I'm not sure if it has technical relevance, I still am at a loss why we check the position of the ignition during every approach checklist iaw with airbus sop's.
I thank you for admiring my diligence but I'm just referring to airbus SOP's from the FCOM. I haven't seen you quote from any reference material yet?
Where do those 5 degrees come from?
I guess airbus could have designed something that eliminates this problem. After all in no-wind conditions the fmgs track and MH should be the same. The difference between MH and Loc can then be deducted from the correction that is applied during decrab. But again this would assume that the track calculation is correct and does not drift.
I think this FCTM quote reveals that the track line is subject to extra unreliablility's beyond magnetic variation. But maybe I'm reading into it.
SI-020 FPV
The FPV appears on the PFD as a symbol, known as "the bird". The bird indicates the track and flight path angle in relation to the ground. The track is indicated on the PFD by a green diamond on the compass, in addition to the lateral movement of the bird in relation to the fixed aircraft symbol. On the ND, the track is indicated by a green diamond on the compass scale. The difference in angle between track and heading indicates the drift. The flight path angle is indicated on the PFD by the vertical movement of the bird in relation to the pitch scale.
RELIABILITY The FPV is computed from IRS data, therefore, it is affected by ADIRS errors. An error may be indicated by a small track error, usually of up to ± 2 °. This can be easily determined during the approach.