ils Localizer vs. mcp course selector
 

Hi ,

first of all I would like to thank you for your patience and knowledge. It was always a surprise how deep your a linked with the systems.

Yesterday we had a discussion in the flight deck about the Localizer beam and the Course selector.
Because the ILS Localizer is a single beam for a given Rwy, does the aircraft track the localizer also in case you have set the wrong course in the MCP, after LOC capture?

Cheers, Hoschi

This sounds a bit type specific. Both glass cockpit type I have flown didn't have a course selector on the MCP. The course is selected on the FMC CDU nas usually it's automatic with the selection of the approach. The LOC needs no course info, it is a pure fly L/R command and does not need heading integration as does the VOR. The only exception to this is when flying a back course ILS and then it's just to have the signal display reversed.

In an ol' fashioned steam gauge flightdeck or on the still common small planes the AP and FD will follow the LOC regardless what you dial in on the CRS selector. As you said it's a single beam.

You could even do it manually when flying raw data, but this requires a lot of menta acrobatics.

To bring the CRS in line with the published LOC course looks better on the display and aids in position awareness.

But if the CRS needle is set backwards the right/left commands work in reverse, very confusing!

"The LOC needs no course info, it is a pure fly L/R command and does not need heading integration as does the VOR. "

According to the Boeing Maintenance Manual, the A/P uses the selected CRS to refine its intercept calculations. For those with access to the manuals, use "delta runway track" as a search phrase.

The capture arc varies depending on the initial intercept angle. It may be a two arc capture if the angle is too great or too small.

Rgds.
NSEU

True.





True.





Exactly the reason why.




Well, strictly speaking a localiser is actually two beams (or "lobes"). They both are transmitted on the same carrier frequency (108.10 - 111.95) with the left hand lobe modulated at 90Hz and the right hand lobe modulated at 150Hz. Both lobes overlap each other down the extended runway centre line out to around 25 miles. When on centre line the ILS receiver receives equal amounts of 90 and 150Hz signals and your localiser deviation pointer will be centred. If you drift to the left then more 90Hz signal will be received and the ILS receiver will give you a "fly right" signal one your deviation pointer. Drift right and the opposite happens.

Glideslope works in a similar way. i.e two beams but obviously in the vertical plane.

- absolutely! If you have ever forgotten, like I have, to reset the ILS QDM after a hold and watched the a/p try to spear off into the bundu looking for a localiser, you will realise that. Leaving aside the appearance of the display, I guess that if one established on the Loc in heading select and then asked it to track the Loc it would, but I have not tried that. I have a suspicion that somewhere in the 'works' is an input bias of 'expected' track and that it might just fly an offset centreline, however.

We used to 'play' with the BAC Lightning ILS on the occasional 'offset' ILS beam and found that you could make the machine track down one edge of the cone and fly the centreline, but that was O L D technology:)

Correct. If you don't select the correct CRS the autopilot will track the localiser but offset to one side or the other. The bigger the CRS error, the bigger the offset. It will be trying to maintain the localiser with the selected course.

Well, I can tell you from real experience on the B737 that the a/p will initially try to follow the CRS selector on respective side (Left for AP1 and right for AP2). The captains side was roughly 140 deg off but mine was set. AP1 engaged and on intercept hdg when HDG turned to VOR/LOC aircraft turned the wrong way...

I believe the more important issue (and the reason QDM is checked on the glass cockpit) is that the flare laws and particularly roll-out in auto-land leave a lot to be desired if the wrong figure is set for QDM....

Okay tried it out during approach with LOC captured (737 NG) and turned the Course selector 10degrees offset. The Autopilot turned the aircraft in the direction with the LOC centered.
So you can fly an Localizer with a wrong course, set on the course selector. Is it right?

Never tried it in the NG but during a runway change in the classic the respective course selector was set 180° wrong. Not a pretty sight on intercept, it tried to turn outbound instead of inbound at first, of course we just switched everything off and flew manually but i dont want that to happen in marginal weather with some problems on board, so since then i allways double check the course selectors.

And that was the right thing to do. It also makes a mockery of those automatic monkeys who state loudly that manual skills in today's airliners are a thing of the past and that only full automation will stop accidents.

Steady as she goes Tee Emm!
some my counter your point by stating that the automatics would have set the correct course!!

Here is a report on a serious incident on Nov 23rd 1997 with an MD83.
http://www.bea-fr.org/docspa/1997/f-...-mc971123a.pdf

The main issue has more to do with not monitoring properly the vertical path.
However the selection of a wrong CRS when intercepting the ILS and the subsequent AP inability to intercept the localizer were key factors in the incident.

Within the course of this investigation, the manufacturer has researched what is the maximum erroneous ILS course selection from which the localizer capture is no longer possible.
This is 40° on MD83.

Please someone confirm my menory yes/no before I go and try it again: NG; HDG SEL on intercept HDG; wrong CRS set for LOC; VOR/LOC armed; a/c did not capture LOC on reaching centreline.