(b)
Re-initialization The pilot may re-initialize the IRS in ATT mode by entering a new (or re-entering the existing) SET HDG via MCDU when IRS HDG initialization option activated is set to Y, or ISDU.
(c)
One IRS is set to ATT mode later If an IRS is set to ATT mode from a different mode and a displayed SET HDG exists, this heading is not automatically burst to the IRS in ATT mode. The pilot must enter the SET HDG value via the MCDU (on the corresponding IRS MONITOR page) or ISDU in order to initialize the new IRS.
(d)
Fault detection Unlike IRS alignment, there is no fault indication for heading initialization. However, if a difference is detected between the displayed SET HDG value, on the IRS MONITOR page and the value echoed back from the IRS on label 043, the echo is displayed. The IRS priority order for the echo to overwrite is: IRS OWN, IRS 3, IRS OPP. The pilot, then, has the responsibility to verify proper initialization of each IRS. If a SET HDG is burst to the IRS and the IRS do not receive the burst, the FM takes no further action. The ENTER HEADING status message still exists on the IRS MONITOR page for any un-initialized IRS. The pilot, then, has the responsibility to resolve the situation by re-entering the SET HDG value.
F. Average Drift Computation The FM computes an average drift on the ground at the end of the flight, for each IRS. This drift is then displayed on the IRS page.
(1) Computation principle
At the end of the flight, after landing (i.e. with aircraft on ground R condition (main landing gear pressed)), the FM takes a snapshot of:
-each IRS position (for IRS in NAV mode)
-landing time
-FM position
-aircraft heading. The FM then computes for each IRS the total drift, which is the difference between the IRS position at landing and the geographic landing position. This geographic landing position is the position of the destination runway threshold plus a shift of 400 m in the axis of the runway. (This can be considered as a good approximation of the landing point). The FM then computes the time since last align which is: landing time-starting time. Starting time is the time at which the first one of the 3 IRS transitions from ALIGN to NAV mode. The average drift computed for each IRS in NAV mode is then:
Total drift -----------------------------------max (time since last align, 1 hour)
NOTE : This assumes that the pilot has performed the proper IRS
____ initialization. The FM does not verify proper initialization.
(2)
Reasonableness test The average drift for a given IRS is not computed if any of the following conditions are true:
-There is no destination runway selected in the flight plan.
-The IRS is not in NAV mode. This assumes that the aircraft has landed on the runway selected in the F-PLN. The runway identifier is however reminded on the IRS MONITOR page.
(3)
Operation The IRS drift value displayed on the IRS MONITOR page is the computed value for the last landing; that means if the aircraft performs one or more touch and go's, the displayed average drift value is the computed value for the last landing, before DONE phase. In dual, if one FM does not compute any average drift, the value computed by the other FM is displayed.
In independent operation, each FM displays its own computed average drift values for each IRS.
G. GPS Initialization
(1) General Since the GPS uses the IRS position (label 310/311) for position initialization, no particular process has to be done by the FM. The GPS is initialized by the IRS alignment process. As soon as the clock is valid the clock data (UTC, date) are sent on the FM GEN bus. Since the GPS can be initialized without clock data, no particular process nor MCDU mechanization is provided if the clock is invalid. In that case the time needed for GPS to init obviously increases.
H. Leg Transitions
(1)
General The FMGC guides the aircraft in both lateral and vertical axes. The lateral guidance is responsible for the construction of a lateral path of the ACTIVE primary flight plan. This lateral path consists of the active leg path geometry to be used to control the aircraft and the multiple leg geometry which is used for generating EIS displays. The lateral path is made of geometric legs and transitions between the legs. The available types of legs for which guidance may be provided are defined in 22-71-00 Para. 2.A. and are as follows:
-great circle course
-planar magnetic course
-magnetic heading
-circular arc (AF leg, HX legs).
(2)
Transition types
There are five types of transitions.
(a)
Type I is a curved path transition between two fix referenced legs (FX or XF) that terminate and begin at the same point. (Ref. Fig. 019) It is performed with a constant turn radius. The nominal bank angle is then corrected to take ground speed evolution into account.
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