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4 red but on glide slope

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4 red but on glide slope

Old 25th Jul 2022, 23:24
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4 red but on glide slope

Hi all,

Im currently doing some line training and I was flying an approach the other day, where we got 4 red on shortish final, but strangely enough we were bang on the glide slope on the ILS/pfd. I reviewed the data when I got home from the Ofdm and it shows that we were on the glide slope the whole way down, the diamond was pretty much locked in. What would cause this? The aircraft was a B738 and the Papis were 61ft and specifically set up for a 738. Would I have been better off trusting the Papis or the glide slope? I dont understand how I could both get 4 reds and be on the GS.
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Old 26th Jul 2022, 00:47
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I have seen that many times, in many different places. PAPI's that would make you cross the threshold at 80 feet, or that would show 4 reds while on the glide slope (as in your case)...

Basically, below minimum you may disregard the G/S and PAPI and visually go for the threshold, crossing it at 50' above. If you are landing in an runway fitted for widebodies you may see that keeping two reds and two whites will leave you high on the threshold (followed by a long landing), and if the ILS is not certified for autoland, following the G/S down to touchdown may not be the best. Below minima it is you, the threshold, and the airplane. The rest is just for guidance.

I have been doing this for a long while and never got a problem.
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Old 26th Jul 2022, 02:53
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The Threshold Crossing Height (TCH) for the ILS and the Minimum Eye Height (MEHt) for the PAPI have to match for the PAPI to line up when you are on the GS. The glideslope antenna position on the aircraft may also be a factor. If the PAPI MEHt is higher than the GS TCH, you'll get what you see when close in because the ground point of intercept of the PAPI path is further down the runway.

I doubt that a 61ft MEHt is "specifically set up for a 738". The 737 is not that big/long. More likely the MEHt has been increased so that the PAPI matches the GS when in a long-body. Also, given the 61ft MEHt, it is more than likely that the runway is quite long (catering for long-bodies) and so going low under the PAPI or GS at the last second, as Iggy is suggesting, is not necessary.
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Old 26th Jul 2022, 05:07
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You are right, the PAPI is deliberately misplaced, the rest is geometry.

An old topic albeit not discussed frequently, explanation blends into landing performance a bit.

Firstly, the stable approach criteria include landing within the first 900 mtrs or 1/3 runway, this causes much trouble so forget it. The correct landing point is the one calculated from following the profile to 50' over THR, around 450 to 600 mtrs past THR.

The 50' datum and 3 profile create a geometrical aiming point. For my airplane the flight-path (GS) ground intercept point is located 298 mtrs past the threshold. Because the antenna is pretty much same height as the pilot-eye position, the visual aiming point is right there.

Picture:



Green spot = 3 path intercept point
Yellow dot = where the plane is actually going (check g/s dev and v/s, the flare has started
Red line = PAPI distance line aligned with the "aiming point" markers as per ICAO standard for RWYs longer than 2400 mtrs.






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Old 26th Jul 2022, 05:19
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Originally Posted by Capn Bloggs View Post
The Threshold Crossing Height (TCH) for the ILS and the Minimum Eye Height (MEHt) for the PAPI have to match for the PAPI to line up when you are on the GS.
Wish this was true, but it isn't. FAA does not recognize (good!!) the ICAO standard for 2400+ runways and puts the aiming markers on the second position (300 mtrs) - then it works as you describe.

The ICAO standard is 150+150+100 = 400 mtrs where the PAPI becomes too deep. Some airports even have 150+150+150 = 450 and everything is all over the place.




N.b. the PAPI (and markers) displacement does not affect the GP antenna or GS calibration of 50' over TRH.

Yes, the GP antenna is not located at the theoretical 3 intersect point due to asymptomatic GP bending close to the ground (below 10 ft or so, on a modern-day installation which is CATII capable it is darn precise to very very low) but slightly farther out. This is not relevant to the discussion, the GP signal itself is OK.

When the PAPI are not at the 300 meters mark, as suggested by the OP (300*66/50 = 400 m), flying below the PAPI 3 signal is a geometrical necessity as GS and PAPI don't align. From below 100 this will become visually apparent (3 reds) and due to the aperture / parallax / sideways distortion 4 reds are a possibility.
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Old 26th Jul 2022, 05:40
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OTOH, flying below GS is a no-no. 738 might be forgiving a little but the larger aeroplanes not so!



Funnily disturbing fact:
- in order to fly 50' GP profile, the RA callout at THR needs to be "THIRTY".

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Old 26th Jul 2022, 06:29
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FlightDetent is spot on, as typical. It's due to the fact that PAPIs are calibrated for larger (longer) aircraft that you are flying, and/or ILS GS not intersecting the runway where PAPIs are located.

Following the GS is typically the best solution. PAPIs aren't that usable below your typical Cat 1 minima anyway, especially if they aren't fitted to the side you are flying from. Good anticipation is key, noting the TCH and MEHT on the charts prior commencing approach.
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Old 26th Jul 2022, 06:33
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Originally Posted by FD
Wish this was true, but it isn't.
No, it is true. Now if the TCH(ILS) and the MEHt(PAPI) don't match, then you'll have similarly unmatched visual indications from the PAPI. My point has nothing to do with FAA/ICAO markings or anything else. If the TCH and MEHt match, then the PAPI will be 2W2R (ignoring GS antenna location being not at eye level). If they don't match, you'll see something other than 2W2R; probably 1W3R, or, as you say, possibly 4R, because the PAPI, on a long runway, is further down than the GS antenna.

Last edited by Capn Bloggs; 26th Jul 2022 at 07:00. Reason: grammar
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Old 26th Jul 2022, 06:57
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For the OP, here is the explanation. Note your eye position with respect to the PAPI slope.

I have used a typical 400m front edge of the aiming point marking for the location of the PAPI.

It is a pity that whoever did that diagram couldn't get C and B correct; B is longer on the diagram but shorter on the numbers.
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Old 26th Jul 2022, 07:26
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C.Bloggs, most sincere apologies.

I only read your first sentence though not as a pre-condition the way you placed it but as a statement. It seemed to re-inforce the OPs initial confusion. Instead, your post #3 says all that is needed.

And love your graphics!

Contributing factor: Often I don't read your posts carefully on known topics because you are simply right and the adrenaline rush of disagreement is never there [human factors, subconscious]

Update to training syllabus: Adopt brevity as a teaching technique [Standards development]

Safety recommendation: Do not post before breakfast. [Operational]

Last edited by FlightDetent; 26th Jul 2022 at 09:19.
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Old 26th Jul 2022, 08:01
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Funny fact, not disturbing:
- the A380 looks deliberately designed for MEHt = 66 feet = PAPI @ 400 m = ICAO 2400+.






What's the pilot eye-height for the 748? IIRC their antenna is in the MLG area...
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Old 26th Jul 2022, 08:16
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Cheers Flight Detent.
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Old 26th Jul 2022, 09:15
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Thanks all for the replies. The MEHT on the chart specifically said MEHT 738 61ft. So basically Im confused how we got into 4 reds. I noted that the approach is not approved for CAT2 approaches or auto land. Basically I got written up as though I got into 4 red, and afterwards I saw I was on the glide the whole way downnow Im a bit confused regarding where I should have been.I suppose looking outside more.
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Old 26th Jul 2022, 09:29
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Ive had similar issues and confusion in a larger airplane.
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Old 26th Jul 2022, 10:05
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@PS upthread we used the MEHt acronym in lieu of "height over the threshold of PAPI 3 ideal beam".

Perhaps in your case it means a restriction, literally? I.e. for 738 do not cross THR below 61' pilot eye-height. Having said that, I grow a strong hunch you might be interested in one of the shirts..

C.B's resistance to the term 'FAA runway' (fairly new to me too) turned into research.

a) ICAO Annex 14
- PAPI to align with Aiming Point Markings
- APMs are 300 meters for LDA < 2400m
- APMs are 400 m for LDA > 2400 m

b) FAA AIM only specifies the Aiming Point Markings to be around 1000'
- https://www.faa.gov/air_traffic/publ...section_3.html

c) FAA AIM does not specify the PAPI location.
- https://www.faa.gov/air_traffic/publ...section_1.html

Expect to see similar to this (runway NOT in the US):
3000 x 45 m
LDA beyond GS = 2704 m
PAPI aligned to PAMs at 330m from THR.



all fair and square.

Until I looked at LAX



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Old 26th Jul 2022, 10:21
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Originally Posted by FlightDetent View Post
@PS upthread we used the MEHt acronym in lieu of "height over the threshold of PAPI 3 ideal beam".

Perhaps in your case it means a restriction, literally? I.e. for 738 do not cross THR below 61' pilot eye-height. Having said that, I grow a strong hunch you might be interested in one of the shirts..

C.B's resistance to the term 'FAA runway' (fairly new to me too) turned into research.

a) ICAO Annex 14
- PAPI to align with Aiming Point Markings
- APMs are 300 meters for LDA < 2400m
- APMs are 400 m for LDA > 2400 m

b) FAA AIM only specifies the Aiming Point Markings to be around 1000'
- https://www.faa.gov/air_traffic/publ...section_3.html

c) FAA AIM does not specify the PAPI location.
- https://www.faa.gov/air_traffic/publ...section_1.html

Expect to see similar to this (runway NOT in the US):
3000 x 45 m
LDA beyond GS = 2704 m
PAPI aligned to PAMs at 330m from THR.



all fair and square.

Until I looked at LAX

thanks for this. I guess in conclusion I was wrong to follow the glide down so longI remember seeing 3 reds on short and looked down and saw we were on glide. Those 3 reds must have gone to 4 reds at some pointI suppose I need to scan outside more at this point.
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Old 26th Jul 2022, 10:49
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PelicanSquawk , IMO you have been unjustly criticised. We need some more info though.

- You say "the chart says MEHt 738 61ft". What chart was this? If it is an official, state chart, I'm surprised it says 738. MEHT for a PAPI/VASI is not contingent on an aircraft type. The PAPI will merely place the eyeballs of who is looking at it at 61ft over the threshold, nothing more. It doesn't matter if it is a Concorde or a Cessna 172.

- What does your ops manual say about transferring to the Visual Aim Point off an ILS? Any mention of transitioning to the PAPI? Mind you, such a thing is, IMO, a stupid idea because it would severely destabilise your approach below the ILS DA. Can your instructor imagine you hauling back on the stick at 200ft AGL to correct the 4 reds you saw?

- What's your stabilised approach criteria? Does it mention that the PAPI overrides the ILS GS?

- What airport was this at? Perhaps the "61ft" is incorrect. It is quite easy to use Google Earth to verify the MEHt if the PAPI can be seen.

- In any case, if the FDAP shows you were on the GS the whole way down, anybody who criticises you or worse, marks you down, is on shaky ground, in my view.

Flight Detent , I don't expect the Yanks to have ICAO markings but the runway above, 05L, has got incorrect markings as well. There should be markings at 150m, 300m then the aiming point markings at 400m. Where is that airport?
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Old 26th Jul 2022, 11:03
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Originally Posted by Capn Bloggs View Post

It is a pity that whoever did that diagram couldn't get C and B correct; B is longer on the diagram but shorter on the numbers.
Boeing did that diagram!
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Old 26th Jul 2022, 11:04
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Yes, if you follow the ILS GP down, you will be a bit below the PAPI, progressively getting lower and lower, because the PAPI beams fan out from one point. So what you will see is exactly what you did, 2W2R into 1W3R, then 4R very close in.

As I said up-thread, the PAPI is further into the runway to cater for long-bodies (as per FD's diagrams). The 738 does not need it, and people criticising others for being off the PAPI when they are spot on the GS are off the mark. At the very least, your trainer should have explained what you have read here so you understand the issue.

And as Flying Stone mentions, using the PAPI for active flight path control below the ILS DA isn't really practical anyway.

Our AIP has this:

Note the difference between 3W and 3R; the equivalent of 2W2R to 4R. That's why you were low on the PAPI, because that's where the GS is.
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Old 26th Jul 2022, 11:11
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Boeing did that diagram!
Of an A340. Figures!
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