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EGPWS alerts below ILS minima

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EGPWS alerts below ILS minima

Old 20th Jun 2021, 18:35
  #1 (permalink)  
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EGPWS alerts below ILS minima

Dear flight ops professionals!

Have an issue and deep discussion with safety manager about the triggering of “G/S” alert EGPWS Mode 5 between 150 and 50 feet AGL as he interpreted it as a serious fact of glide path (glide slope) deviation. However I do insist that below the published and applicable ILS minima the published GS is no more considered to be reliable source and in case if you are visual with the RW you should continue visually to touchdown disregarding the glide slope indication (pointer or deflector). PAPI lights are no option coz often they are not available or turned off by notam. The published ILS minima as I understand it is the limit until which the ILS equipment guarantees the accuracy and at which if you are visual you continue (visually to land) otherwise go around.

I am told that you have to continue ILS even below minima, but where is the end of ILS in this case?!

I surfed 100 of documents however useless coz no certain answer to the question.

Hope my question reaches the guys who are well informed on the issue and could provide the official source of the answer.

thanks to all!
Hido is offline  
Old 20th Jun 2021, 20:00
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Can’t help with the document you are looking for .
what I can say is that the mode 5 is desensitised below 150agl .
Below 150 feet you would need to be 3.5 dots low to get just a soft warning .4 dots low to get a hard warning .
so based on this , if said pilot was getting warnings below 100feet , he was seriously low and seriously in danger , so I would say your safety manager has a point .
if there were no warnings below 100 feet , then all good for me , irrespective of the GS deviation on the instruments.
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Old 20th Jun 2021, 20:38
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Can’t help with documentation, however I can tell you that in 20 years and 14k+ hours I have never seen a glide slope warning below mimima, and above minima only a handful all caused by entertaining weather.

Maybe your safety manager has a point?
Locked door is offline  
Old 20th Jun 2021, 21:00
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The glideslope doesn’t stop at 200’ AGL.
When you follow the glideslope guidance down to the ground you’ll impact in the touchdown zone. There is no reason to deviate from the glideslope once at minima.
I’ll need to look it up but I believe the GS is certified to half its minima, so 100’AGL in case of a CAT 1 ILS.
Even then the ILS critical area should be clear.
You’re supposed to be flying a stable approach, aka configured on speed on glide path with the appropriate power setting.
Deviating (significantly) from your vertical path to the extend of a GPWS alert just made your approach unstable and cause for a go around.
Company I work for it’s a mandatory go-around on your second “Glideslope” warning.
There is a minimum runway length for ILS approaches. There is no ILS to a “short” runway. Your performance calculations do not include a “tuck under”.
I’m with your safety manager.
Weather at minimums is not the time to F around.
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Old 20th Jun 2021, 21:36
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Hido,
Documents - search for ‘EGPWS pilots guide’, add manufacturer if not Honeywell. Note some systems / authorities allow GS cancellation - GS button / annunciation.
A GS amber (soft alert), is generally considered to be advisory, requiring attention and correction of flight path as necessary - don't over react - check with PAPI.
Which airport; there could be difference in beam sensitivity - angular splay. Cat 2 - 3 are generally more sensitive, but depends on installation.
ILS indications below minima are generally considered advisory - depends on flight calibration.

From the above it should be possible to calculate the vertical height deviation for 1 dot low for given range / height - relatively small and decreasing, but the difference should be checked against the obstacle clearance boundary for the runway concerned (1:40 slope back from 50ft ??) Might be something of interest in https://www.ecac-ceac.org/images/doc...3_Sep_1988.pdf

B2N2 “When you follow the glideslope guidance down to the ground you’ll impact in the touchdown zone.”
Depends on transmission type, aerial, and location. Old style reflective beams will not touch the runway due to the parabolic geometry.

Last edited by alf5071h; 20th Jun 2021 at 21:47.
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Old 21st Jun 2021, 00:30
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Flew a lot of the “enhanced” Cat 1’s and the first cat 2’s that really just required a tougher check ride and a RA back in the day. (Flying medium pistons and light turboprops.)
I never ever let the autopilot drive after I had the environment in sight.

Maybe Part 121 is different from 135 or 91
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Old 21st Jun 2021, 15:45
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Hi,

The glide slope transmitter is located between 750 feet and 1,250 feet from the approach end of the runway (down the runway) and offset 250 to 650 feet from the runway centerline. It transmits a glide path beam 1.4 degrees wide (vertically). The signal provides descent information for navigation down to the lowest authorized decision height (DH) specified in the approved ILS approach procedure. The glidepath may not be suitable for navigation below the lowest authorized DH and any reference to glidepath indications below that height must be supplemented by visual reference to the runway environment. Glidepaths with no published DH are usable to runway threshold.

Ref. Jeppesen - General . Can't find You a better reference right now.
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Old 21st Jun 2021, 16:03
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Locked door

I've seen a few. None on an ILS though, all on non precision approaches where Boeing, in its endless wisdom, decided that IANs FMC computed GP gives the same warnings as a "real" ILS, not only the same indications. Which, with just half a hP off the real QNH can very easily lead to GPWS warnings on short final.

Wrote an ASR on the first one and got a response back: yes, it is a known issue and can happen easily, simply continue visually and land in the touch down zone.
Denti is offline  
Old 21st Jun 2021, 18:23
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Dear all my dear Colleagues!

First of all let me thank you for your reply and commitment to drop some light on the issue I posted!

I am analyzing all your responses and all of them are very helpful for me!
Hido is offline  
Old 21st Jun 2021, 23:30
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Regarding your question, G/S coverage is provided at least 100ft below your cat I minima.

Taking into account the desensitization of the G/S mode bellow the minima, for an average speed of 140kts, 2 possibilities exist to trigger the warning :

1st to be in the lower limit of the envelope just before the minima and diverging (G/S déviation not meeting the stabilized app criteria).

2nd Being on G/S passing the minima and increasing the V/S above -1000ft/ min (unstable approach below minima).

Triggering the G/S mode at such low heigh means, that you do not comply with either the visual aids (Papi/ vasi lights, aiming point) or the minimum threshold crossing height (RA).

I agree with you that below the minima and with positive visual contact, you can “adjust” your flight path in order to touch down as close as possible to the aiming point. Nevertheless, I always respect the minimum threshold crossing height of my type(did you check yours?).

You didn’t mention if the situation occurs during a special type of operation (LDA performance limited field) or if it is just a way to vacate the runway earlier.

Last edited by pilot-737; 22nd Jun 2021 at 00:24.
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Old 22nd Jun 2021, 10:36
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Thank you all, seems like my 5th point is saved after all your comments😂
Hido is offline  

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