I don't get it, Penko. From what I see people seem to think that the aircraft can fly a ground track that is different from the localizer course while the aircraft is locked on to the Localizer and that this is evidenced by the track not matching the Localizer course. Which would then lead to the aircraft mowing the grass.
While in fact it is just evidence that the FMGS is not calculating the ground track accurately, thus that the GPIRS has drifted and is not giving accurate information.
In answer to 737jock, the aircraft indeed aligns the IRS/FMGS computed track with the course you check at 350. Which other track can it use?
Penko: The drift is the difference between the ground track and the heading/axis (nose), drift is what is removed during decrab maneuvers through the ALIGN SUB-MODE (as you said DSC 22-30-80-30 Precision Approach Modes)
Thus the aircraft aligns the Nose of the aircraft/axis (heading) with the course of the localizer during decrab!
OR does it align the nose with the FMGS computed track which is assumed to be equal to the LOC course? In thiscase it would obviously be very important that the FMGS track matches the LOC course, as the aircraft would not use the LOC signal during decrab.
However if the aircraft would align the FMGS track with the course of the localizer it would not be doing anything (certainly not decrabbing), as on serviceable aircraft the FMGS computed track is equal to the LOC course. .
Once the aircraft has captured the Localizer it adjusts the heading to maintain the localizer course/localizer ground track.
The FMGS updates it's lateral position when using the LOC beam during an ILS approach and for quick update in GPS/IRS mode(DSC 22-20-20-10 Radio Position and DSC 22-20-20-10 Navigation Modes)
So my concern if any would be that if the FMGS track does not match the LOC course at 350ft is my displayed heading actually correct? Does the aircraft know the direction the axis is facing? Or is it off by X degrees as well?
People are saying that if at 350RA the FMGS track deviates more than 5 degrees from the RWY QFU/LOC course (we are not discussing offset approaches here) than we should go-around or fly to CAT2 minima for a manual landing. I don't get it:
PRO-NOR-SRP-01-70:
AUTOLAND
CHECK that the FMA displays the aircraft capability (CAT 2 or CAT 3) for the intended ILS approach. MONITOR the radio automatic callout.
At 350 ft RA: CHECK that “LAND” is displayed on the FMA. CHECK ILS course. If LAND is not displayed or if the ILS course is not correct, do not perform an autoland. The flight crew should perform a go-around, if visual references are not sufficient.
Between 50 and 40 ft RA: CHECK that “FLARE” is displayed on the FMA.
At approximately 30 ft RA: CHECK that “IDLE” is displayed on the FMA, and that autothrust starts to reduce thrust toward IDLE.
At 10 ft, “RETARD” callout comes up: MOVE the thrust levers to IDLE. Autothrust disconnects.
So where does the FCOM refer to checking the Track vs the ILS course?