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2 Answers

Timing on an ILS approach

Asked by: 2757 views Instrument Rating

Hello I have a question, when shooting an LOC ILS approach. Normally the FAF for the LOC part is at the same point where I intercept the glideslope. Should I take the time from the FAF to MAP when doing the ILS as a reference or backup in case I loose the glideslope in order to continue the approach? 

2 Answers



  1. John D Collins on Jul 06, 2021

    The GS intercept point on the ILS often is before crossing the FAF and is somewhat dependent on the temperature. On a hot day, the altimeter will indicate lower than your actual altitude, so you will intercept the fixed in space GS later than on a cold day where your indicated altitude is above your actual altitude. The LOC procedure begins the final descent at the FAF and there may be step down fixes on the final approach segment. The timing to the MAP begins at the FAF for the localizer option. So if you are going to use timing to determine the MAP, you would need to start your timing over the FAF.

    Is it legal to switch from the full ILS to a localizer procedure? Yes. Is it smart, that can debated. You can’t switch once below the MDA. You won’t have time to brief the Localizer option after you commence the approach, so you need to do this well before the approach. What is the benefit if the reported weather is below straight in localizer minimums? Also, timing the procedure to locate the MAP is not needed when the MAP can be determined by a GPS or DME. If the MAP is a straight out procedure to the hold, timing to determine the MAP location is not needed. But if the ILS has no other options to determine the MAP other than timing and there are turns involved, then it is certainly a good idea to hack the time passing the FAF.

    My personal opinion is that I would rather miss the approach and re-evaluate and brief a different approach, including setting the bug for the MDA rather than the ILS DA if I decided to fly the LOC procedure. There is also likely to be an LPV to the same runway and if your aircraft is equipped to fly it, it may be a better choice. Also what is the likelihood of the GS failing on the few minute period of your ILS.

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  2. Mark Kolber on Jul 11, 2021

    This is an ancient question and I agree with the general proposition that expecting to switch to LOC-only is not a good reason to time an ILS.

    Having seen this one argued for the almost 30 years I’ve been instrument rated, I’ve come to the conclusion there is only one good reason to time the ILS – to develop the SOP timing habit. Time every approach whether or not you need to and you won’t forget it when you do need to.

    But with GPS marking the MAP for us, even that reason is gone with modern avionics.

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