Latitude limitations for the use of IRS
 

Hello, during one of my interviews I was asked about the N and S latitude limitations for the use of IRS. Does any one know the exact latitude N and S values? Apparently they aren't the same latitudes and South latitude is lesser than the north. What is the reason for this? What is the reason for the limitation in the first place? I cant seem to find the answer in any of the books I have.

I cant answer the question directly, but knowing how IRS works, i cant see why there would be any latitude limits. A gyro based IRS would track movements in 3 axes wherever it is on the planet surely?

The exact latitude limitations will depend on the aircraft type. Most manufacturers offer "polar" option, which removes the latitude limitations and I believe the limitation is more related to the FMC and autoflight rather than the IRS, as the FMC has to transit into the "polar" mode and use true headings/tracks/courses for navigation rather than magnetic ones. Same goes for the autoflight system.

This Boeing article on navigation in polar regions might be useful.

Thanks, that is what I thought too but apparently there is a limit, at least according to the person interviewing me.
I remember there being a limit for INS. Approximately 70 degrees N/S because the INS cant figure out E/W from N/S due to high latitudes. Any idea about that. Cant seem to find the reference anywhere though.

I agree. Correct me if I'm wrong but doesn't the IRS always give true track/GS/ True heading?

DJF, not really. The machine does calculate with TRUE track inside, however, the information supplied out to the various other aeroplane systems is converted to MAGNETIC heading for compatibility. There is a magnetic variation table (hopefully well updated) inside the IRS boxes.

What FlyingStone means is that some aeroplanes have a switch to change all navigational instruments / computers into TRUE track, which enables polar area navigation. The IRS do not care as long as they had been built to operate at high latitudes.

At least on the CEO Airbus generation.

Thanks, I was trying to imply exactly that.

So basically there are no limitation then apart from the switching to true navigation in polar regions? As in no machine (accelerometer) restrictions?

As has been said by FlyingStone the limit is going to be equipment specific..FWIW a 70N limit would be very limiting for a lot of Long Haul operators since it would rule out many Polar/Arctic routes taken by many IRS equipped aircraft, and we certainly operate well north of 70N on some of our routes.

Probably not, but nevertheless limitation prevents you from operating outside of that area.

It could also be the case that certain aircraft are fully able of polar flight from the factory, but require an AFM supplement or modification enabling operation over the poles, which obviously costs a lot of money.

Ahh, got you. Any idea of the latitude limitations for the second point? Thanks.

Thanks everyone for the answers. Much appreciated

Thats complete rubbish, you clearly dont understand how IRS works with that statement.

hoss183 The higher the LAT, the longer the align time still, not? Spill the beans or at least a link for a nice read.

I'm hazarding a guess at Gyrocompassing being possibly more problematic at high latitudes but that's possibly ancient history and since a long time since I heard a specialist navigator moaning about the dark arts of Inertial alignments so I'll stand by to be corrected and getting the full story.

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.

Boeing Polar Route navigation by model

Other information: High Latitude Operations

Jepp

https://cimg7.ibsrv.net/gimg/pprune....ee1f841cbb.png

The Fokker 70/100 had a limit, something like 78N. No TRUE/POLAR mode, which is probably the reason.

[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

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.

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 laser 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.

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.

I always thought it was set at 78 degrees because of the wandering.

https://cimg4.ibsrv.net/gimg/pprune....90667f0736.png

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.

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?)

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.

Most U.S. Operators in the region are Part 121, not 135.

Alignment has nothing to do with variation.
Next?

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.

Not sure when the charts and procedures show grid vs mag. For SCC we use grid.

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.

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

GnD
Many thanks for that, especially the second ref.
Cheers JO.