Re: how does the ILS really work on the B757/767

Date:         06 Apr 99 11:46:18 
From:         Ralf Sipple <sipple@gmx.de>
Organization: Sipple Aviation & Engineering
References:   1
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Paul MacDougall <pmpub@simcoe.igs.net> wrote:
> just wondering if anyone is familiar with the inner workings of the ILS
> system and how it integrates with the FMC....ie if you dial in the correct
> localizer frequency but mis-set the front course setting how does the system
> work or not work... does it matter if you mis-set it only 10 degrees verses
> 80 degrees....my AOM is not very helpful and I know this system has screwed
> a few people...

sorry, don't know, but I have a similar question:

Today I was flying an ATR72. We were cleared ~30 nm from FCO
to intercept the localizer 16L and as the the indication
this far far out is often somewhat inreliable I decided to
follow the extended centerline with LNAV. I monitored the
ILS indication on the captain's CRT (in the ATR you have
either RNAV or conventional NAV indication, but not both)
and confirmed by that that I was precisely on LOC and GS.

Closer to the field I switched my display to L/V (conventional NAV).
This caused the LNAV mode to go off, the AP was in basic lateral
mode (maintain bank). I armed APP. The course selector was
set 50 deg off to the right fron the correct course (162deg).
As I was already precisely on LOC and GS they both captured
immediately. But then the unexpected happened: The A/C banked sharply
to the right (the direction the course selector was pointing to)
and came off the LOC to the right. I then turned the course
selector to the correct approach course and the AP re-intercepted
the LOC.

It appeared to me that the AP used the set course as an initial
value for a heading which may be appropiate to follow the
LOC (in this case it wasn't).

I once discussed a maater with a B737 captain. He said that
his company rules called for flying an approach with the ND
in APP mode. He said he likes to keep the ND in MAP mode
to improve situational awareness. The reason for the rule
to have APP mode displayed is that he couldn't see the course
dialed in MAP mode.

However, the deviation signal from the localizer (opposed to VOR)
is completely indepent fom the selected course.

Has anyone tried similar things? How does your plane behave?

Viele Gruesse,

Ralf

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