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Asked by: Dave
On an ILS DME (LOC offset by 1 degree from ext. RWY CTRL) with a MAP thats defined by a certain DME distance,
showing a DA and MDA, would you go missed arriving at the DA or the MAP, or whatever occurs first?
Sam Dawson on Nov 15, 2012
I would probably go missed at DA- missed approach point on an ILS is DA- but would need to see the approach. I can’t think of any ILS approaches off the top of my head where the MAP was not DA.
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Dave on Nov 15, 2012
Your thinking was my first thought as well. But i think i figured it out.
They dont provide a stopping time from the FAF. But of course, you need a point that tells you when to go missed from your MDA for the LOC approach or in case the GS fails during the ILS.
So that must be the reason why there’s an addt. MAP at a defined dist. from the LOC (1.3) i assume.
Thank’s anyway! ; )
on Nov 16, 2012
You may also have an airplane not equipped with a GS, or GS may be out of service from the get go. Most airlines do not permit you to revert to the LOC. approach if the GS goes out during an ILS- you must come back around and brief the LOC approach.
Also remember that the FAF for the ILS is glideslope intercept, not a DME point. The DME point is the FAF for the LOC approach.
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on Nov 16, 2012
There is not normally an FAF on an ILS (emphasis on the word FIX).
The “fix” that is depicted is, like mentioned above, for non-precision, LOC only approaches. What is important to know is the Final Approach Segment, which doesn’t necessarily begin at glideslope intercept (i.e. you intercept the glideslope 30 miles out at 10,000 feet). The final approach segment begins at glideslope intercept “at the G/S intercept altitude” and that altitude coincides with where the “feather” on the planview intercepts the G/S.
on Nov 16, 2012
I think it is important for instrument students reading this that a typical ILS approach plate will have the words in the title “ILS or LOC 26” meaning that the instrument approach plate in your hands has two quite similar but different instrument approaches. (On older approach plates not updated in the last five or so years… it may only say ILS 26 but have two separate approaches… these are being updated to the new format).
The new TERPS (US) and PAN-OPS standard (rest of the world) is to list all the required equipment in the title of the approach. In this example, an ILS DME approach requires a localizer, glideslope and DME. The LOC DME requires a localizer and a DME.
With that in mind, the plan view for both approaches are overlaid on each other. The FAF for an ILS is glide slope intercept altitude, as previously stated, and is depicted by a lightning bolt. It is not necessarily colocated at the maltese cross. An ILS only approach does not have a maltese cross. See KRNO ILS 16R approach. The FAF for the LOC approach is at the maltese cross.
The MAP for the ILS is at “decision altitude on glide slope” and the MAP for the LOC is usually a time from FAF or DME distance.
Two separate approaches with separate FAFs, separate MAPs and just as importantly, separate minima and visibility requirements.
Some instructors have taught, that if the glideslope becomes inoperative inside the FAF to “switch over” to localizer minimums. I think, if properly briefed and executed, that would be fine, but for instrument students just getting the hang of how things work, an immediate missed approach and time to brief the LOC approach is in order. It will also solidify that the approach plate has two separate approaches on it and show excellent single pilot resource management and situational awareness.
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