ILS course and frequency question

[Tim013]

This is more of a developer/Jan question;

In a simple aircraft with, say, a BendixKing KX-155, dialing in the localizer frequency will allow you to track the localizer, regardless of how you have the OBS set.  In other words, if the ILS front course is 275 degrees, even if you have the OBS set to 010, the KX-155 will still track the localizer, and the needle will center when you fly the correct heading.  The rotating OBS setting means nothing to the receiver, and is irrelevant.

My question is this.  In the IXEG 737-300, what role does the course setting on the MCP play?  If I have the correct localizer frequency tuned in the VHF radio, does the course setting in the MCP have any relevance, or is it just a reminder to you what the published course is?  If the published front course has to be dialed in, how far can you deviate from  the published course for the A/P to still track the front course?  Is there a front course + or - a certain number of degrees wiggle room for the A/P to capture and track?

 

Tim

[mmerelles]

The ILS antenna produces 2 lobes, one to each side of the centerline being overlapped at the centerline. Left side of the centerline is modulated at 90Hz while right side of the centerline is modulated to 150hz. By the received signals the aircraft known being flying on the right, needs to go left, or being flying on the left zone needs to go right. When both signals equals it knows being on the centerline.

Course selector is to capture and to know to which track to align initially (course is not available on the signal) so how the aircraft will know to turn to or away from the runway?

Also course selector is useful to refine A/P movements because it has a target to compare when the signal says right or left? yes but how far?

 

On modern avionics, course comes from the navdatabase automatically, but on the classic pilot needs to manipulate the course selector.

 

Hope this clarifies. Captain Jan can surely dig further on this.

[Tim013]

That is very helpful.  So it is needed for the AP to fly the couse, and the closer the course is set to the published course, the better.  And if you are hand flying, then course setting doesn't matter, and you will still be able to follow the raw data of the localizer and glide slope in the form of the magenta diamonds.

The reason I ask is I was watching a streamer who has a basic understanding of flying IFR, and he forgot to set the course for the ILS.  Upon LNAV intercepting the localizer, the AP then proceeded to turn away from the localizer.  He had the approach course dialed 150 degrees off of the correct course, so it got me thinking about "modern" AP's, FMC's, and PFD's, with regard to the raw data of a simple VHF receiver and pilot controlled OBS knob.

 

Tim

[Litjan]

Mmerelles has it right. The indication will be "on centerline" if you are on centerline, no matter what the CRS is set to. However, the autopilot needs the CRS information initially to make the correct turn when capturing.

You can try and deliberately set the course "a little bit" wrong, maybe 20 degrees - the plane will still track the centerline perfectly. The principle is the same as if there was a strong crosswind.

Cheers, Jan