MCDU messages (A320)

[Neupielot]

Hi all,
Anyone seen this mcdu " Chk IRS 1 or 2 or 3/ FM pos" message before?
from Fcom 4.03.38- Each IRS position is compared to the FM position. The difference is greater than a threshold function of time
I have no idea what that means.....Can someone help plz?
I gather u check the position of that particular IRS vs FM pos, then? its in lat/long. what's the limit ( obviously its out of limit otherwise there would not be a mcdu amber message) .
If u are in the middle of taxying, what are the correct procedures to carry out?

Many Thanks.:)

[Bidalot]

a quick IRS realign

[Superpilot]

Clearly the full alignment just 15 minutes prior wasn't enough then?

Why would this be?

[PappyJ]

This is likely where you've already performed the alignment, taxied, then changed the runway and mistakenly entered the wrong runway/airport (ie: activated secondary with different flight plan or opposite runway which is far enough away to set of the warning) Of course, there are other possible reasons

Basically, the two systems are separate - IR (ADIRU) and FM - but they communicate with each other and spit out messages when they get confused.

You'd get a theoretically similar message if you entered the wrong ILS frequency for a departure runway which had an ILS in the database.

Understand?

[shortfuel]

Neupielot,

You'll get that message on MCDU if your position error is greater than a certain threshold.
That threshold is a function of Nav time in hours (since last alignment)
Position error or drift (unit is Nm) = deviation of each IRS position from the onside FMGC position.

Where can you get an idea of threshold values?
Go to FCOM 3.3.25 page 2:

CASE 1:
Flight time < 1.5H: drift at or below 5 Nm.
1.5H < flight time: drift below min(2T+2,22), T in Hours.
=> No further action action required
(That case should not trigger the "Check IRSx/FM pos" msg in the first place)

CASE 2:
Flight time < 1.5H: drift between 5 Nm and 7.5 Nm
1.5H < flight time: drift between min(2T+2,22) and min(3T+3,33)
=> Check on next flight, if still in this case or CASE 3, consider IRU as failed

CASE 3:
Flight time < 1.5H: drift above 7.5 Nm
1.5H < flight time: drift above min(3T+3,33)
=> Consider IRU as failed

Note: the actual thresholds that triggers that MCDU msg might be different from those (above) used to determine (post-flight) IRS drift check.

So if that happen during taxi...well it all comes to your commander's decision, if you are GPS-equipped, type of route flown, approach capability required...etc.
MEL is a good tool for your decision making in that case.
If you really want to align again...do it with ENG not running....


min(a,b): smallest element btw a and b