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A puzzle: strange glide slope indication


0TreeLemur

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Returning from a trip today, ceilings at 500-600 ft, while shooting the ILS 04 into KTCL, with KX155 and 430W(VLOC) showing the same indication, the glide slope dropped out.  Twice.   

By dropped out, one moment I was established on LOC and GS, the next, the GS indications went haywire, indicating that I was way high.  After 10 sec. or so they came back.  My VS didn't change, I was still well above DH, and it seemed like a reception issue.  After 10 sec. or so, they dropped out (down) again.    I was not about to go chasing them so I declared a missed, and came around and shot the RNAV 22, which transpired without issue.

This funky GS behavior happened simultaneously on both the 155 and 430W.   I'm guessing that there is something wrong.   Maybe the splitter between the two receivers and the GS antenna?  Somehow there is some strange interaction or feedback that is affecting both, or my GS receiver is bad in both???   That seems very unlikely.   Anybody seen this kind of behavior before?

If we had an avionics shop on field at KTCL I would have them look at it, but sadly there is none.   All the avionics shops nearby are too busy to give a little ol' 52 y.o. Mooney the time of day.   Thanks.

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I just had a thought from somewhere back in my apparently paying more attention and better trained reptilian brain. The BHM approach controller held me at 4,000 and vectored me to a point outside the FAF, and had me keep up "maximum forward speed to 5 mile final".   I'd bet that in my haste to follow instructions, I locked onto the false GS that is above the real one.   As I tried to descend along it, it was too steep for my established 500 fpm 90 kias glide, and it dropped out.   I did see the runway as I passed over it after I declared missed.  

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Just now, carusoam said:

Did you get the ILS data on the audio? 4 and 1... 4 time is the ID of the VOR, once is the ID of the ILS... if there is one missing, the ILS is out...

Old school audio details related to signal quality of the VOR/ILS... (old fuzzy memory)

Best regards,

-a-

Yes.  Verified the dots and dashes, on both ILS receivers.

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Was the localizer needle steady?  Missed was probably the best option, but with solid loc needles you could have descended to localizer mins to see if you break. out.  Like I said, missed maybe best option to get away from the ground to sort out the issue.

You gotta be real high to get a false indication.  More likely a reception or transmission issue.

My local ILS at KSFF will S turn you down final like a big dog.  It’s FAA checked and passes, but it’s sporty with a good autopilot that tries to stick right on the line (gfc 700)!  Some of them have some gremlins...

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On 12/30/2019 at 9:28 PM, Ragsf15e said:

Was the localizer needle steady?  Missed was probably the best option, but with solid loc needles you could have descended to localizer mins to see if you break. out.  Like I said, missed maybe best option to get away from the ground to sort out the issue.

You gotta be real high to get a false indication.  More likely a reception or transmission issue.

My local ILS at KSFF will S turn you down final like a big dog.  It’s FAA checked and passes, but it’s sporty with a good autopilot that tries to stick right on the line (gfc 700)!  Some of them have some gremlins...

Yes, the loc was steady, but the gs was really erratic.  The MSA in our area is 3700.  They held me at 4k until established, and max fwd. speed until 5 mile final.  More I think about it with me being a relatively new IR pilot, this was a setup for me to have missed the real GS and then momentarily caught the false GS, which I quickly went above.

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6 hours ago, 0TreeLemur said:

Yes, the loc was steady, but the gs was really erratic.  The MSA in our area is 3700.  They held me at 4k until established, and max fwd. speed until 5 mile final.  More I think about it with me being a relatively new IR pilot, this was a setup for me to have missed the real GS and then momentarily caught the false GS, which I quickly went above.

I don’t doubt that it could happen to some degree, but it’s pretty darn tough to get a false gs indication on the front course of an ils unless you’re really far above the glide slope. The backcourse does occasionally have some erratic indications but you expect those.  The conventional wisdom with false indications on the front course are that they can occur in 3 degree increments above the glideslope, so if your normal 3 degree, 5 miles out faf is at 1500’ above tdze, you could see a false indication at 3,000’ or 4,500’ above tdze (6 degrees or 9 degrees).  

If you’re on a 6 degree glide slope to the runway in imc, you need to go missed anyway.

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Yes, the loc was steady, but the gs was really erratic.  The MSA in our area is 3700.  They held me at 4k until established, and max fwd. speed until 5 mile final.  More I think about it with me being a relatively new IR pilot, this was a setup for me to have missed the real GS and then momentarily caught the false GS, which I quickly went above.


I didn’t see any mention of NOTAMs for the ILS. Could it be there was an issue with the hardware? There is a NOTAM for today.

74c5ffd95c2d384f6aa3686b3a77bf2a.plist


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16 minutes ago, Marauder said:

I didn’t see any mention of NOTAMs for the ILS. Could it be there was an issue with the hardware? There is a NOTAM for today.
 

There was nothing in the NOTAMs on that day for the ILS.   The more I think about what happened, I think it was an attempted slam dunk that went missed because of a new IR pilot. 

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3 hours ago, 0TreeLemur said:

There was nothing in the NOTAMs on that day for the ILS.   The more I think about what happened, I think it was an attempted slam dunk that went missed because of a new IR pilot. 

That I would definitely buy, and there’s absolutely nothing wrong with having to go missed.  Talking about the experience on MS is much better than us trying to figure out why you turned into a smoking hole after getting disoriented during a rushed approach in imc!

Also, going missed is real good for you.  You’ll be more likely to do it in the future if you need to instead of pushing on in a bad situation.

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