Course selector discrepancy 737NG

[WhiteKnight]

Well, just had an interesting experience.
We were cleared for an ILS approach on intercept heading in a 737NG. Unfortunately we missed setting the correct inbound course in the course selectors, it was 30° offset to the published inbound course.

Well we got a VOR/LOC capture but the airplane just flew through the localicer. Well we managed to get quickly back on course but anyway, I was always under the impression that it really shouldn`t matter what you have in the course selector on an ILS approach. Why is the autopilot not just tracking the localicer inbound no matter what course you have dialed in?


Any ideas?
Thank you!

[john_tullamarine]

I suspect that the problem had naught to do with the course selected.

I don't know the NG so I can only speculate.

(a) intercept angle outside system boundaries - I've seen that one on previous occasions with other systems

(b) some other mis-switching or setup problem

(c) an ad hoc gremlin on the day

[Zeffy]

I don't know the NG, either -- but doesn't the course selector provide course datum to the FD?

[FCS Explorer]

:} always wanted to try that! but people keep telling me it's no good idea!

no seriously:
the FMS contains the correct front course for the ILS **BUT** there's no protection from selecting a wrong one on the course windows.
IF you've tuned the wrong ILS-freq the FMS will give you "RWY TUNE DISAGREE" (or if the freq was changed an the data base was not updated ;))

IF the two course selectors DIFFER the course will be crossed out yellow on both PFD's. but still there's no "correct course"-protection, merely a difference-thing.

[TolTol]

I think it does matter whats dialled in the course selector window. But it has to differ by more than 90 degrees or something. It gives incorrect guidance, eg left of runway centreline and the dot on the PFD will be to the left. I seem to remember being shown this during LVO sim.

[posi+iveLanding]

Not only in NG.
The same think can happen in classic as well.

[happybiker]

WhiteNight

There may be something in this previous thread to explain the effect.
http://www.pprune.org/tech-log/352043-crs-selectors-b737-800-a.html

[BOAC]

Endorsing that you read the other thread, WN.

In simple terms, for any A/P - F/D ILS, the receiver gets a 'left' or 'right' signal from the LOC. L or R of what? It has to have some datum. If you mis-set the CI, then its 'datum' is mis-set too. So, suppose you have the CI 45 degrees right of datum and you are closing from the left of the C/Line on the same 45 - the a/c gets a 'you are left of the C/Line' signal. All OK. Now as you approach the LOC so that the intercept geometry is triggered, the a/c will look at the numbers and say 'Cracking! I'm on the LOC'. Next it will get a hard fly left signal which will spoil its and your day. You can think of it in simple terms as being like a VOR intercept, although the 'mechanics' are quite different, but - which heading does the a/c turn to on intercepting? The heading set in the CI window?

I'm not sure what will happen in an LNAV intercept, since I have never been 'authorised' to do one. Why the 737, (Classic and NG) cannot put up a warning if the CI differs from the database QDM for the selected runway I do not know. Other a/c automatically set the ILS QDM on the CI when the runway is selected in the FMC - even easier!