IRS

rici

New Member
Hiya all!
I was wondering if you could help me with a quick question the IRS on the A340?
How is the IRS initialised? I know that there is a prompt on the MCDU on the INIT A page that says ‘align IRS’ but then there are the switches on the overhead panel as well. I have an A340 FCOM and it says that pushing the LSK next to the prompt sends the coordinates to the IRS and completes the alignment process. It says that this should be done 3 minutes before pushback, yet the alignment should take 10 minutes! So, I take it that when you move the IRS switches from ‘OFF’ to ‘NAV’ on the ADIRS panel, the IRS aligns itself and after 10 minutes all of the data on the PFD and ND becomes available. When the ALIGN IRS prompt on the MCDU is pushed, the coordinates are sent to the IRS and the FMGS now knows exactly were it is. Is this correct? Can you help at all?
Thanks
Richard
 

A300Capt

Freight Dawg
The process is similar to the A300. After selecting NAV on the individual overhead IRS units, the initialization process begins with a series of self testing and alignment process. Most interaction between the pilots and the IRS's is usually done between the FMS's. During battery only operation communication to the remaining IRS is done through the overhead ISDU.

Normally, the current known position of the aircraft is initially entered into one of the FMS's (airport, gate, ramp etc) in either Lat/Long format, a combination of alpha/num ID's and company route/active rwy. The FMC's cross-talk so you only have to enter the data into one since it will automatically synchronize the other. The IRS's already know what their last know position was after parking the aircraft on the previous flight.

We generally select a company route which are stored in the database of the FMC's. This gives a general starting point of reference for the initial alignment. We can narrow or enter a more exact point by inputing a gate or ramp position for a more precise alignment.

The aircraft shouldn't be moved or bumped during the alignment process which takes up-wards of 10min or the process has to start all over again. Once the alignment is complete the PFD for attitude reference and the ND (compass card and moving map) will become available along with a few other instruments which get info from the IRS's.

After selecting the TOGA switches on takeoff, the FMC's are "position updated" with the active rwy you've selected in your FMC route. The FMC's are auto updated through a triple mix of 3 IRS's and, for our aircraft, dual GPS.
 

rici

New Member
Ok, thanks for the reply.
So, correct me if I am wrong: The ADIRS switch is moved from OFF to NAV, they then start spining up, crosschecking one another and aligning themsleves. During this process the information on the PFD and ND is unavailable like in this pic:

http://www.airliners.net/open.file?id=371667&WxsIERv=QWlyYnVzIEEzNDAtMjEx&WdsYXMg=QWVyb2xpbmVhcyBBcmdlbnRpbmFz&QtODMg=UG9ydG8gQWxlZ3JlIC0gU2FsZ2FkbyBGaWxobyAoUE9BIC8gU0JQQSk%3D&ERDLTkt=QnJhemls&ktODMp=SnVuZSAxOCwgMjAwMw%3D%3D&BP=0&WNEb25u=UGF1bG8gUnVkb2xmIEhlcnJlbg%3D%3D&xsIERvdWdsY=TFYtWlBY&MgTUQtODMgKE=Q2FwdGFpbidzIHNpZGUgb2YgdGhlIGZyb250IHBhbmVsLi4u&YXMgTUQtODMgKERD=MTY4MA%3D%3D&NEb25uZWxs=MjAwMy0wNi0yMg%3D%3D&static=yes&size=L

When this is finished, the V/S indicator and ATT indicator become available as the IRS are running, but the ND is unavailable because they do not know were they are, like here:

http://www.airliners.net/open.file?id=369525&WxsIERv=QWlyYnVzIEEzNDAtMjEx&WdsYXMg=QWVyb2xpbmVhcyBBcmdlbnRpbmFz&QtODMg=UG9ydG8gQWxlZ3JlIC0gU2FsZ2FkbyBGaWxobyAoUE9BIC8gU0JQQSk%3D&ERDLTkt=QnJhemls&ktODMp=SnVuZSAxOCwgMjAwMw%3D%3D&BP=0&WNEb25u=UGF1bG8gUnVkb2xmIEhlcnJlbg%3D%3D&xsIERvdWdsY=TFYtWlBY&MgTUQtODMgKE=V2hpbGUgcGFya2VkIGF0IGdhdGUgMkMsIHRoZSBmaXJzdCBvZmZpY2VyIGlzIHByZXBhcmluZyB0aGUgY29ja3BpdCBmb3IgdGhlIGNoYXJ0ZXIgZmxpZ2h0IGJhY2sgdG8gQnVlbm9zIEFpcmVzIGFzIEFyZ2VudGluYSAxOTU3Li4u&YXMgTUQtODMgKERD=NTY2Mw%3D%3D&NEb25uZWxs=MjAwMy0wNi0xOQ%3D%3D&static=yes&size=L

So, then 3 minutes before pushback, the coordianates are sent to the IRS by pushing the 'align IRS' prompt on the CDU, after 3mins, the IRS know were they are and the ND becomes available. Then it the mix IRS is updated with the GPS position.
I think.
So is this right? (Sorry for the long links)
Richard
 

A300Capt

Freight Dawg
[ QUOTE ]
The ADIRS switch is moved from OFF to NAV, they then start spining up, crosschecking one another and aligning themsleves. During this process the information on the PFD and ND is unavailable like in this pic:


[/ QUOTE ]

Correct, except they work independently of one another. One IRS provides att/heading info to the Capt's instrument while another provides info to the F/O's. The third IRS is a backup to either pilot. The IRS's also check to make sure they can operate on DC power during the self-testing phase in case of AC power failure. In that case, normally two will drop off line after about 5 mins while the third will operate until the main battery dies.

[ QUOTE ]
When this is finished, the V/S indicator and ATT indicator become available as the IRS are running, but the ND is unavailable because they do not know were they are, like here:


[/ QUOTE ]

The IRS's provide attitude and heading information, among other things, so, until they are aligned and provide that info the PFD and ND they will not function. The VSI receives info from the IRS's which makes it more accurate, but if there is no IRS info it'll still work using the air data computer. It just won't be as instantaneous.

[ QUOTE ]
So, then 3 minutes before pushback, the coordianates are sent to the IRS by pushing the 'align IRS' prompt on the CDU, after 3mins, the IRS know were they are and the ND becomes available. Then it the mix IRS is updated with the GPS position.
I think.


[/ QUOTE ]

Not sure where you got this 3 minute thing you keep referring to. The coordinates are sent to the IRS's as soon as possible during the initial cockpit setup usually by the f/o. You want to have them aligned before pushback, that's all. Doesn't matter if it's 10 seconds, 3 minutes or an hour. just so long as they're aligned. You shouldn't do a "quick align" (doesn't erase all the drift errors)within three minutes of a full alignment. Maybe that's where you're getting the 3 minutes question.

The IRS's are never updated once aligned and will begin to drift over time. That's why it's important to input the correct coordinates initially. They know about where they are and will warn you if you put a gross error while inputing coordinates. The FMC's are being updated using a triple mix of IRS position and/or GPS position, whichever is present.
 
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