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ILS approach. Some questions.

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ILS approach. Some questions.

Old 3rd Jul 2012, 16:19
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ILS approach. Some questions.

Excuse my silly questions lads, but im a newbie looking for some answers.

On an ILS approach, with approch mode amed while you are slightly below platform altitude, things should be okay correct?
If you are above platform altitude, wether or not you will catch the glide depends on distance.
If you were at 4500 feet on long final 17 miles, you should catch the glide correct?
If you were at 10 miles (for example), you'd have to catch it from above.
Is the GS usually accurate if showing you that you're above or below it, even from a far distance?

Also. If you were coming 15 miles from touchdown, and 3000 feet, with a platform altitude of 2000 feet. You are coming from the opposite side of the runway, and ATC has cleared you for the approach (hypothetically), in this case surely the GS will never bring you down coming in from that angle correct?

Apologies once again.
Just seem a little confused.

Last edited by meatlover; 3rd Jul 2012 at 16:22.
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Old 3rd Jul 2012, 19:58
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Here's an idea

turn off the autopilot and hand fly it.
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Old 3rd Jul 2012, 22:39
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Commissioned GP coverage is usually only ten miles, so 4500 above the facility is technically a no-go. Most equipment will pick it up even if it is below the tolerance signal levels, but the facility isn't approved further out and there are no guarantees. You should get flag warnings if it is not to be trusted...

There are GP "back courses". They usually aren't anything you could approve as a GP, but if you are approaching the reciprocal runway with the ILS tuned, you may pick one up and see it on the needle... with no flag. They could be perfectly flyable and lead you straight to the departure end of the runway.

The procedure designers with all the expensive training are the ones who are allowed to be creative in this case.
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Old 4th Jul 2012, 01:03
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On an ILS approach, with approch mode amed while you are slightly below platform altitude, things should be okay correct?
I'd say so.. but still need to be cautious of your speed and configuration (it might not work if you are clean, 20 kts tailwind and at 290kts..)

Having said that, do you mean platform altitude?
If you are cleared to descend 3thousand and also cleared for the approach, you shouldn't be going below the platform of 3thousand until you have captured the glide.


If you are above platform altitude, wether or not you will catch the glide depends on distance.
If you were at 4500 feet on long final 17 miles, you should catch the glide correct?
If you were at 10 miles (for example), you'd have to catch it from above.
Is the GS usually accurate if showing you that you're above or below it, even from a far distance?
It's roughly the 3x table.
2000ft - 6 miles, 3000 - 9, 4000 - 12 etc
You of course need allowance for the airfields height above sea level.. if threshold elev is 12 feet it doesn't matter, but if it is 1800 feet then your readings on the altimeters are going to be higher and you may get caught out with configuration etc.

As mentioned above, you have areas of coverage for the ILS. Caution when intercepting from above due to false glide paths etc, if it is IMC then you need to be really careful about this.. one of the reasons we have a check height on the way down (usually the OM or alike).

Also. If you were coming 15 miles from touchdown, and 3000 feet, with a platform altitude of 2000 feet. You are coming from the opposite side of the runway, and ATC has cleared you for the approach (hypothetically), in this case surely the GS will never bring you down coming in from that angle correct?
Don't understand that?

Last edited by i_like_tea; 4th Jul 2012 at 01:06.
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Old 4th Jul 2012, 01:07
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There can also be "false" GSs. They are generated at multiples of the GS angle. So there could be a GS with a 6 degree angle, 9 degree angle, 12 degrees, etc. Why you check your altitude and distance at GS intercept.
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Old 4th Jul 2012, 01:36
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I_like_tea,

Thanks mate.
Yes understand the problem with elevation being 1800 feet, my whole times 3 should be added to that.
12 miles - 5800 this case.
Easier to configure and do everything based on radio altimeter for now as a newbie?
2500 feet at least 7.5 miles.
Before intercepting LOC I could always do flaps one, especially if it's a tight turn, and once radio alt comes alive, 2500 to 1500 could be enough to be in landing config more or less.
Of course this is only to avoid confusion in high elevation airfields just for now.
And it's an idea.

Any more advice is welcome.

Also got one more ques.
On a VOR approach for example. Performing a fully selected approach. Platform altitude being 2500 feet.
ATC has cleared me for the approach while I'm at 4000 feet.
Just open descent and get myself to 2500, and just as always I'd pull FPA etc.
What happens if while trying to descend down to 2500 ive passed my FAF.
On an ILS of course it's capturing the GS from above like we said.
What happens in this case?

Thanks again.
Now at 10 miles and I'm at
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Old 4th Jul 2012, 02:19
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Yes and no, Rad Alt can be a good clue but in areas with terrain it should not be relied upon. Remember that in a lot of places the terrain on the approach can be very different to that of the runway, so 2500 on the rad alt could still be 1000ft AAL.

If you struggle in the early days you could always make a note of the heights you're looking for and stick it somewhere in front of you.. it sounds a bit silly but it can help and can increase situational awareness greatly.
Also useful for tricky EO - SIDs.

Configuring will be down to your airlines SOP.
If you're doing a decelerated ILS approach then it's going to be done on height about airfield and also DME distance (maybe ATC want 160 to 4?)
it also depends on your platform altitude.

Flap 1 on the LOC is a good idea but may not be suitable in some situations.. perhaps you're miles alt with a speed constraint from ATC.
Certainly tight vectors with a tail wind behind you pushing you through your lock, you will want to be reducing speed... it all really depends on the environment and how god your vectors are, which you would learn with time.

If you have a low platform altitude (for example, 2000ft in BCN) then we'd be conf 2 before descending on the glide as per our SOP.
Or a higher platform (standard 3000ft) with a chuffing tailwind, you could take flap 2 and select speed 180, so you have the drag when you need it, then descend on the glide)

SOP here is conf2 by 2000 AAL, (SO if theshold 1800AGL = 3800 on the altimeter) and fully configured by 1000AAL.
When you select that will be again down to conditions.. tail wind, weight etc. A light 319 is going to slow down a lot quicker than a heavy 320!

A good rule of thumb a few people use for gear selection is GS + thres elev... so if you have 190kts groundspeed and 300 feet thres elev.. they reckon on taking the gear at 2200ft in standard conditions...


As for the VOR question, again depends a bit on the day IMO.
If you had "seen it happening" got yourself fully configured and at VAPP, you could quite happily descend and re-capture the profile as you would if you were high during the approach
A useful guide for FPA corrections is that 0.1 change in FPA will affect the altitude by 10 feet in the next mile.
Remember though, if 2500 is stil the tagret altitude, it's going to try and capture it which will get you even more behind for the approach. Wind it up if the missed approach altitude is below you so you don't ALT* (I assume you're after Airbus info..) but don't forget to reset the missed approach altitude later.

This is my own experience and opinion,
There are SO many variables it is impossible to give you a set solution.
You learn from experience.. once the skipper has suggested you start configuring a few times due to that 10kt tailwind you didn't notice.... you might remember next time!

I hope that helps sort of, I'm no expert and you should fly as you learn it and study it.. don't take my word for anything, ever!

Our company has some pretty good guides to a lot of this stuff, not sure if it is the same company as yours though?

Last edited by i_like_tea; 4th Jul 2012 at 02:30.
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