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10 Answers

Glide slope failed after passing OM

Asked by: 2981 views Instrument Rating

The case: 1. Clearded for the ILS approach at night time 2. Just after passing the OM, the Glide slope failed 3. The pilot can see VASI very clearly What is the correct action of the following: A. Go missed B. Request LOC clearance and fly to the LOC minimum C. Continue the approach using VASI without even notifying the ATC Provide ref please.

10 Answers

  1. Wes Beard on Jul 14, 2013

    If you can see the runway then I would continue and land.

    If you can’t see the runway I would go around unless I also prepped myself for the localizer approach.

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  2. Mark Kolber on Jul 14, 2013

    Agree completely with Wes. There is no reg and no FAA reference I am aware of that requires you to continue or to go missed, so the choice of whether to do one of the other is essentially yours (unless, of course, there is something about that specific approach that requires some other action).

    The only thing I would add is that 91.187 would require you to report the GS failure “as soon as practical.”

    In your scenario, to me, “as soon as practical” means:

    • As part of reporting the missed in your choice A.

    • In choices B & C, once I made any needed configuration changes to be stabilized for the visual or LOC only approach (applying the aviate-navigate-communicate priority order).

    IMO the timely notification is important here because it’s always important to be on the same page as ATC when on an approach. That necessarily means is this were a quiz, I would not not select “C” since it says that you would not notify ATC. Technically, I would also not “ask” ATC for the LOC clearance as stated in “B”; I would “advise” ATC I was flying the ILS to localizer minimums due to the failed GS as part of my 91.187 report.

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  3. Ale on Jul 14, 2013

    1. VASI provides a safe glide-path. I personally trust a visual glide-path, such as VASI and PAPI, more than an electronic glide-path, such as GS and VNAV.

    2. When you see the VASI, you have enter the visual portion of your instrument app (vertically). You must use the LOC for lateral guidance till you see a lateral visual guidance such as the runway or its lights.

    3. Legally, pilots permitted to use VASI for vertical guidance during approach not only above the DA but also below the DA (it is one of ten visual references permitting descending below the DA). So if it is safe enough to let’s continue descending below the DA, then surly it is safe to use it above the DA (after passing the OM).

    4. No hurry to report to ATC the malfunction of the electronic GS; do that after you you exit the runway (aviate, navigate, communicate).

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  4. Mark Kolber on Jul 15, 2013

    I would generally prefer to advise ATC on my next communication with them as opposed to receiving an alert from them if my flight path is not what they are expecting. If I waited until I landed and taxied off the runway, I wouldn’t bother notifying them at all since safety of flight and the mutually-expected flight path would no longer be even a minuscule factor.

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  5. Wes Beard on Jul 15, 2013


    The VASI or PAPI only guarantees obstruction clearance out to 4 nm and within 10 degrees of the centerline. Reference AIM 2-1-2.

    The glideslope provides obstruction clearance from the FAF inbound. Which is typically 5NM.

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  6. Mark Kolber on Jul 15, 2013

    Wes is quite correct. As I recall, there was even a recent article in IFR regarding the dangers of coming off the electronic glideslope and relying on visual cues, with specific examples of approaches into airports with nearby terrain. The electronic GS guarantees clearance; the visual versions, no.

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  7. Ale on Jul 15, 2013

    In addition to wes’s last comment, the following must be considered as well:

    1. In certain circumstances, the safe obstruction clearance area (i.e. +/- 10 deg & 4 NM ) may be reduced due to local limitations, or the VASI may be offset from the extended runway centerline. This will be noted in the Airport/ Facility Directory.

    2. Pilots of high performance aircraft are cautioned that use of VASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and rollout.

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  8. Ale on Jul 15, 2013

    A pilot is making an ILS approach and is past the OM to a runway which has a VASI. What action should the pilot take if an electronic glide slope malfunction occurs and the pilot has the VASI in sight?

    A) The pilot should inform ATC of the malfunction and then descend immediately to the localizer DH and make a localizer approach.

    B) The pilot may continue the approach and use the VASI glide slope in place of the electronic glide slope.

    C) The pilot must request an LOC approach, and may descend below the VASI at the pilot’s discretion.

    Correct answer: B

    Reference: http://www.faa.gov/training_testing/testing/test_questions/media/ira.pdf

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  9. Fly92020 on Jul 15, 2013

    Tower doesn’t care what line of minima you’re using on the approach. You would just revert to the LOC mins if you hadn’t already reached MDA. In the real world, in the soup, I would probably go missed just so I could regroup and figure out what just happened (did my G/S go out, or did the ground-based component fail?). That’s just me. Any surprises that occur on an approach are easier to sort out anywhere other than a few hundred feet off the ground.

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  10. RLewis on Jul 17, 2013


    Wes and Ale have provided some great resources to answer the question, however I still find fault with all 3 options the FAA provides.

    Answer 1 and 3, which mentions notifying ATC, goes against Aviate, Navigate, Communicate. Once inside the FAF, the pilot’s only task should be focusing on the approach.

    Answer 2, continuing with the VASI or even a PAPI may be technically accurate, experience has told me not to do this. I’ve had a similar scenario happen to me – localizer approach at night with the PAPI’s in sight when crossing the FAF. When descending on the approach, we went into clouds, and at that point had to rely on instruments – talk about disorienting! My recommendation is if you feel comfortable with changing your game plan to localizer minimums, fly the instruments regardless of what you see outside until you are at minimums and nearing your VDP.


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