ILS glideslope integrity

[GADD45B]

When identifying the localizer after it is tuned in to ensure signal and integrity, how does one ensure that the glideslope is also being received? Let's assume that the VOR indicator does not have a Glideslope flag. This situation happened to me yesterday. I was receiving the morsecode crysal clear and still had a GS flag at the glideslope intercept position. I had a GS flag up, but if my particular indicator did not have a flag/no flag indicator how would I know?

[Dugie8]

I have not seen a LOC/GS indicator that does not have a flag, I believe they are required, but I would be hard pressed to find that in writing. As to verifying the integrity, that is why you have the glide slope intercept altitude published on the chart, ie "Final approach fix, instruments and altimeters cross checked...". Don't stop their either, using DME, either ground or space based, cross check your postion from the runway with your current AGL altitude and compare to the glideslope. For example, you are 3 miles from the runway and the glideslope says your are centered but you are at 300 AGL (reference the TDZE), rut ro raggy, sumpin wrong!

[GADD45B]

Good points. So identifying that the glideslope signal is reliable must come only from indirect means with either DME, VOR intersection, or LOM, in the absense of an indicator lacking a GS flag.

[Dugie8]

Pretty much. The morse code ID is for the freq, 109.7 for example. Now for every ILS that uses 109.7, it has the exact same GS freq, they are paired. To be perfectly honest, in all of the ILS approaches I have shot, I have never had a false GS, or had one lead me astray, not saying it can't happen, just food for thought.

[DE727UPS]

You can't have a glideslope indicator without a glideslope flag. Not sure what you're flying but it sounds familiar.

I used to fly an old Cessna that had a rock solid LOC but a wimpy GS. LOC would ident fine but the GS flag wouldn't go away, even when the GS needle seemed to be doing the right thing. Then, when you got in closer, the GS flag would disappear and the needle would steady out.

I wouldn't take a plane like that into actual until I knew it's limitations.

Checking your altitude at the OM/FAF on the LOC is more like confirming that your GS indication is correct. Not really "identifying" the GS, so to speak.

[EatSleepFly]

Originally Posted by Dugie8

Now for every ILS that uses 109.7, it has the exact same GS freq, they are paired.

Actually they're not the same. The localizer frequency is VHF, and is paired with a UHF glide slope frequency for a total of 40 possible ILS frequencies. The glide slope freq. range and the localizer/glide slope frequency pairings are in the AIM.

[Dugie8]

Originally Posted by EatSleepFly

Actually they're not the same. The localizer frequency is VHF, and is paired with a UHF glide slope frequency for a total of 40 possible ILS frequencies. The glide slope freq. range and the localizer/glide slope frequency pairings are in the AIM.

Oh good lord, that is what I meant, thank you

[awacs94]

Frequency Pairs Allocated for ILS Localizer MHz
Glide Slope

108.10
334.70

108.15
334.55

108.3
334.10

108.35
333.95

108.5
329.90

108.55
329.75

108.7
330.50

108.75
330.35

108.9
329.30

108.95
329.15

109.1
331.40

109.15
331.25

109.3
332.00

109.35
331.85

109.50
332.60

109.55
332.45

109.70
333.20

109.75
333.05

109.90
333.80

109.95
333.65

110.1
334.40

110.15
334.25

110.3
335.00

110.35
334.85

110.5
329.60

110.55
329.45

110.70
330.20

110.75
330.05

110.90
330.80

110.95
330.65

111.10
331.70

111.15
331.55

111.30
332.30

111.35
332.15

111.50
332.9

111.55
332.75

111.70
333.5

111.75
333.35

111.90
331.1

111.95
330.95

Reference: AIM
www.faa.gov/ATPUBS/AIM/Chap1/aim0101.html

[seagull]

This is why ICAO requires that the FAF crossing altitude be published and cross checked on the approach.

[BobDDuck]

Originally Posted by seagull

This is why ICAO requires that the FAF crossing altitude be published and cross checked on the approach.

There also can be multiple ghost glide slopes at angles other then 3degrees. You can sometimes get them if you break off an approach and fly straight ahead. The glide slope will go below you (as it should) then at some point jump the top of the display and then drop down towards the bottom again as you fly through it. It is *in theory* possible to capture one of these slopes but hopefully you would note that you were decending at WAY more then 3 degrees and would break off before you hit the FAF and saw you were much higher then the published FAF crossing altitude.

[seagull]

While it is true about false glideslopes, in reality they often have reverse sensing and are so extremely sensitive and erratic that you would not be confused for very long!

[Dugie8]

Originally Posted by seagull

While it is true about false glideslopes, in reality they often have reverse sensing and are so extremely sensitive and erratic that you would not be confused for very long!

I've read that too about the reversing of the glideslope if it is a false one, can't remember where, but to be honest, of all the accident reports (and I haven't read that many) that involved following a false glideslope, it was always too steep of an approach, which seems to say the glideslope wasn't inverted but was simply too steep.

Do you happen to remember where you read that about the false glideslopes and they being inverted?

[BobDDuck]

An interesting safety video care of Air NZ about glide slope problems. I remember reading the report (which was much more detailed) but I can't seem to track it down.

[seagull]

Will have to research, but I seem to recall that the alternate, one normal, the next reverse, etc. I think there is more risk of following what you think is a GS and having no GS signal at all. That is what KAL did into GUM a few years ago. Nasty accident, a site which I had the unfortunate pleasure of being at.

[Jason]

Originally Posted by BobDDuck

An interesting safety video care of Air NZ about glide slope problems. I remember reading the report (which was much more detailed) but I can't seem to track it down.

I show this video to my Citation recurrent classes...it's a real eye opener. The moral of the story is that there are multiple 'safeguards' built into an ILS transmission system but they can fail in various modes and from the cockpit the ONLY way for you to verify that you're on the correct GS is to do MULTIPLE distance/altitude checks. The ident. on the frequency is only for the localizer signal and only doing one altitude check at the 'FAF' is not valid.

It's a good video and everyone should definatley watch it.

Jason

[seagull]

I am not sure you need to go that far. If you are at the right altitude at the OM/FAF, and your rate of descent is about right (or display flight path angle, if you have it), that will be a very good indicator that your GS is doing its job.