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Latitude limitations for the use of IRS

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Latitude limitations for the use of IRS

Old 6th May 2019, 23:18
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[14 CFR 135, §135.98 Operations in the North Polar Area.] After February 15, 2008, no certificate holder may operate an aircraft in the region north of 78° N latitude (“North Polar Area”), other than intrastate operations wholly within the state of Alaska, unless authorized by the FAA
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Old 7th May 2019, 02:03
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Magnetic Variation varies SIGNIFICANTLY near the North and South Poles within a short distance. The Variation Table is a data set containing variation information for geoid positions. The limited range of that table basically applies variation to compass headings to produce true headings and therefore reliable NAV data. Due to minor variation differences nearer the equator and sub-tropics, it can be pretty vague and therefore small in size so takes up less RAM in the archaic A320 FMGC. The A320 wasn't really designed to operate trans-polar, so this wasn't an important feature. In the A330, which came a few years later, RAM had come down in price and due to the nature of long-haul, it would be assumed that trans-polar routes would be planned, therefore it has a better variation table.

I didn't realise that it would cause the AP/FD to trip off without the mod, as compressor stall said, but obviously that makes sense as the FMGC is basically saying the NAV data is unreliable. I assume then the 'Polar Mod' is either a RAM upgrade, or simply a better optimisation of the FMGC data, eg less airports/routes/waypoints filling up the RAM to enable a larger VAR Table? Additionally, I assume this is why autoland is prohibited in certain airports with specific FMGCs. On my fleet, autoland is prohibited in odd places like Johannesburg or Keflavik as the autoland system would presumably align itself in the flare with whatever was tuned as the ILS CRS (a rapidly varying magnetic heading). I haven't tried it but i'm told it aligns itself with the CRS during FLARE so if you were to put a wrong figure into the RAD NAV page, you'd end up with a pretty awful landing, hence the reason we check the CRS during the LAND FMA.
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Old 7th May 2019, 04:13
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I heard this story many, many years back when I was flying S-2s in the Canadian military. This was back in the days before GPS, and when IRS had spining gyros and gimbles - no fancy ring [email protected] gryo stuff.

There was a CP-140 Aurora on a northern patrol, way up north, close to the North Pole. Some bright guy suggested it might be neat to fly right over the North Pole. The crew commander agrees this is right fine idea, so one of the navigators gives the pilots vectors to try to get right over the NP. First pass, and the head nav stated that they had missed by a tiny bit. Second pass, same story. Third pass, and they nailed it, and all the gimbles line up and they toppled all three IRS. All the nav systems and heading references gone. Every direction is south, but some of the souths are a better choice than the others.

They climb, hoping to get on top of the cloud to spy the sun. The cloud goes up higher than they can climb. No radio navaids in range. Crew crapping bricks. Finally one of the navs, running the radar in ground mapping mode thinks he recognizes some of the Artic Islands, and they roll those dice and start heading towards what they hope is Canada. Finally they get far enough south that the ADF picks up an NDB in northern Canada and all ends well. The beer tasted especially good that night.
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Old 7th May 2019, 04:34
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Originally Posted by khorton View Post
Every direction is south, but some of the souths are a better choice than the others.
This is a key point. Neither the concept of heading nor the concept of longitude make any mathematical sense at the north pole. That's not an engineering limitation of any compass, gyro, INS, or GPS system: it's a limitation in the underlying mathematical geometry. To navigate at the pole you need to use a coordinate system which is something other than latitude/longitude, with courses and headings defined relative to that coordinate system.
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Old 7th May 2019, 04:43
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I always thought it was set at 78 degrees because of the wandering.

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Old 7th May 2019, 08:36
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Originally Posted by FE Hoppy View Post
Every AFM Iv'e read has alignment limits for IRS. Often pan handled around the North Pole.
Some also include operational regions for older units.

Would love to hear Hoss183's secret info about how a gyro (laser or otherwise) works. I've only read Honeywells description.
I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.
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Old 7th May 2019, 09:05
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Originally Posted by hoss183 View Post
I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
Genuine question - Is that actually the case? As far as I'm aware (emphasis on that!!) that certainly wasn't the situation with the old steam driven INSs.

The old systems levelled the platform to local horizontal and then ( and I'm very very much simplifying here) "found" and torqued the platform to align to True North by sensing the effect of the Earth's rotation on the platform.. mag var didn't come into the align process, am I wrong?/is it done differently now?

(Very belated edit to add that I'm assuming strap down systems must align in a different manner to that used in gimballed platforms - but the underlying question still stands - do they use variation in their alignment process?)

Last edited by wiggy; 7th May 2019 at 11:31.
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Old 7th May 2019, 10:54
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Originally Posted by hoss183 View Post
I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.
You might not have been talking about alignment, but I was. That’s why I even put a “1. Alignment “ at the start of my paragraph and you even quoted it.

And I’m eagerly awaiting your answer to Wiggy.
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Old 7th May 2019, 11:20
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wiggy,

You are right about the old INS - nothing whatever to do with magnetic anything, everything to do with sensing the rotation of the Earth. I am long out of date, but I would guess it is the same today with modern systems.
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Old 7th May 2019, 14:00
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Originally Posted by Smythe View Post
[[url=http://code7700.com/high_latitude.htm#references]14 CFR 135, §135.98 Operations in the North Polar Area.] After February 15, 2008, no certificate holder may operate an aircraft in the region north of 78° N latitude (“North Polar Area”), other than intrastate operations wholly within the state of Alaska, unless authorized by the FAA
Most U.S. Operators in the region are Part 121, not 135.

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Old 7th May 2019, 14:19
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As far as I remember, the 747-400 had an operational limitation which prohibited flight at latitudes exceeding 89 degrees. In other words, one had to miss the pole by at least 60nm. My guess is that this was to avoid the problems experienced by the unfortunate CAF Aurora crew.

Cessna Citation and CJ models with which I am familiar have a “keyhole” shape around the North Pole and Hudson Bay within which flight is prohibited. As they have AHRS but not INS/IRS, they rely on magnetic sensing to feed the compasses and I think that the large magnetic dip values in this “keyhole” reduce the magnitude of the horizontal component below a value that is required by the flux sensors.

I did once accept a direct routing from YFB towards YYQ which just nibbled inside the edge of this keyhole, but the wings didn’t fall off.

There is a “DG mode” which removes the magnetic sensing input and then requires the pilot to manually align the displays with the wet compass, just like a Cessna 150.
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Old 7th May 2019, 14:22
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Originally Posted by hoss183 View Post
I'm not talking about the align accuracy or time, that is clearly effected by the magnetic / true variation.
​​​​​CS claimed that the gyro itself wouldn't work at or near the pole because of velocity, that bit is rubbish, a gyro works anywhere including in space.
Alignment has nothing to do with variation.
Next?
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Old 7th May 2019, 17:11
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As far as I remember, the 747-400 had an operational limitation which prohibited flight at latitudes exceeding 89 degrees.
Boeing reference in Post #18
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Old 7th May 2019, 17:24
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Originally Posted by Smythe View Post
Boeing reference in Post #18
Thanks for that..like Bergerie I recall the 89N limitation being stated in our (company produced) FCOM for the 744.
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Old 7th May 2019, 20:01
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Not sure when the charts and procedures show grid vs mag. For SCC we use grid.
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Old 7th May 2019, 23:58
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Australia operates an A319 from Tasmania to its Antarctic research station which is outside the normal latitude limits for the type.
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Old 8th May 2019, 01:06
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Australia operates an A319 from Tasmania to its Antarctic research station which is outside the normal latitude limits for the type
And we have the Captain of said aircraft commenting in this thread, should he want to put his hand up.
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Old 8th May 2019, 08:33
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How on earth (sorry for the pun) did the USS Nautilus manage to do it so accurately all those years ago?
Remember, they were underneath the polar icecap.
Just asking.
JO.
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Old 8th May 2019, 09:21
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Originally Posted by judge.oversteer View Post
How on earth (sorry for the pun) did the USS Nautilus manage to do it so accurately all those years ago?
Remember, they were underneath the polar icecap.
Just asking.
JO.
Getting to the pole is easy, just keep steering 000 True. ;-)
There's a tiny bit on the navigation here: https://en.wikipedia.org/wiki/USS_Na...the_North_Pole
More detail here: https://www.ion.org/publications/abs...ticleID=101690

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Old 8th May 2019, 11:37
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GnD
Many thanks for that, especially the second ref.
Cheers JO.
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