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
Last edited by shortfuel; 19th July 2010 at 00:41.
Reason: explanation