B744 Flight Director Approach Mode

[SB4200]

G'day there

Re: B744...

I have noticed slight discrepancies in the sim between FD and LLZ indications on the ILS with the FD in APP mode. For example the aircraft is on LLZ with a slight fly left FD command.

I have looked in the applicable manuals to clarify my understanding of the system to no avail, which is not to say it isn't there!

In this case the QDM was manually tuned and I suspect was 1 degree out. My understanding is that this would account for the discrepancy. Correct?

Additionally, what factor determines the amount of FD lateral deflection? Is it simply distance from desired track? Does the rate at which one is deviating from or approaching the desired track have any influence?

Comments from other Boeing products are welcome as I suspect the systems are similar if not the same.

Thanks in advance

[Short_Circuit]

possibly because when manually selected you have to enter RWY HDG. So for eg. RWY 250 may actually point 249 deg thus a standing HDG error whilst on LOC. The Sim may not be so cleaver and just use 250 deg as RWY 250 HDG (as it is programmed not real mag tables)
S_cct

[Pontius]

Probably (but not definitely) because the FD was taking wind into account, whereas your LOC display is simply telling you your relative position to it. You were on the LOC but with (in this example) a wind from the left you wouldn't have stayed there without aiming left a bit, as per the FD.

Clearly the LOC QDM in the NAV/RAD page should match the QDM on the approach chart otherwise it will make a nonsense of the LOC display on your EHSI/EADI. You may well have the LOC needle in the middle (i.e. that being transmitted from the ground and aligned to the runway) but if you were to match up the track line and the incorrect QDM that you have manually entered, then obviously you will drift off the 'real' course being beamed up from the ground.

I like to adopt a 'big picture menatlity' and explore both sides of LOC deflection scale, thereby giving the examiner something to talk about whilst at the same time ensuring my mistakes with wrong QDMs etc are covered up :)

[SB4200]

The 'big picture' - I like it. :)

I have found a note here in the ops manual which relates although is not exactly what I'm referring to.

Note: After localizer capture, flight director roll commands may appear inconsistent with A/P roll manoeuvres for one to two minutes.

Hmmm... :hmm: