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In what situation will an aircraft show you "LOC*" in FMA? What signal does the aircraft received when LOC*?(how does the aircraft know it is on LOC*) And which heading will the aircraft choose,is there any formula for the heading?

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  • $\begingroup$ The aircraft is a machine, incapable of making a choice. Generally the pilot will choose the heading to fly. The formula is zero plus the number of degrees of the chosen heading. $\endgroup$ Sep 20, 2022 at 14:33
  • $\begingroup$ You're asking multiple questions at once, some of them quite broad. I suggest you do some reading on Instrument Landing System or the Localizer and then come back to ask specific questions when something is still unclear. $\endgroup$
    – Bianfable
    Sep 21, 2022 at 8:40

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The Localizer signal is the horizontal (lateral) component of the full ILS signal (top portion of the image). It consists of two "lobes" which are amplitude modulated from each other, allowing the receiver to distinguish between left and right. The FMA reporting LOC has identified what it believes to be the ILS localizer signal, but NOT the glideslope signal (which is the same technique, but rotated 90-degrees so that the system is able to distinguish up from down, instead). You have side-to-side guidance along the runway centerline, but no guidance as to what altitude you should be flying at.

LOC* (as opposed to LOC) specifically refers to the "capture" phase. It's mechanically no different from LOC mode, but having a specific mode for the capture phase allows for a different set of logic to control the capture vs. the hold phase of the approach. The capture phase essentially exists between the moment you pick up the ILS signal until you've stabilized on the centerline the first time. After that I would expect the FMA to say "LOC" instead of "LOC*".

As to how the aircraft will maneuver itself in response to that signal, that will vary by aircraft model, possibly operator, and possibly even specific configurations of a given aircraft. Generally the heading for the capture maneuver is whatever vector you're given by Approach, until you've intercepted the beam itself. At that point the aircraft is turning to match the approach heading for the given runway but will shift as needed to stay centered on the beam. The maths for that are fairly complex. But if you're curious, here's an example method using H-infinity synthesis, if you want to dig into it. enter image description here

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  • $\begingroup$ "LOC"=in the centre of the two "lobes". "LOC*"=in one lobe,and at the edge of the other one,is that correct? is it the same logic for glide path? $\endgroup$ Sep 20, 2022 at 15:05
  • $\begingroup$ "LOC" = have been in the centre of the two lobes at some point; "LOC*" = consistently in one lobe and haven't yet seen the centre of both. I would expect it's similar for glideslope, but the consequences of missing the GS too much downwards are MUCH more likely to be severe than right or left so I'd wager the programming is much more conservative in how it executes. $\endgroup$ Sep 20, 2022 at 18:52

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