>Two of them I like to know about are "NAV"
Autopilot navigates using flight plan data. Most commonly limited to path-over-the-ground [no vertical guidance control]. What actually computes this ground track varies with installed equipment.
>and "HEADING HOLD".
Autopilot will hold the current heading without compensation for wind or ground track. Can be either manually set heading or whatever heading the autopilot senses when it rolls wings level --again, it depends upon installed equipment.
>How about GPS holding? Is that what the button is for?
Personally, I've never heard/seen this function. Suspect it is just another form of Flight Management System functionality that permits creation of a holding fix based upon GPS data only.
>NAV hold I assume is used for ILS landings,
>to lock on to the localizer.
I've never seen/heard of "NAV hold" either. ILS approaches normally have a function switch(es) labeled "ILS"; "LOC" and "G/S"; or "APP" (for approach).
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