YMML MEHT
 

Hi,

Anyone can explain why is it that the MEHT for YMML is 74ft when the MEHT for the majority of heavies is around 50ft? Am I missing something?

With Regards

Not, just YMML. Take a look at YSSY (64 feet 16R/34L) and YPPH (71 feet for 03/21 and 24). As to why, it is to cater for the A380 so I've been told by checkies.

Yes you are. Every "long" runway in Oz, ie those which might accept "long" or "high" bodies jets, has a MEHT of at least 60ft. The MEHT, of course relates only to the PAPI. The Glideslope threshold crossing height is a different kettle of fish altogether and is always 50ft.

The difference exists because, if the MEHT (for the PAPI) was 50ft, when the jet was on the GS, you'd see 4 whites out of your long/high body. Looking at it from another angle, if you did a visual approach without the GS, if you had a MEHT of 50ft and followed it, the wheels of your long/high jet would be very low over the threshold.

What this all means is that, when on the ILS GS, the PAPI will look basically OK (2W2R) when you're on short final. If you are in a tiddler, then, at the DA, you'll probably see 3R because your eyes and the GS are closer together.

A further clue is the physical location of the PAPI. It will be on the front edge of the aiming point marking, which, on a longer runway, is well past the 300m marker (which is where the GS touches the ground).

Crikey Bloggs! Just when I thought I knew how to land…. and you throw this at me!

Because Australians have to do everything slightly differently for no good reason.

Not just in a tiddler, a 737 will dip below the PAPI 'on slope' indication if it follows the ILS glide-slope indication, which is probably why a lot of 737s can't make November on 27 anymore, whereas most of the international heavies make it regularly. So at the minima it's better to follow the electronic glide-slope and use visual ques for the 300m touchdown marks rather than follow the PAPI and end up touching down at 450m.

What a moronic statement. Have a look at the MEHTs at say EGLL.

Oohhh, touchy.

No, stop posting tripe. "P"Prune.

It’s a few years ago that I did the trig but IIRC (and if I was right !) every foot your aircraft’s MEHT is less than the PAPI’s MEHT, if you follow the PAPI and land like for like (same flare same AoD) you’ll land 6 metres longer.

737 = "tiddler"

This may not be very intuitive to a larger pool of us, and seeing through it (pun intended) requires just a little education, beyond the typical frontline curriculum.

Key takeaways:

+ 3 deg nominal profile (and a commonly adjusted GP) from 50' intersects the ground plane 291 m past the threshold.
+ Annex 14 ICAO standard marking LDA < 2400m have a so-called "aiming point" located at 300 m
+ Annex 14 ICAO standard markings for LDA > 2400 have the "aiming point" located at 400 m
+ ICAO Annex 14 standard for PAPI location is "abeam the aiming point markers" that yields 2 nominal MEHTs (as per above) of 50 and 66' feet
+ in P.A.P.I. the first P. stands for "electronic vertical plane", not that it needs to be aligned with GP of an ILS installation exactly
+ in P.A.P.I. the A stands for APPROACH and not LANDING.
+ any plane should be flown i.a.w. its AFM and not according to any painted rectangles / lightbeams

Sounds a bit rough but not intentionally, a small price for clarity and brevity.

https://cimg1.ibsrv.net/gimg/pprune....2fe997277f.png


https://cimg2.ibsrv.net/gimg/pprune....8acaac3ff2.png

pictures:


https://cimg8.ibsrv.net/gimg/pprune....146dac02ba.jpg




https://cimg4.ibsrv.net/gimg/pprune....c018ba66f7.jpg


https://cimg9.ibsrv.net/gimg/pprune....a2e5f64c63.jpg

FD; [ Pedant on ]

PAPI - Precision Approach Path Indicator as per RAE Bedford 'invention' (actually the idea was French).
Regulators or world wide interpretations may differ.
[ Pedant off ]

A long time ago - ILS electronic glide paths were created by reflecting the beam off the ground in front of the transmitting aerial. Thus GS origin was the distance proportional to x deg in front of the aerial position (not exactly correct because the beam was subject to parabolic distortion proportional to the aerial lateral offset).

Do modern ILS GS transmitters still use a reflected beam?

If so then the diagrams require amendment / relabelling.

If not, how is the beam origin and thus runway interception defined?

Yes, shows my age a bit. The term "precision" is today (well inside the era of 2D/3D nomenclature) defined by the delivered performance (result) as opposed to my education time when it was directly describing the technology. Precision = vertical guidance provided by electronic means, and the light beam does exactly that. I may have used the equal sign there illegally.

To my knowledge, EW propagation don't change over time, surely still the first 1-6m of GP pattern is bent and the GP plane ground intercept is not exactly co-located with GP antenna spot. In that respect the graphic (from "flight crew" manual) is good enough to suggest a box where the ideally-straight GP comes from, upwards. The ground around the antenna needs special focus, some pictures show it.

https://cimg8.ibsrv.net/gimg/pprune....a4fa879bd8.png

The Airplane Flight Manual's 50-feet&3-deg ground intercept is defined as 50/[tan(3deg)] = 291,78 m (past the threshold) for the geometrical plane - and the engineers do their magic to align the GP signal to ... ehm ... close to there most of the time.

Well visible on the following :
+ distance marker 300 m
+ 3 deg plane somewhat shorter than 300 m
+ GP antenna anchored a little further to allow for GP bending,

https://cimg3.ibsrv.net/gimg/pprune....d826b1ab0d.png

The idea here was rather to avoid splitting the hair on 50ft-3 deg VS. GP signal VS 300 marker VS antenna anchor,

and instead, share insight on the geometry why PAPI indication for the little aeroplanes like the A340 and smaller will be offset both from the GP indication and the correct visual aiming point (AFM).

The problem of pilot perception is corroborated by ICAO standard marking for LDA2400+ where the big rectangle is painted unnecessarily far down, to allow for the very large planes ( height offset between pilot pilot eye and GP antenna), and the PAPI getting stuck with that displacement.

https://cimg5.ibsrv.net/gimg/pprune....3f928a7cbf.png

In the US, the distance marker stays at 305 m (=1000') but PAPI can still be further down to accommodate the 747 and assure MLG clearance.

https://cimg6.ibsrv.net/gimg/pprune....09cae89e5c.jpg

Thus, the old adage "PAPI is unreliable (for landing) below 200ft" is only correct by coincidence. Light-beams don't bend, yet for most but very few select airplanes they are just coming from the right spot on the "international" runways.

And that's OK,

https://cimg0.ibsrv.net/gimg/pprune....80c0e87dd2.png

I wouldn't call it "unnecessarily". The aim is to stop long-bodies touching down too close to the threshold. "Tiddlers" on that same runway (by definition, >2400m) will still have plenty of strip to land on, even if they do use the PAPI. The whole point of having a big white square is to use that as the visual aim point. If it was always at 300m in your long/big jet, you'd have to do trick flying to aim for ~400m so you didn't run the risk of scraping the mains through the grass if you got a tad low. ICAO has obviously decided that having a big white square in a realistic position is good. The Yanks, not (at least not on KLAX 07R).

This post by Dan Winterland is informative:

https://www.pprune.org/tech-log/2738...ml#post3259750

With the gear down in the 747, the pilots eyes would definitely be heading somewhere in the vicinity of your 440m, way down the runway compared to the GS path.

You mean "TCH" (as opposed to MEHT).

That "P" just means I get paid to fly, which I do and don't have to spend time worrying about what the other Austronauts do.