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Ils approach height

Asked by: 540 views Commercial Pilot, Instrument Rating

on an approach to an ILS facility, ATC clears the aircraft initially to 5000' , the platform intercept alt of the ILS (FAF) is at 2000' The aircraft has captured the localiser. There are no procedures or height constrictions between the aircraft and the FAF - thus on a long final.

ATC now clears the aircraft for the ILS approach. What is the more correct procedure- to stay at 5000' (at this stage still below the glide slope) and intercept from 5000' or to descend to 2000', and intercept the glide slope from the published platform height?

1 Answers

  1. John D Collins on Nov 13, 2017

    The normal service volume of the GS is 10 NM from the threshold. The typical GS intercept altitude will intercept the GS in the vicinity of the FAF, usually just before crossing the FAF. Intercepting the GS when 3000 feet above the GS intercept altitude, puts the aircraft approximately 10 NM outside of the FAF or 15 NM from the threshold and well outside the standard service volume of the GS. So as long as you are on a published route portion of the procedure, you should be able to descend to the GS intercept altitude and intercept the GS from below at the GS intercept point shown on the chart.

    Intercepting the GS outside of the service volume may not work and you might not get a usable GS indication at the point the theoretical 3.0 degree GS intercepts the 5000 foot altitude. You may find yourself above the GS when you get a reliable indication. Assuming the following: Runway elevation 335 MSL, GS 3.0 degrees, TCH 50 feet, FAF 5 NM – GS elevation 1999 MSL, 10 NM GS elevation 3570 MSL. So at the point you were getting a reliable signal, you would be 1430 feet above the GS at 5000 MSL.

    I don’t have an issue with using the GS to descend if you are inside the service volume, intercepting the GS from below, rather than doing a 500 foot descent to a GS intercept altitude, but intercepting the GS from 3000 feet above the GS would not be my choice for the reasons stated.

    You also implied that there were no intervening step downs on the final approach course. Just to be clear, if there were, the GS may not be followed if it puts the aircraft below any of the step down minimums outside of the GS intercept point.

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