IRS Position

[rich49]

Hi all,
Can someone answer a quick question on IRS alignment on the A340? When you push the 'ALIGN IRS' promt on the CDU, does this start the 10 min align process of the IRS or does it simply send the coordinates from the CDU to the IRS which are already aligned? I ask because I always thought that when you moved the switch from OFF to NAV on the ADIRS panel, the 10 min align process started and then 3 min before pushback, the 'ALIGN IRS' promt was pushed on the CDU to send the coordiantes form the CDU to the IRS. So how exactly is the IRS aligned?
Cheers
richard

[Rynopilot]

Richard,

When the "ALIGN IRS" respective key is pushed the coordinates in the LAT and LONG fields are sent and it completes the alignment process.

Best Regards,

[oxford blue]

I'm not familiar with the operation of the A340 system, but with most modern FMS/IRS combinations, when you put the initial airport ICAO identifier in the "Position Initialisation" page, the IRS uses the airport ARP as an initial present position for alignment. That gets the 10 minute alignment sequence going. The fact that the lat and long might be a mile or two out is not significant for the purpose of alignment.

At some later stage, but before take-off, the FMC position is updated to an accurate present position. In the Boeing systems, for instance, this happens when the throttles are advanced for take-off by operation of the TO/GA lever. The lat and long of the runway threshold is automatically passed to the FMC at commencement of the take-off roll. The raw IRS position stays a couple of miles out for the whole flight, but it is never used in its raw form. The updated FMC position is then kept up to date by a Kalman-filtered combination of IRS velocities and DME/DME fixes, or possibly GPS mixing.