A320 IRS Alignment Question

[FLY BY WIRE]

The A320 IRS Full alignment cycle requires a present position (usually the gate co-ordinates) to be entered within 10 minutes of switching the IRS switches to nav. If it dosn't get the position the align lights flash.
Are the align lights flashing just an attention getter to prompt position input, or is the subsequent IRS accuracy affected by slow input of the present position.
I think they're just an attention getter but views in my company vary, so if anyone has more indepth knowledge i'd be grateful for any info.
Many thanks
FBW

[mcdude]

I can't specifically answer your question, but the FCOM says in at least two places;

"Enter the present position, just after switching the three mode selectors to NAV, to avoid excessive ADIRS drift."

It then goes on to say;

"The IRSs are usually aligned to the departure airport reference point coordinates. It is recommended to use these coordinates, as stored in the navigation database, for transit flights, or for any flight with GPS. When published, use of the gate coordinates (insert/slew in the INIT A page) should be reserved for aircraft without GPS, and when flying long segments without radio updates."

mcdude

[alexban]

The flashing IRS means that you should enter position.They won't affect the IRS accuracy. The IRS platforms are aligned and only thing the IRS computer require now is the present position,from which it can make all movement calculations.
Brgds Alex

[Nozzles]

Whilst I have no experience with the A320s system, alexban's statement strikes me as a little odd. No INS system can be aligned without it knowing its latitude. If a gyro is aligned without a latitude bias, the gyro will translate Earth rates into a non-existant movement over the Earth's surface. That is why some form of position (manual or via GPS) must be entered in order to achieve an accurate platform alignment.

My a/c type has a RLG INS whose nav solution is damped by (amongst other things) an embedded GPS. Upon selecting align the INS will take the GPS position if the GPS is tracking, or the last known position if not. However, the manual recommends that the align postion be manually keyed in by the pilot within 2 minutes of commencement of the align process, otherwise the system will record the alignment as 'degraded'. Why does it do this? Because the INS trims itself based on it's previous performance, and will not use the INS performance of a flight based on a 'degraded' alignment for trimming purposes. Thus, if every pilot relies on the GPS to automatically enter the align pos, the system will never trim itself to fine-tune its performance. Why does it want the pilot to enter the position? Two reasons: first, if the GPS does not track before the align is complete, the system will use the last known position. If the aircraft has been moved with the INS switched off, or has just received a brand new INS, the align position will be incorrect. Second, the system wants a human quality control check of the GPS positional input in case of GPS spoofing/jamming/failure.

FBW, it's possible that your system has similar requirements.

[Pegasus77]

In my company the procedure since a few weeks emphasizes entering the coordinates directly after switching the IRSs on. This not for accuracy, but apparently for refueling. The fuel-indicators use the IRS for reference as well.
I was taught in flight school that you should enter the position asap to prevent excessive drift. In real life I never noticed a difference between aligning the IRSs after 1 minute or only after 15.

P77

[oxford blue]

Although I don't presently fly any aircraft with FMC, so I can't quote any specific system, I have used one before and I do teach general FMC theory. I think some contributors may not be drawing a distinction between input of present position used for alignment and present position used for starting the navigation.

Most IRSs remember where they were shut down at the end of the last flight and use this as the default position for alignment. Even if there was some nav error at the end of the trip or if the aircraft has been moved in the meantime with the IRS off, the last remembered present position will be within a couple of miles, and this is good enough for getting sin latitude and cos latitude, which are the terms needed for alignment. Any error introduced by use of the wrong latitude for alignment will normally be insignificant.

This is the raw IRS position which, in most IRSs, is never updated. As the aircraft moves, any initial error is simply forward carried as a vector and is added to by the position drift rate of the IRS in flight.

However, the navigation present position is corrected within the FMC, downstream from the IRS. This is where your "ENTER PRESENT POS" lat and long goes. It then gets further refined by whatever system the FMC uses. In the Boeing 737-400, for instance, as the TO/GA lever is advance on initial take-off roll, the present position of the piano keys of that particular runway is fed into the FMC to obtain an initial update fix for the start of the navigation. The FMC position is then further refined in flight by Kalman filtering it against DME/DME or GPS.

Other systems may vary in detail, but I think that they all work on much the same principle. I'm happy to be corrected by a current operator, though, because things may have changed as technology moves on.