View Full Version : 744 Fmc - Q

cool blue
4th Apr 2002, 23:50
Iím puzzled by this photo of a 744 EADI/PND. 3rd Photo down at: http://www.aerowinx.com/html/gallery03.html
Could somebody answer the following, as Iím not FMC experienced [yet] but would like to understand whats going on.

1.Is the white inverted triangle the actual a/c hdg?
2.Is the pink bug a selectable FMC hdg bug, and therefore the pink line from GGC the predicted track that it would fly if it followed the active route beyond GCC?
3. Am I correct in thinking that the FD is on [ie cross-hairs visible], the AP is engaged [ie CMD-green], and the FMC is active[ie LNAV+VNAV-green]?

By what means then could the trend vector be indicating such a left turn, the MAG-TRK not be consisitent with the FMC TRK, and why is the a/c not following the active route? Have I missed/not understood something[probably:D]. Could this all be due to the turbulence?

5th Apr 2002, 00:09
Yes, Yes and Yes! but can only download pics. 1 and 2, probably a problem at my end.

5th Apr 2002, 03:15
2. The Pink (Magenta- as Boeing calls it ) bug is the HDG bug from the Mode Control Panel (MCP). ie in picture 4 the HDG bug (top left) reads 132 on the MCP and would read this on the ND and PFD.
In pic 3 it reads 118 on the PFD therefore would read 118 on the MCP, spin the HDG bug on the MCP and the the Magenta bugs will follow it on the ND/PFD. It is unrelated to the FMC ie raw data.

The Magenta lines are what the FMC is programmed to follow while in LNAV (if I remember correctly they are actually Great circle routes between waypoints), the Magenta HDG bug would be followed if in HDG SEL ie inverted triangle over Magenta hdg bug.

The long thin white line is actual A/C track and should lay over/under the magenta line when on track. ( BA use HDG up instead of TRK up therfore pic 3 ND would be rotated approx 3 degrees left with HDG 111 MAG displayed at the top).

The turbulence as you said probably caused it to go off track and the A/P is making a correction to return to track, you can see how far off track you are on the FMC(not shown) ie. R 0.2nm

Hope this helps.

cool blue
5th Apr 2002, 23:18
Ah yes - the 'penny' has finally dropped. A combination of surprise that turbulence could 'show' up as badly as that, plus the MAG-TRK being 'in the wrong place' [to a steam pilot:D] ie up the page, and the[my] 'FMC' bug in fact being an MCP bug - was what the trouble was all about. Sorted. Many thanks - both of you.

Ford Airlane
6th Apr 2002, 05:50
I doubt it was turbulence, the second officer was probably flying.

;) ;) :p :eek: