RobertL18C wrote:
The regulations are quite straightforward – if the ILS is a CAT 1 ILS the glideslope is not reliable below the system or published minima (200 feet), and the approach continues using visual references
Do the regulations actual say that the glideslope is unreliable? I would expect them to state the performance that must be met or exceeded, but not to actually require the ILS to be unreliable.
ortac will track down the FAA source, but thought this document might help understand some of the points Dave Phillips made. The antenna is offset and aiming at DA. The signal also suffers from variability, reflections, terrain roughness, scalloping.
That would be my suggestion as to the likely fire behind this smoke; that there is no requirement to calibrate or certify below 200’, so the assumption must be made that they are unreliable, because no-one can produce proof that they are not.
This is very similar to the argument that no-one has certified overlay approaches (whereas they have certified and approved the underlying raw-data approach) and therefore the overlay may as well not exist.
Or the argument that no-one has certified the BaroVNAV slope to work as SBAS VNAV, so we must switch it off.
However, certified and approved or not, it is perfectly clear to us all that flying the overlay is safer than flying the ADF, and flying with an LNAV glideslope if safer than flying a 2D approach. But that clarity of thought and common sense is not applied by the regulator.
My view, based on long and very regular experience, is that the ILS and LPV glideslope is completely usable and reliable between 200’ and 100’, and is the safest way to remain on slope until really clear (as opposed to adequate) visual references are available.
This may not be the view of a calibrator, who has not calibrated that part of the profile, because he is not required or asked, possibly even allowed, to do so, but it is the view of every professional pilot I have spoken to about it.
Well I suggest that we all go and fly one ILS or LPV each, to 100ft, and report back on whether the practical performance is indeed better than the regulatory/calibration requirements, as Timothy is suggesting.
ortac wrote:
Well I suggest that we all go and fly one ILS or LPV each, to 100ft, and report back on whether the practical performance is indeed better than the regulatory/calibration requirements, as Timothy is suggesting.
I don’t think that will really achieve anything. LPV slopes work below the minima of course – they are GPS derived. ILS might, or might not as Dave has said. Personally, I wouldn’t rely on it in IMC. If I couldn’t be sure I am on the right slope after mimima by visual reference, how would I know if the GS is showing incorrect info?
I agree from a practical point of view and I don’t have any appetite for doing approaches where I can’t fly visual at 200ft.
But it’s always good to know what is possible if needed in a tight spot, and we have one person saying it’s straightforward and another saying it’s impossible.
It is an interesting thought experiment to ask yourself what you would do if everything within range of your aircraft were unforecast R100.
I have seen such a situation twice in my career.
The first time the whole of southern England went below minima, but I found out from VOLMET early enough not to cross the Channel and to land, sit and wait it out in northern France.
The second time was similar, but I was approaching Biggin from the north, when everywhere I got the weather from began to fall like nine-pins. I ducked quickly into Cranfield, which was still open, but the curtains closed behind me.
Both events were unforecast. So, although a thought experiment, it is far from impossible.
This was why the great Dai Heather Hayes taught me to land blind in my initial IR training. One day it just might save your life.
Although the GS may not be certified below DA, it is definitely there and definitely usable.
JasonC wrote:
If I couldn’t be sure I am on the right slope after mimima by visual reference, how would I know if the GS is showing incorrect info?
You are stable, and your stability includes RoD and speed.
This is the same question as “how would I know I was on the FAT of an overlay NDB approach if I had no ADF?” or “How do I know I can rely on an SBAS VNAV slope when it was defined for BaroVNAV?” The answer is that no-one in officialdom will agree that you can, but you know perfectly well you can.
Timothy wrote:
This may not be the view of a calibrator, who has not calibrated that part of the profile, because he is not required or asked, possibly even allowed, to do so, but it is the view of every professional pilot I have spoken to about it.
The calibrator hand flies down to 50ft on all ILS taking that to the threshold or, for a CAT III the localiser. GP calibration profiles also check the GP +/- 8 degrees laterally and from 0.3 Theta to 1.75 Theta (Theta being the notional GP angle). The simple fact is that the RF, DDM and Signal Strength tolerances for a CAT I are not required to meet (and often don’t) the minimum specification necessary to give you reliable vertical guidance beyond Point B (system minima). A really good example of a bad GP below system minima is/was the old one at Biggin Hill. Indeed, other specific glidepaths that do bite include RW31L at Abu Dhabi which will drive you into the undershoot and that is on a CAT IIIB system!!
The suggested trial is of little value in an ordinary aircraft. If you accurately fly the notional ILS GP down to 100ft (or less) in a PA31 the PAPIs, the only mechanism of cross reference in most aircraft, will indicate three-reds-one-white (possibly one of the reds being in a pink transition) due to the MEHT correction which is set at value far greater than needed for the PA31. Meanwhile, the calibration aircraft knows where it is in space to within 20mm accuracy (rather ironically, due to a fancy DGPS 
) and can accurately judge the GP signal.
Although the GS may not be certified below DA, it is definitely there and definitely usable.
…..and that is my major concern as a professional pilot – I don’t test the boundaries.
I suppose seeing the GS bang on when you are on glideslope, might not necessarily mean that it would show an accurate deviation if you were to deviate from the parameters.
Some more interesting references are found here.
