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To do a VOR, ILS or RNP approach, I am turning to establish the final approach course. The approach chart shows I should descend to a lower altitude for intercepting the final descent path.

So, my question is how many degrees deviation from the defined final course can I have to start this descent?

Is 0.8 degree for ILS, 5 degrees for VOR and NDB, 0.1NM for RNP? And the same issue when intercept a VOR radial. Where can I find the official answers about those?

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  • $\begingroup$ I think it is within half-scale deflection (whatever type of approach) and within 30 degrees heading of final approach course. [This could be wrong, I'm looking into it]. Also in what context - ICAO? FAA? EASA? $\endgroup$ Commented Sep 16, 2021 at 10:59
  • $\begingroup$ It would depend on air traffic instructions (ask), local conditions, and traffic (fly where others fly). You are describing a common descending turn. For final approach it would seem logical to get on the glide path ASAP, as the longer one waits, the worse it can be. $\endgroup$ Commented Sep 16, 2021 at 13:17
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    $\begingroup$ Versions of this question have been around for many, many years without a definitive/conclusive answer. Diverse aircraft course guidance equipment (various designs of CDI's, FMS, Flight Directors, etc.) and many types of approaches available (GPS, LNAV, RNP, ILS, VOR, etc.) make a specific answer (one size fits all) difficult. The basic requirement is to remain within the obstacle protected airspace designed for the approach. How you determine the boundaries of the obstacle protected airspace using a "one size fits all" methodology may not be possible. $\endgroup$
    – user22445
    Commented Sep 16, 2021 at 18:48
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    $\begingroup$ Not all courses have angular (degrees) deviation. For example, enroute GPS courses are fixed-width unlike a VOR or Localizer scales that narrow down at a fixed angle as the station is approached. $\endgroup$ Commented Sep 16, 2021 at 20:36

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You can descend to the minimum altitude for a segment as soon as you are established on that segment.

However, I can’t find a definitive answer as to when exactly you become established. Some folks say less than full-scale deflection while others say at half-scale deflection. I’m not sure how clearly it applies, but the ACS standard is that you must maintain no more than 3/4 scale deflection, so it seems like half-scale deflection is the safest choice.

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  • $\begingroup$ At least for RNAV/RNP, in AIM 5-5-16, "Definition of 'established'" is defined. $\endgroup$
    – user14897
    Commented Sep 17, 2021 at 14:16
  • $\begingroup$ @ymb1 FAA regulation is not universal for all procedures globally. Don’t presume the person asking this is operating in America. $\endgroup$ Commented Sep 18, 2021 at 6:01
  • $\begingroup$ @ob318: Please don't presume that I presumed OP is asking about USA. I did not post an answer, or direct my comment at OP. I always look for country tags when doing so, and I had already seen you've asked OP for jurisdiction clarification, but no reply as of yet. On the other hand StephenS is well-versed in FAA regs, so my comment was just a tip that such definition exists in the AIM at least for RNAV/RNP. Hope that clarifies it. $\endgroup$
    – user14897
    Commented Sep 18, 2021 at 13:16

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