Intercepting from Above
Join Date: May 2002
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Like keithl, I still can't find anything documented.
Lest anyone be tempted still, as OzEx points out, be tempted to follow it even once they have identified it as a false GP, they need to consider that the aircraft's energy will be significantly higher than when on a "normal" ILS approach.
Kinetic energy is significantly higher, speed often ditto, and all that neergy has to be lost on the round-out. This will involve landing longer or faster (or both) than normal, and your landing distance performance tables are then useless. If you are heavy, chances of going off the far end (at worst) or heavy brake use (at best) will be the inevitable result.
Not to be recommended, as you could (a) bend it or (b) find yourself in the FM's office with hat and without tea and biccies.
Lest anyone be tempted still, as OzEx points out, be tempted to follow it even once they have identified it as a false GP, they need to consider that the aircraft's energy will be significantly higher than when on a "normal" ILS approach.
Kinetic energy is significantly higher, speed often ditto, and all that neergy has to be lost on the round-out. This will involve landing longer or faster (or both) than normal, and your landing distance performance tables are then useless. If you are heavy, chances of going off the far end (at worst) or heavy brake use (at best) will be the inevitable result.
Not to be recommended, as you could (a) bend it or (b) find yourself in the FM's office with hat and without tea and biccies.
Join Date: Sep 2000
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Keithl,
I tried to find documentation on where the false glidepaths appear, but haven't succeeded so far. So I would like to see what you can find.
And, sorry, I haven't either found an approach where such an approach would be the normal procedure. Would like to hear if there is such, as well.
And then a correction to my own posting after having read it a number of times; Setting the localizer minima when flying an ILS approach could create a big hazard (B-737-700). If you at any point decide to do a missed approach, the Flight Director/Auto Pilot will aim for the altitude set in the MCP panel.
It will be a judgement call if you are intercepting the glidepath from above, but latest, as my SOP says, the approach should be stabilized from 1000' RA meaning:
Aircraft in landing configuration
Aircraft on the correct fight path
Airspeed within target speed +10/-5 kts
Sink rate maximum 1200 fpm
Engines at stabilizid forward thrust
Landing check list completed.
I am sorry for the lengthy post, but I didn't want anyone to get new ideas based on my previous posting.
I haven't come up with any answers to your question but I will follow this thread, as I am curious myself.
I tried to find documentation on where the false glidepaths appear, but haven't succeeded so far. So I would like to see what you can find.
And, sorry, I haven't either found an approach where such an approach would be the normal procedure. Would like to hear if there is such, as well.
And then a correction to my own posting after having read it a number of times; Setting the localizer minima when flying an ILS approach could create a big hazard (B-737-700). If you at any point decide to do a missed approach, the Flight Director/Auto Pilot will aim for the altitude set in the MCP panel.
It will be a judgement call if you are intercepting the glidepath from above, but latest, as my SOP says, the approach should be stabilized from 1000' RA meaning:
Aircraft in landing configuration
Aircraft on the correct fight path
Airspeed within target speed +10/-5 kts
Sink rate maximum 1200 fpm
Engines at stabilizid forward thrust
Landing check list completed.
I am sorry for the lengthy post, but I didn't want anyone to get new ideas based on my previous posting.
I haven't come up with any answers to your question but I will follow this thread, as I am curious myself.
Last edited by bluethunder; 5th Jan 2004 at 07:38.
Thread Starter
BT and all interested parties: I have now found three relevant references.
1. My old CFS notes (circa1980) say that the first false GP is "not below 10deg".
2. "Ground Studies for Pilots", Taylor & Parmar, 1990 quotes "above 6deg".
3. "Air Pilot's Manual", Thom, 1990 says "above 12.5deg".
So, I don't think I've proved anything, but it would be wise to assume the lowest figure until we've proved otherwise. Is there a "Calibrator" out there?
1. My old CFS notes (circa1980) say that the first false GP is "not below 10deg".
2. "Ground Studies for Pilots", Taylor & Parmar, 1990 quotes "above 6deg".
3. "Air Pilot's Manual", Thom, 1990 says "above 12.5deg".
So, I don't think I've proved anything, but it would be wise to assume the lowest figure until we've proved otherwise. Is there a "Calibrator" out there?
Some of the older ILS that I flew on, gave false glide slopes at the harmonics of the GS angle i.e. 6 deg, 9 deg, and 12 deg, for a 3 deg slope. The signals were reversed at alternating angles; thus although a false path existed at 6 deg the fly up / down indications indicated in the opposite sense. Some 9 deg GS were use for experimental work on steep approaches. Some military ILS with GS as low as 2.5 deg had correspondingly lower false slopes at 5, 7.5 and 9 deg, but again with alternating reversals of the fly up / down indications.
There is nothing to say that you must not join a GS from above, but it is not good practice. Good situation awareness is always required, but with joins from above or when flying a CDA a mental plot of altitude vs range is essential. Remember 3 deg slope is approx 300 ft/nm thus you should be on the slope by 1500 ft (aal) at 5nm.
Also, remember that there are similar side lobe effects on the LOC, thus automatic / FD systems should not be armed for capture until close to the centreline, which in turn must be verified by other means. ADF locator, map, crosscut DME, etc. For me this is a far greater hazard than a false GS; early LOC capture and turn in can quickly erode safe altitude margins.
There is nothing to say that you must not join a GS from above, but it is not good practice. Good situation awareness is always required, but with joins from above or when flying a CDA a mental plot of altitude vs range is essential. Remember 3 deg slope is approx 300 ft/nm thus you should be on the slope by 1500 ft (aal) at 5nm.
Also, remember that there are similar side lobe effects on the LOC, thus automatic / FD systems should not be armed for capture until close to the centreline, which in turn must be verified by other means. ADF locator, map, crosscut DME, etc. For me this is a far greater hazard than a false GS; early LOC capture and turn in can quickly erode safe altitude margins.
Thread Starter
That's most interesting, safetypee. So a consistent argument that would draw all the evidence together would be that:
For a standard 3deg GP:
a. The first false GP is at 6 deg, but
b. the first false one that gives logical demands is at 9deg
c. Falsies do appear at 12deg and upwards but may be discounted due to the extreme error needed to find them.
Anyone unhappy with that?
For the record I am therefore happier about intercepting from above (within reason), but still have a problem with the unclear wording in GASIL.
For a standard 3deg GP:
a. The first false GP is at 6 deg, but
b. the first false one that gives logical demands is at 9deg
c. Falsies do appear at 12deg and upwards but may be discounted due to the extreme error needed to find them.
Anyone unhappy with that?
For the record I am therefore happier about intercepting from above (within reason), but still have a problem with the unclear wording in GASIL.
keithl, don’t be too quick with the logical conclusion. My experiences were with older ILS (many still in service), but I suspect that even the latest equipment has many pitfalls for the unwary aviator.
I also note that at least one of the major manufacturers, whilst accepting joins from above, cautions against arming the automatic capture systems until the aircraft is safely clear of false GS or LOC signals. Although the human can deduce that the indications are reversed, automation just blindly follows the received signal.
The UK NATS give advice on the use of ILS in AIC Pink 141 (AIC 34/1997) the link may have restricted access. If registration is required start with: www.ais.org.uk
The following extracts are relevant:
“The GS coverage is 10 nm +- 8 deg from LOC centerline. Vertical coverage is between 0.45 x and 1.75 x GS angle (i.e. 1.35 deg to 5.25 deg above the horizon for 3 deg slopes)”. I assume this is the calibrated (guaranteed) signal, thus does not exclude false GS beams outside of these limits.
“Where special approach procedures have been devised in which the GS may be joined from above, pilots must be aware that false GS may exist at an angle of about an angle of 2 x GS angle (i.e. 6 deg for 3 deg slope). This false GS must not be used for descent guidance. Establishment on the promulgated GS should be confirmed by the relationship between aircraft height and the distance to runway threshold.” i.e. approx 300 ft/ nm for 3 deg GS.
For LOC the useable coverage is 25 nm for +- 10 deg and 17 nm for +-35 deg, both from the front course. (Does this imply no back course ILS in UK, I think so?) These values are less for steep approach ILS installations. There is a caution about the risk of early LOC capture even within these limits.
I also note that at least one of the major manufacturers, whilst accepting joins from above, cautions against arming the automatic capture systems until the aircraft is safely clear of false GS or LOC signals. Although the human can deduce that the indications are reversed, automation just blindly follows the received signal.
The UK NATS give advice on the use of ILS in AIC Pink 141 (AIC 34/1997) the link may have restricted access. If registration is required start with: www.ais.org.uk
The following extracts are relevant:
“The GS coverage is 10 nm +- 8 deg from LOC centerline. Vertical coverage is between 0.45 x and 1.75 x GS angle (i.e. 1.35 deg to 5.25 deg above the horizon for 3 deg slopes)”. I assume this is the calibrated (guaranteed) signal, thus does not exclude false GS beams outside of these limits.
“Where special approach procedures have been devised in which the GS may be joined from above, pilots must be aware that false GS may exist at an angle of about an angle of 2 x GS angle (i.e. 6 deg for 3 deg slope). This false GS must not be used for descent guidance. Establishment on the promulgated GS should be confirmed by the relationship between aircraft height and the distance to runway threshold.” i.e. approx 300 ft/ nm for 3 deg GS.
For LOC the useable coverage is 25 nm for +- 10 deg and 17 nm for +-35 deg, both from the front course. (Does this imply no back course ILS in UK, I think so?) These values are less for steep approach ILS installations. There is a caution about the risk of early LOC capture even within these limits.
Thread Starter
Thanks again, sp. Your point about the crew's responsibility for being clear of false signals is one I referred to earlier in this thread (23 Dec 10:53) so I agree there. I do have the AIC you refer to, but it is not quite detailed enough. Interestingly, the reference to "where special approach procedures have been devised in which the Glide Path may be joined from above..." takes us right back to my original question, i.e. "Where might that be then?"
