IR Alignement Architecture
 Figure 018

 (3)
 One MSU is set on NAV mode later The alignment is not achieved on the other IRS. If the position has not yet been sent to these IRS, we are in the first case, which is explained above. Else the position which has been sent to the other IRS (via MCDU or via ISDU), is also sent automatically to the third IRS from the FM. The alignment is achieved on the 2 IRS, thus a position has to be sent to the third one, which is now in Alignment Status. In this case, the ALIGN IRS prompt is displayed on the MCDU and may be cleared after either a pressure on the 3 RLS key on INIT A page (ALIGN IRS prompt selected) or a position entry on the ISDU. After one of these two actions, the calculations of the third IRS can be achieved.

 (4)
 Position modification This modification may be done in the FMS by slewing actions on the INIT A page. The modification is done before the previous position has been sent to the IRS; we are in the first case. A position has already been sent and alignment is not yet achieved, then the ALIGN IRS prompt re-appears and the calculations go on until the new position is sent either via the MCDU (by pressing the prompt) or via the ISDU. The alignment is achieved. If the modification is done on the INIT A page, it means the crew needs to modify the reference position; on the MCDU, the RESET IRS TO NAV message is displayed in the scratchpad. This message can be cleared either by a clear action on the MCDU or by resetting one IR at least in NAV mode. Then the ALIGN IRS prompt is displayed on the MCDU until the new position is sent (either via MCDU or ISDU). If a position is modified on ISDU, and no IRS is set on NAV mode, this action has no effect. It is necessary to set one of the IRS on NAV before modifying the position on ISDU; thus on the MCDU, the ALIGN IRS prompt is displayed and may be cleared as already described.

 (5) Reset of one MSU at least in NAV mode R This is the case where one IRS in NAV mode is set off and then back R to NAV mode (NAV - OFF - NAV), while the calculations are achieved on R the three IRS. The ALIGN IRS prompt re-appears on the MCDU and new
 IRS alignment is initiated.

 (6) Fault detection
 (a)
 Align fault Anytime one of three IRS indicates that an ALIGN FAULT has occurred (i.e. bit 21 of label 270 is 1), the alignment prompt is redisplayed on the INIT A page allowing the pilot to attempt realignment of the failed IRS. The text of this prompt is REALIGN IRS. Upon selection of this prompt, the position indicated in line 4 of the INIT A page is reburst to the IRS.

 (b)
 Misreceived data If any one of the three IRS echoes a latitude and/or longitude (on label(s) 041 and 042, respectively) that is not identical to the position displayed on the INIT A page, then the echoed latitude and/or longitude overwrites the respective display in line 4 of the INIT A page. The IRS priority order for the echo to overwrite is: IRS OWN, IRS3, IRS OPP. It is, then, the pilot responsibility to review and resolve all discrepancies.

 (c)
 Non-received data If a position is burst to the IRS from the FMS and an IRS does not reset its NO IRS initialization discrete to 0 and an ALIGN FAULT has not occurred on that IRS, then the FM takes no further action. A status message, ENTER PPOS, still exists on the IRS MONITOR page for the un-initialized IRS. It is the pilot responsibility to resolve the problem by re-entering the position or by switching the indicated IRS from NAV to OFF to NAV again.

 E. IRS HDG Initialization (if OPC option activation)
 (1)
 General When any one of three IRS is set to ATT mode (bit 12 set to 1 on label 270), the pilot must initialize the appropriate IRS with a heading. This initialization may occur in flight or on ground, in case of failed IRS in NAV mode. The entry is performed on the MCDU or on the ISDU, if an optional ISDU is installed. Due to IRS drift while in ATT mode, it is possible to re-initialize any IRS in ATT mode. Re-initialization is manually specified at pilot discretion.

 (2)
 MCDU initialization The MCDU initialization is only allowed if "IRS HDG initialization option activated" is set to Y. If not , the MCDU field selected to the SET HDG function will remain blank as long as the HDG has not been initialized through the ISDU. When ATT mode is selected for any one of three IRS, the MCDU prompts the pilot for heading initialization by displaying a SET HDG field with amber boxes on the IRS MONITOR page. Entry of a heading into the SET HDG field results in the burst of this entry on label 043 to the IRS.

 (3)
 ISDU initialization

 (a)
 General When a heading is entered via the optional ISDU (for first or subsequent initializations), the IRS are initialized directly by the ISDU. The ISDU entered HDG is echoed to the FMGC on label 043 and, thus, is displayed on the MCDU in the SET HDG field of the IRS MONITOR page. Normal SET HDG display rules then apply.
　
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