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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, 09:56
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Question 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.
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Old 6th May 2019, 10:12
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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?
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Old 6th May 2019, 10:25
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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.

Last edited by FlyingStone; 6th May 2019 at 10:27. Reason: Added link
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Old 6th May 2019, 10:26
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Originally Posted by hoss183 View Post
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?
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.
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Old 6th May 2019, 10:28
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Originally Posted by FlyingStone View Post
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.
I agree. Correct me if I'm wrong but doesn't the IRS always give true track/GS/ True heading?
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Old 6th May 2019, 10:53
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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.
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Old 6th May 2019, 11:11
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Originally Posted by FlightDetent View Post
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.
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?

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Old 6th May 2019, 11:13
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Originally Posted by DJ Flyboy View Post
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.
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.
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Old 6th May 2019, 11:36
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Originally Posted by DJ Flyboy View Post
So basically there are no limitation then apart from the switching to true navigation in polar regions? As in no machine (accelerometer) restrictions?
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.
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Old 6th May 2019, 11:54
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DJ Flyboy Dont confuse the two limitations.

1. Alignment. Most IRSs cant align above the low 80s N/S as there is not enough change in velocity to align the gyro. Think a gyro at the pole is just rotating like a record player. These limitations are invariant of hemisphere.

2. Operational areas that give accurate mag track for the ADIRUs. This is dependant on the magnetic variation tables which are no symmetrical in each hemisphere.
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Old 6th May 2019, 12:22
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Originally Posted by compressor stall View Post
2. Operational areas that give accurate mag track for the ADIRUs. This is dependant on the magnetic variation tables which are no symmetrical in each hemisphere.
Ahh, got you. Any idea of the latitude limitations for the second point? Thanks.

Thanks everyone for the answers. Much appreciated
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Old 6th May 2019, 13:04
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Originally Posted by compressor stall View Post
DJ Flyboy Dont confuse the two limitations.

1. Alignment. Most IRSs cant align above the low 80s N/S as there is not enough change in velocity to align the gyro. Think a gyro at the pole is just rotating like a record player. These limitations are invariant of hemisphere.
Thats complete rubbish, you clearly dont understand how IRS works with that statement.
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Old 6th May 2019, 13:34
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Well, the Boeing answer is in the FCOM:
ADIRU
ADIRU alignment must not be attempted at latitudes greater than 78 degrees, 14.75 minutes.

This is the latitude of Svalbard/Longyear ENSB.
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Old 6th May 2019, 13:50
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hoss183 The higher the LAT, the longer the align time still, not? Spill the beans or at least a link for a nice read.
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Old 6th May 2019, 14:48
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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.

Last edited by wiggy; 6th May 2019 at 15:09.
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Old 6th May 2019, 15:09
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IIRC A320 has limitations both for alignment and operation at higher latitudes - dont have the FCOM at hand to give specific numbers. Anyhow, not possible to operate in Polar regions and use True REF

OTOH A330 has only alignment limitations and has to be used in true ref above certain LAT.
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Old 6th May 2019, 15:26
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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.
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Old 6th May 2019, 17:54
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Boeing Polar Route navigation by model

Other information: High Latitude Operations

Jepp


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Old 6th May 2019, 18:38
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The Fokker 70/100 had a limit, something like 78N. No TRUE/POLAR mode, which is probably the reason.
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Old 6th May 2019, 21:20
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A320F has a latitude limit for alignment of 82 N/S with or without the polar nav mod. And between 60 and 82 it takes significantly extra time.

One of the advantages of the polar nav mod is that it is selected before the boundary of the Mag database area to avoid tripping the AP and ATHR off as the ADIRUs swap from Mag to True automatically as usually they don’t swap in exact sync. Additionally there is a Grid reference on the screen above 65.

Originally Posted by DJ Flyboy View Post
Ahh, got you. Any idea of the latitude limitations for the second point? Thanks.
From the Generic A330 FCOM:
The ADIRU gives true heading instead of Magnetic Heading
Above 82N
Above 73N between 90W and 120W
Above 60S (make sure you understand that above in the Southern Hemisphere is higher latitudes, ie below on the map!)

Originally Posted by Sidestick_n_Rudder View Post
IIRC A320 has limitations both for alignment and operation at higher latitudes - don’t have the FCOM at hand to give specific numbers. Anyhow, not possible to operate in Polar regions and use True REF

OTOH A330 has only alignment limitations and has to be used in true ref above certain LAT.
The limitation on the A320 and the A330 are the same if the Polar Nav mod is fitted. Obviously most narrow bodies don't go into the polar region so it's not fitted as standard. A while ago I was told that there was only one narrow body Airbus in existence with the Polar Nav Mod, although with the growth of the LR and CJ narrow body fleet, I imagine that would no longer be true. The operational limits are the same as I quoted above.

Semantics are an issue here- don't mix up the geographic term Polar Regions with the polar regions area as determined by Honeywell et al. If the system is capable to navigate in True up to 82N, then the same is true for up to 82S, but for the IRS manufacturers the Polar Region definition starts at 60S due to magnetic database absence so (legalities aside) you could practically operate in True in the polar region at say 65S. The same logic for that 90W-120W sector.

Last edited by compressor stall; 7th May 2019 at 00:34.
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