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![aircraft localizer aircraft localizer](https://d3i71xaburhd42.cloudfront.net/b724074e2145c2331820c4d7642cf87ab55255c9/2-Figure1-1.png)
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That means that from full deflection to centerline is 1.5 to 3 degrees. The course width on the localizer is from 3 to 6 degrees, depending on the length of the runway. I’m guessing that we have adequate margins for the entire course width since we are not necessarily able to exactly maintain the centerline for the duration of the approach. I’m sure John can weigh in on what obstacle clearance is guaranteed in the design of an approach. That leaves terrain/obstacle clearance as the big consideration. There would probably be no legal consequences as I doubt that the controller is sitting at his scope waiting to detect that we have left 5000 feet before being exactly on the centerline of the localizer on his scope. The question at hand appears to be “What would be the consequences of descending before the needle is centered?” The question addressed ILS/LOC approaches.Ī typical clearance might contain language like “Maintain 5000 till established”. The AIM talks about being established on course for RNAV – “within 1 times the required accuracy for the segment being flown.”īottom line is that, in the US, it’s English but a good guideline is the one John mentioned when the needle deflection is 1/2 scale or less and your heading is aligned with the desired course (remember – the needle will be absolutely centered at come point as you cross a course by 90 degrees – that’s hardly “established” by anyone’s definition). The ICAO procedures document lists half deflection for an ILS and VOR and withing 5° of the required bearing for an NDB. The only official metrics are the one in the ICAO Document 8168 “Procedures for Air Navigation” and out AIM. Besides, the ATP PTS standard for the same maneuver is 1/4 scale deflection, so it’s a testing standard, not a definition. Yes, the Instrument PTS requires maintaining less than 3/4 scale deflection but that doesn’t tell us anything about what it means to become established to begin with. From the Pilot/Controller glossary, we can see it simply means “To be stable or fixed on a route, route segment, altitude, heading, etc.” In other words, simple, plain English (which is IMO why it gets debated so much! How dare the FAA use English! :D).Įven the PTS isn’t really helpful. What “established” means is one of those great debates. (If you had to have the needle exactly in the center, now how precisely would you define “center”?) But this is what I use, and what I teach, and I think it is the safest reasonable definition. You could easily be through the course quickly if you get distracted.īut once you turn the airplane so that it’s following the final approach course and are just trying to bracket the course, then you know that you’re “safe” and can descend to the next altitude.Īs John says, this isn’t published anywhere and there is no formal definition. If you’re intercepting a course at 45 degrees (like a procedure turn), just getting in to half scale deflection isn’t enough – you’re still pointing 45 degrees off the course and haven’t actually intercepted it yet. Your heading (or ground track if available) is basically along the final approach course. What I teach for any time you need to consider yourself “established” on a course (such as descending from a procedure turn, intercepting an ILS, turning onto a GPS final approach course, etc.) is that you are “established” when two conditions are met:Ģ.