I can think of beyond distance to alternates.

- What happens if the electric jets fail to produce any electric at those latitudes/ worst case scenario's? If all electrics fail, the aircraft is toast wherever we are in the world. Same if we lose all hydraulics.



AstroNav wrote:
Thank you GF - it leads me to a few other questions:

Does this mean that magnetic is primary, therefore?
I assumed that position fixing and bearings are electronic primarily.
True is primary and Magnetic is backup? Or the other way around?


There is no "True" or "Magnetic" hierarchy in that way. It is purely a display convention. Position fixing and bearings are entirely electronic. The backup compass doesn't feed any instruments beyond the eyeballs of the pilots. Position is derived a combination of GPS, radio beacons and inertial navigation.

As GalaxyFlyer says, magnetic variation is calculated based on position and applied to the directional reference to give magnetic headings. The aircraft does not rely on magnetic reference to navigate.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 12:52 am

I can’t say about the Gulfstreams, but Citations, Hawkers Lears, Challengers and Globals all have magnetic compasses. I thought they were required as back-up. We checked it as a pre-flight item against the ND headings.
 
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Starlionblue
Posts: 19934
Joined: Fri Feb 27, 2004 9:54 pm

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 7:40 am

GalaxyFlyer wrote:
I can’t say about the Gulfstreams, but Citations, Hawkers Lears, Challengers and Globals all have magnetic compasses. I thought they were required as back-up. We checked it as a pre-flight item against the ND headings.


Heard it from a G550 pilot.

Some years later, I had the chance to look into both the G550 and the G650 cockpits. Guess what I completely forgot to check or ask the company rep... ;)

Can't find one in any cockpit pics. But it could be folded away, or in the ceiling like on the MD-80.

Image

Image
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
AstroNav
Topic Author
Posts: 28
Joined: Wed Jun 03, 2020 8:51 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 12:43 pm

Thank you, everyone. I have a keen interest in all aspects of navigation.

I guess runway designations are in magnetic, too in spite of any variation/ declination drift.
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 1:28 pm

IIRC, Thule’s runways are in True.
 
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Starlionblue
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Joined: Fri Feb 27, 2004 9:54 pm

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 3:13 pm

GalaxyFlyer wrote:
IIRC, Thule’s runways are in True.


Indeed. As are some runways in northern Canada. Same with the local VORs.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
kalvado
Posts: 2723
Joined: Wed Mar 01, 2006 4:29 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 4:15 pm

GalaxyFlyer wrote:
IIRC, Thule’s runways are in True.

According to the map, they would have 45 deg difference between magnetic and true..
https://maps.ngdc.noaa.gov/viewers/hist ... clination/
I wonder how it is handled in southern New Zealand - they have about 25 degrees. Still using magnetic headings, as far as I can tell:
https://skyvector.com/airport/NZNV/Invercargill-Airport
 
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dennypayne
Posts: 309
Joined: Tue Mar 13, 2012 3:38 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 4:58 pm

AstroNav wrote:
I guess runway designations are in magnetic, too in spite of any variation/ declination drift.


I don't remember where off the top of my head, but runways have been renumbered before as a result of magnetic drift.
A300/310/319/320/321/332/333/343/380 AN24/28/38/148 ARJ AT6/7 B190
B717/722/732/3/4/5/7/8/9 741/744/752/753/762/763/764/772/773/788/789
CR1/2/7/9 D8S D93/4/5 DHC2/3/7/8 D28/38 EMB/EM2/ER3/D/4/E70/75/90
F50/100 J31 L10 L4T M11/80/87/90 SF3 SU9 TU3/TU5 YK2
 
CosmicCruiser
Posts: 2415
Joined: Tue Feb 22, 2005 3:01 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 5:12 pm

It happens a lot. Here in MEM the N/S runways were once 18L&R and now are 17L&R.
 
Woodreau
Posts: 1899
Joined: Mon Sep 24, 2001 6:44 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 5:53 pm

In the Northern Domestic Airspace in Canada, since you have to use True for navigation, for all of the airports located in the Canadian NDA, their runways are numbered in TRUE as well instead of magnetic.

Here is something from an ICAO air navigation conference less than 2 years ago which proposes switching everything from magnetic to TRUE by 2030.
https://www.icao.int/Meetings/anconf13/ ... 114_en.pdf

have no idea if it got anywhere or got tabled without action...
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
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zeke
Posts: 14974
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Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 7:57 pm

AstroNav wrote:
They said that >60 deg S, magnetic bearings don't work. Announced by the crew.


That is not quite correct, the issue is not the latitude it is the proximity to the magnetic north or south poles. As you get closer to them the Isogonals (where magnetic declination is identical) change too rapidly.


GalaxyFlyer wrote:
The FMS switches over to true, stops computing magnetic outputs, so courses, tracks hand heading are displayed in True referenced to the North Pole. The old INSs systems often used True all the time regardless of latitude.


This is not correct, the IRS/INS/FMC do all their navigation based upon true, and then their either calculate or lookup the magnetic variation.

AstroNav wrote:
It is very interesting that aircraft navigate magnetically. I assumed that it was electronic fix/ bearing with GNSS - TRUE bearings.

Thanks Woodreau.


Under the hood everything is done with true bearings. A FMC for example when navigating between waypoints would lookup the latitude and longitude of each waypoint and calculate the true great circle track between them using spherical mathematics. It then either uses a magnetic field lookup table (tables get updated with database cycles as the magnetic poles move) or calculates the the variation using a world magnetic model algorithm.
Human rights lawyers are "ambulance chasers of the very worst kind.'" - Sky News
 
GalaxyFlyer
Posts: 5613
Joined: Fri Jan 01, 2016 4:44 am

Re: Magnetic Navigation >60 deg lat N/S

Tue Jun 30, 2020 8:47 pm

zeke wrote:
AstroNav wrote:
They said that >60 deg S, magnetic bearings don't work. Announced by the crew.


That is not quite correct, the issue is not the latitude it is the proximity to the magnetic north or south poles. As you get closer to them the Isogonals (where magnetic declination is identical) change too rapidly.


GalaxyFlyer wrote:
The FMS switches over to true, stops computing magnetic outputs, so courses, tracks hand heading are displayed in True referenced to the North Pole. The old INSs systems often used True all the time regardless of latitude.


This is not correct, the IRS/INS/FMC do all their navigation based upon true, and then their either calculate or lookup the magnetic variation.

AstroNav wrote:
It is very interesting that aircraft navigate magnetically. I assumed that it was electronic fix/ bearing with GNSS - TRUE bearings.

Thanks Woodreau.


Under the hood everything is done with true bearings. A FMC for example when navigating between waypoints would lookup the latitude and longitude of each waypoint and calculate the true great circle track between them using spherical mathematics. It then either uses a magnetic field lookup table (tables get updated with database cycles as the magnetic poles move) or calculates the the variation using a world magnetic model algorithm.


May not have been clear, but in post #7 I said that the FMS and the sensors work in true, but use an algorithm to compute magnetic for display

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