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What happens with an ILS (or LPV) glideslope below the DH?

Dave_Phillips wrote:

… and my experience is that a CAT I GP below system minima may not even reach the dizzy heights of adequate

Please tell us where this experience has been gleaned. You mentioned somewhere in the Middle East. Anywhere closer to home?

EGKB Biggin Hill

I am a lowly CAT 1 Single Pilot IR holder, but for CAT II the operator needs to demonstrate the equipment, and operating procedures over a minimum of 50 approaches and at least three ILS systems. While the standard is that at least 90% of the approaches are successful (in simulated IMC), we all probably agree that we prefer 100%.

I don’t know if Dave also calibrates special CAT 1 or CAT II, but I imagine his job and equipment provides him with a robust statistical sample, which from an engineering perspective he then can make the statement that the glide slope on CAT 1 may and on occasion will deviate below system DA.

I for one am glad that these regular calibration flights take place. Whether a GS is stable down to 100 feet on a CAT 1 system on a given date is not something you can then take to the bank on every occasion.

Oxford (EGTK), United Kingdom

Absolutely. That’s why I would love to know where he has seen errors between 200 and 100’ on a Cat 1 ILS, for all the same reasons as you.

EGKB Biggin Hill

Why should I, not least because there is a contractural relationship between supplier and customer (i.e. airport)? It really is quite easy and I don’t understand why the argument is perpetuating. A system is either CAT I, II or III. The system incorporates all sorts of criteria such as lighting, pilot quals, operations manual approvals, crew procedures, A/P, Flight Director, taxiways holding positions etc etc, but lets just stick with the signal in space.

If you read my earlier response regarding propagation, DDM, signal strength etc you would get it. In very simple words – a system which is only specified as CAT I does not guarantee the integrity of the GP signal beyond system minima (ordinarily 200ft). The signal may be good enough for further descent but the tolerances to which it has been designed, installed and calibrated simply does not assure that. You may get a flag, a fly-down or, more likely a fly up. I’ve seen enough of those on the 13 -18 runs I do on every ILS, every 6 months to convince me that following the GP below minima is not a great idea.

RobertL18C wrote:

I don’t know if Dave also calibrates special CAT 1 or CAT II, but I imagine his job and equipment provides him with a robust statistical sample, which from an engineering perspective he then can make the statement that the glide slope on CAT 1 may and on occasion will deviate below system DA.

And we do. That is precisely the information that is presented to the customer. “Your system is in/out of tolerance for a CAT I, II or III approach.” The airport’s part of the deal is to tell the pilot what standard the system reaches; the pilot’s part of the deal is to stick with the standard.

Last Edited by Dave_Phillips at 30 Jan 18:22
Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

So, the same argument as why you wouldn’t use an overlay approach, absent an ADF?

Providing that’s right, we are completely in accord.

EGKB Biggin Hill

Timothy, I haven’t a clue what you meant by that last post. I’m done here.

Fly safely
Various UK. Operate throughout Europe and Middle East, United Kingdom

Apart for the fact that I have learned a lot about the ILS system, it seems to me like the mere fact that there is a debate around this is that a number of people may be advocating or experimenting what one may often call “stupid pilot tricks” which become widely publicised when they go wrong.

The tendency to question and transgress limitations that are imposed by the regulator, by the equipment manufacturer and against the advice from people whose job is to test the limits of the system always surprises me, whether those are weather minima, distance from clouds, DA etc.

Maybe it is my Germanic side…

LFPT, LFPN

I think you’ll find that it is SOP for many/most pro pilots.

Dave has, quite rightly, pointed out a tiny theoretical risk, not in any way borne out by practical experience in the real world, and not encompassed in testing. He has labeled exposure to this tiny risk as significant, whereas experience shows that the risk of not using the slope in considerably more real (just like using an uncertified overlay approach is safer than relying on an ADF).

To label these as “stupid pilot risks” is to misunderstand the risk profile quite considerably, but is the habit of regulators, in my unhappy experience. It is dancing on pinheads.

But, as I keep saying, each of us can, happily, take our own decision.

But let us be quite clear, I am only talking about using the ILS below DA once visual reference has been confirmed at or before DA.

EGKB Biggin Hill

OK, this is the end of this “competition”. It’s been done to death.

Both sides have made good points and there is no need to have the last word, Timothy.

Administrator
Shoreham EGKA, United Kingdom

From here

Mooney_Driver wrote:

To follow the GS below minima may well cause rather unwanted results. CAT II/III approaches rely on additional sensor input such as RA. Probably the best solution is indeed to fly to the minimum and then keep the rate of descent you had on the glide and keep the loc centered with a nose up attitude and wait for it. Also: check RVR on the opposite side, often enough the ILS side of the runway is worse than the opposite end.

That is what I meant. You obviously can’t and shouldn’t follow it to touchdown

EGTK Oxford
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