High Latitude IRS Alignment

[QAVION]

I've read that on your typical Boeing 767/747-400/etc, it's recommended that you leave your IRU's in ALIGN for as long as possible before selecting NAV when departing from an airport in high latitudes. A minimum of 18 minutes is quoted.

Would selecting NAV and not entering PPOS for a similar period of time achieve the same goal, or is PPOS required as soon as possible to achieve the "fine tuning" required for high latitude navigation.

Thanks.
Regards.
Q.

[turbynetrip]

Our book gives no other guidance other than a minimum 17 minutes' align time...then enter PPOS and then selectors to NAV. I would assume that the extra time in ALIGN is indeed for fine-tuning. Perhaps not necessary with the GPSs updating, but wouldn't want to experiment without the OK from Mr. Boeing....hopefully an IRS expert will explain!

TT

[oxford blue]

It would not give you as good a result. The initial alignment is from the airport ARP, and is downloaded when you enter the airport four-letter ICAO identifier. That gets the alignment process going, even though you may be parked in a bay a couple of miles from the reference point. This is insignificant for the purposes of getting the right latitude for alignment.

If you select NAV right away, as soon as the minimum time necessary for alignment has finished (including the fine tuning), the system will automatically go into NAV mode. If you remain in ALIGN, it will carry on aligning until you select NAV. This may give you extra time aligning, which will give you a more accurate alignment.

Selecting PPOS, or operating the TO/GA lever on take-off, updates the present position in the FMC. It does not alter the raw IRS position, which stays a couple of miles out for the whole trip. However, it is not used in its raw form. Navigation is carried out using the FMC position, which is the final updated position before take-off, carried forward by IRS velocities Kalman-filtered against DME/DME or GPS.

[QAVION]

"The initial alignment is from the airport ARP, and is downloaded when you enter the airport four-letter ICAO identifier. "

Sounds a little odd, Oxford, but I'll take it on board. During maintenance, I've aligned the IRS's many times without entering anything in the FMC... other than the present position. If I forget to enter the position, I get a prompt in the scratchpad 10 minutes after switching on the IRS's (747-400). In this case, the full alignment takes only a few seconds after entering the position. Perhaps the IRS's take last known FMC position in lieu of the ARP?

What would happen in the case of a dual FMC failure on a 767? I can't see the alignment process being degraded or slowed down by this.... You'd simply enter the PPOS on the overhead panel (no ARP necessary for initial alignment).

Cheers.
Q,

[mono]

From my recollections of FMC/IRS alignment The following are true.

Leaving the switches in align will allow a more accurate alignment of the accelerometers.

Failure to insert ppos when align selected and more than 10 mins elapsed, will cause an error code in the IRS CDU (6 I think when STS is selected, but I may be wrong), a prompt on the FMC CDU scratchpad and the align lights on the IRS CDU will flash. This is because the ppos is req'd for full align, at this point the IRS will NOT further reduce errors in accelerometer alignment.

ARP is NOT req'd for alignment. Most operators I have worked with load gate ppos from the jepps, although some which only operate to a few airports have them loaded into the FMS database. Most use gate ppos to reduce map shift on T/O (unsquat) which on most a/c I have worked with is when update takes place. Failure to do this on airbus a/c may give rise to a IRS/FMS position error on the PFR. Of course the quick way to put in ppos is to use the last pos from the FMC (This is how most maint personnel do it.)

FMS uses nav info for position computation in the following order

GPS, DME/DME, DME/VOR, VOR,VOR, IRS.

Raw IRS position IS sometimes used. Our fleet has several a/c that are not GPS equiped but fly over the pond to the 'States. This can result in a large map shift when land is near and the first DME's are picked up (3+3T nm is the rule for IRS/INS error so after 4 hrs with no DME/VOR the IRS could be as much as 15nm out. In practice it is much less)

That's all my foggy memory can dredge up but hope it helps.

[QAVION]

"....at this point the IRS will NOT further reduce errors in accelerometer alignment."

Thanks, mono.

From your description, there appears to be quite a few differences between the aircraft I work on and yours, but I can't see the basic operating principles varying too much.

Here are a few of the variations:
The T/O update is inhibited on our 744's because of the installation of GPS (not deemed necessary). Prior to the installation of GPS, the update was triggered by pushing the TOGA switches on the throttles. On our 767's, the T/O update is/was triggered by pushing the N1/EPR button on the A/P Mode Control Panel. To be honest, I would have thought an unsquat signal would have introduced larger errors than taxying from the gate to the T/O position(???). Unsquat can be half way down the runway.... or all the way down the runway (give or take a few meters). This would introduce errors measured in kilometers, wouldn't it?

VOR/VOR updating is not used on our aircraft (I didn't realize it was an option).

We do use Last Position to set up the IRS's. The interesting thing about GPS is that it updates the Last Position continuously. If the aircraft is towed to a different gate, even with the IRS's switched off, the Last Position changes (on the FMC CDU).

3T + T has now been replaced with a chart/graph (which varies from this formula).

Cheers.
Q.

[QAVION]

Found the answer I was looking for... from a man on the inside.

Although his answer was, in parts, too technical for me to digest, I did get a general picture ax well as some interesting detail...

Basically....

Each IRU remembers last position (even on power down) and uses the latitude component of last position (in the absence of a new PPOS latitude) to compute Earth Rate. If the last position was not too far awry, the earth rate computation should not be too far off... at least under normal circumstances.

Earth Rate at high latitudes is minimal, so has to be looked at for a longer period to get an accurate measurement. Withholding a good PPOS latitude will not help. Therefore, switching directly to NAV and not entering PPOS serves no benefit.

Thanks.
Regards.
Q.

[comxtek]

QAVION....your man on the inside is correct. Keeping the IRUs in the align mode will not give a more accurate alignment at all.

[QAVION]

"QAVION....your man on the inside is correct. Keeping the IRUs in the align mode will not give a more accurate alignment at all."

That's not what I said at all, comxtek. In fact it <I>will</I> give a better alignment. That's precisely why it's done for high latitudes. NAV is only selected when 17 or so minutes has elapsed in ALIGN mode. The longer the better. The more precise the earth rate computation during align, the more precise the earth rate compensation in NAV mode.

I don't think anyone here said that it wouldn't???

A misunderstanding, perhaps?

Regards.
Q.

[comxtek]

You turn the mode selectors to the NAV positon and do not enter a position. When the IRUs have completed its alignment and start flashing 'ALIGN' for a position entry, leaving the IRUS in align mode for any longer will not give a more accurate alignment.

That is direct from the people that manufacturer IRUs.

Maybe I misunderstood the question.

[QAVION]

"That is direct from the people that manufacturer IRUs."

Thanks for the confirmation, Comxtek. Sounds like you've found a man on the inside, too :D

Cheers.
Q.

[SOPS]

:O OK I am willing to look stupid... What the hell is Earth Rate?

[comxtek]

Earths rate of rotation.

360 degrees every 24 hrs
15 deg per hr

IRUs can sense rotation easier at some latitudes than it can at others.
For instance, at the North Pole, it is harder to sense the earths rotation rate, so the alignment time is much longer than it would be here in New York

No question is stupid.