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Elsmoko

T7 Premature Localizer capture ?

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  Perhaps just imagining; but seems that after arming Loc, that the " LOC " (captured) annunciation appears earlier

than it should.   Better than never I spose' ~:)  

   Changed to this (seeming) early capture behavior about a month (20+ flights) ago.

Captures before the deviation pointer shows any sign of life and at ~10nm from threshold and ~3nm from runway

centerline (extended) and on 30 degree intercept.

  Is there an easy way to test / verify correct localizer capture. 

Best to all,

Ken Boardman

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at ~10nm from threshold and ~3nm from runway

It sounds normal to me. I don't know of anyway to test it. It seems that the runway and runway threshold should be about the same distance from the A/C, not a difference of 7nm, Maybe I am misreading what you wrote in the quote.

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Thank you for the reply Michael.

To perhaps clarify (or add to the cornfusion) > the "3nm" is not from the runway / airport;  it is 3nm from the extended runway 

centerline on a 30 degree angle of intercept to this extended RW centerline,  with the intercept occurring ~7nm from RW threshold.

  Perhaps, the more (most) puzzling symptom of the possible problem is the appearance of the bold green  "LOC" (capture)

annunciation on the PFD prior to any movement of the localizer deviation pointer (bottom of PFD).  Correct me if I'm

mistaken;  but shouldn't the Loc. deviation pointer be alive and within limits (~2.5 degrees from centered) before capturing

the Localizer? 

Ken  

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Someone who knows the 777 systems better than I could confirm; but I expect the LOC mode engagement is based on closure rate with the localiser, so the faster you are going and/or the greater the intercept angle, the earlier LOC will engage, very possibly before the needle has moved from full scale deflection -- conversely with a very low closure rate LOC may not engage at all (eg if you are in LNAV and the magenta line parallels the localiser track without actually crossing it).

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There's a difference between LOC engagement and LOC capture. LOC doesn't capture until you are more or less lined up on the centreline, but LOC is annunciated on the FMA when LOC mode engages and takes over steering from the previous lateral mode.

 

Also remember that the width of the localiser beam increases with distance from the threshold. So when you are 20 miles out you will be further from the centreline when LOC engages.

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There's a difference between LOC engagement and LOC capture. LOC doesn't capture until you are more or less lined up on the centreline, but LOC is annunciated on the FMA when LOC mode engages and takes over steering from the previous lateral mode.

 

This.

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Based on the previous two replies,  I believe what I may be experiencing is premature LOC capture.

Just wondering if anyone else has reported this. I am aware that width of localizer beam widens

(and sensitivity decreases) as distance from the transmitter increases.

Ken

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I believe what I may be experiencing is premature LOC capture.

 

You didn't understand Kevin's response.  If the bird captures and tracks the LOC then it is not broken.  Another way of saying what Kevin said is that LOC mode is active doesn't mean you are going to turn towards final course.  It will hold heading until it is time to turn to final, then LOC is captured.

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I believe what I may be experiencing is premature LOC capture.

 

No.

 

What you're experiencing is a normal operation of the aircraft. As Kevin mentioned, I confirmed, and Dan further explained:

 

The light status or FMA changing is an indicator that the aircraft is now primed for that mode to take over. There's no such thing as a "premature LOC capture." That makes no sense in the slightest.

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