Flying below the glide slope as a tactic

azure said:
This is a really good point - you are much likelier to see the rabbit below you, looking to the left of the nose or out the side window, than you are to see it in front of you just because the optical depth of the soup is so much less vertically.

And also, Henning is right, do the math and you'll see that at a quarter dot low you're only 3 feet or so low and maybe 60 feet further back, assuming a 3 degree glideslope.

After listening to all the arguments here I'm convinced that it's not just stupid because of the risk, but offers no operational advantage either, even if you went a dot or two low.

Edit: whoops, I see MauleSkinner already posted about the geometry of the glide slope.
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I was almost insulted there for a minute! ;)
 
If you're good enough to hold a steady 1/4 dot below GS all the way down, more power to ya
 
dell30rb said:
If you're good enough to hold a steady 1/4 dot below GS all the way down, more power to ya
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I disagree on many levels...

The idea of following heavy airplanes hasn't even been mentioned yet. I realize a quarter dot, or quarter scale that it likely wouldn't be a consideration, but I can see a few scenarios where it could be.

Besides, I said it before and I'll say it again... Both aircraft will be 200 feet above TDZE elevation looking for lights. The best position to be at that altitude to look for those would be on the glideslope
 
Kritchlow said:
I disagree on many levels...

The idea of following heavy airplanes hasn't even been mentioned yet. I realize a quarter dot, or quarter scale that it likely wouldn't be a consideration, but I can see a few scenarios where it could be.

Besides, I said it before and I'll say it again... Both aircraft will be 200 feet above TDZE elevation looking for lights. The best position to be at that altitude to look for those would be on the glideslope
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That's just it, as Maule Skinner showed in the math, for all intents and purposes, at DH you basically are on the GS at a 1/4 dot off.
 
The GS leads to the runway...fly the GS...and like magic...the runway will appear. You will be a hit at all the parties from then on!
 
I'm still trying to understand why he thought this is useful. If you illegally follow the ILS GS down the ground, where will that put you on the runway assuming everything is perfect with no error? Does anyone know off hand? I know that point will NOT be the numbers on the approach end.
 
Kelvin said:
On a runway with standard markings...1000' is the touchdown point.
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Okay in some twisted logic, if you keep on going down the GS and blow through your DA and all the stars align, you would have more runway in front of you when you finally get down to 20 feet AGL say. Like 1000 feet more runway if you figure out how much below the GS you need to be to hit the numbers. This would be a crazy plan. :dunno:
 
Kelvin said:
On a runway with standard markings...1000' is the aiming point.

In reviewing this doc, I learned a few things..

http://ww1.jeppesen.com/documents/aviation/business/ifr-paper-services/airport-signs.pdf
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Right. Assuming zero float, runway impact will be 1,000 feet past threshold. That may vary slightly. To be exact, look where the glide slope antenna is placed. Usually 1,000 feet down. The antenna used to be big and visible. I think they downsized it now and it's not as visible anymore come to think of it.
 
narchee said:
I'm still trying to understand why he thought this is useful. If you illegally follow the ILS GS down the ground, where will that put you on the runway assuming everything is perfect with no error? Does anyone know off hand? I know that point will NOT be the numbers on the approach end.
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Your glideslope antenna is aiming for the 1000' markers. On a large airplane that could put the landing gear on the runway considerably before that.
 
This sounds about the same as climbing above your target altitude and descending back to it so you can fly on the step.
 
peerlesscowboy said:
Your glideslope antenna is aiming for the 1000' markers. On a large airplane that could put the landing gear on the runway considerably before that.
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It could, but after flare they almost never touch down before the 1000' mark unless they're low. From extensive watching of where they land, I'd say the sweet zone seem to be 1500-1750'.
 
peerlesscowboy said:
Your glideslope antenna is aiming for the 1000' markers. On a large airplane that could put the landing gear on the runway considerably before that.
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Don't forget the flare.

For Part 121 the FAA requires touchdown in the TDZ.

Also, if the airplane is autoland equipped (most of them are these days) the radio altimeters have more input then the G/S as the runway gets close.
 
EdFred said:
This sounds about the same as climbing above your target altitude and descending back to it so you can fly on the step.
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I think it's more like putting english on a cue ball, YMMV.

dtuuri
 
narchee said:
Okay in some twisted logic, if you keep on going down the GS and blow through your DA and all the stars align, you would have more runway in front of you when you finally get down to 20 feet AGL say. Like 1000 feet more runway if you figure out how much below the GS you need to be to hit the numbers. This would be a crazy plan. :dunno:
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No, if you're flying a set "1/4 dot low" or even "1/4 scale low", the glideslope is still leading you to the same point 1000 feet down the runway. You're just coming in shallower, and the vertical displacement from the ILS glideslope gets smaller as you get closer to the antenna.

The way you have described would only work if you could fly some set _height_ below the glideslope, like 50 feet low (which would put you touching down right about on the threshold given a 3 degree glidepath). But that's not possible with current equipment as far as I know. Also realize that 50 feet low at the threshold is going to be more than full-scale deflection of the glideslope.

50 feet is an average threshold crossing height for an ILS. See also the previous comments about wheel height, which I have not accounted for since it was already mentioned.
 
RussR said:
No, if you're flying a set "1/4 dot low" or even "1/4 scale low", the glideslope is still leading you to the same point 1000 feet down the runway. You're just coming in shallower, and the vertical displacement from the ILS glideslope gets smaller as you get closer to the antenna.

The way you have described would only work if you could fly some set _height_ below the glideslope, like 50 feet low (which would put you touching down right about on the threshold given a 3 degree glidepath). But that's not possible with current equipment as far as I know. Also realize that 50 feet low at the threshold is going to be more than full-scale deflection of the glideslope.

50 feet is an average threshold crossing height for an ILS. See also the previous comments about wheel height, which I have not accounted for since it was already mentioned.
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Exactly, regardless where in the scale you keep a steady needle, all paths intersect at the same point. ILS deflections are not parallel, they are just different angles from the same origin.
 
MauleSkinner said:
If he's talking 1/4 dot instead of 1/4 scale, that's only 5 feet below centerline. Either way, it doesn't seem useful to me.:dunno:
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Let's say it's 6 feet below centerline. Staying on the glidepath, by comparison, would require 60' of linear travel for each 3' of vertical elevation. So, 60' x 6' / 3' = 120' delay in spotting the ALS light bars--enough to try to catch one more sequenced flashing light. Might be useful, might not. :dunno:

dtuuri
 
dtuuri said:
Let's say it's 6 feet below centerline. Staying on the glidepath, by comparison, would require 60' of linear travel for each 3' of vertical elevation. So, 60' x 6' / 3' = 120' delay in spotting the ALS light bars--enough to try to catch one more sequenced flashing light. Might be useful, might not. :dunno:

dtuuri
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I don't see how it would be. Cat 1 it's what you see at 200 feet (usually), not what you see at X distance.
In addition, if it's that low a commercial airliner is probably not doing a Cat 1 approach so that number shrinks, and in some cat 3 approaches the approach lights are irrelevant.
 
Kritchlow said:
I don't see how it would be. Cat 1 it's what you see at 200 feet (usually), not what you see at X distance.
In addition, if it's that low a commercial airliner is probably not doing a Cat 1 approach so that number shrinks, and in some cat 3 approaches the approach lights are irrelevant.
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I flew my share of them. At LAX in the early morning fog. You just don't see the approach lights. In fact, in a 1011, you don't see anything until the nose comes down after the mains are on. Not an operation to do unless everything is "in the green."
 
narchee said:
Okay in some twisted logic, if you keep on going down the GS and blow through your DA and all the stars align, you would have more runway in front of you when you finally get down to 20 feet AGL say. Like 1000 feet more runway if you figure out how much below the GS you need to be to hit the numbers. This would be a crazy plan. :dunno:
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An airliner(or any heavy turbine for that matter) isn't trying to hit the numbers, they are aiming for the touchdown zone as others have said. With such a big airplane, one thing you are concerned about is landing short and/or taking out runway lights. I am required to cross the threshold between 50-75 feet. A normal 3 degree glideslope usually has you crossing the threshold around 50 feet. With the deck angle I fly the ILS, and the location of the ILS antenna vs. the main wheels, if I fly the ILS spot on, I usually cross the threshold around 40 feet. If I did it a quarter of a dot lower that would be even lower! Not sure what airframe he uses to fly international but I am guessing it is quite large and he is probably in the same boat I am.
 
dtuuri said:
Let's say it's 6 feet below centerline. Staying on the glidepath, by comparison, would require 60' of linear travel for each 3' of vertical elevation. So, 60' x 6' / 3' = 120' delay in spotting the ALS light bars--enough to try to catch one more sequenced flashing light. Might be useful, might not. :dunno:
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I don't think that's right. The GS is 1.4 deg wide, is that not 8 dots from full scale up to full scale down?

If so, 1/4 dot low is 1/16 of 0.7 deg or 0.04 deg low, hardly worth considering the difference in angle from being perfectly centered on the GS.

How low that is from the center of the beam scales with your distance from the antenna, but at 4000' (a little over a half mile from the threshold) it comes to about 3 feet low, even less if you are closer in.

If the GS is 3 degrees, you lose a little over a foot for each 20 feet horizontally, so by my math that comes to just under 60 feet back from where you would be if you were right on the GS.

Even at 90 kts ground speed, 60 feet go by in less than a half second.

I can't see any significant advantage there.
 
Sounds like a bad tactic.
 
Depending on the type aircraft the friend of the OP flies, he is flirting with dragging his main gear very low at the end of the runway. My present airplane lists these paramenters
Cockpit height at touchdown 40 feet
Main gear height at the threshold 26 feet
Main gear touchdown point 540 feet
These numbers are at a forward CG and more restrictive and at a 2.5 degree GS.
At a 3.0 GS the distance for touchdown of the main gear is 658 feet.
We use an aim point of 1500 feet down the runway due to the main gear being 102 feet behind the cockpit.

I shudder to think how close to the end of the runway the gear would be at 1/4 dot low. We are allowed Zero below GS in the touchdown zone.
 
azure said:
I don't think that's right. The GS is 1.4 deg wide, is that not 8 dots from full scale up to full scale down?

If so, 1/4 dot low is 1/16 of 0.7 deg or 0.04 deg low, hardly worth considering the difference in angle from being perfectly centered on the GS.

How low that is from the center of the beam scales with your distance from the antenna, but at 4000' (a little over a half mile from the threshold) it comes to about 3 feet low, even less if you are closer in.

If the GS is 3 degrees, you lose a little over a foot for each 20 feet horizontally, so by my math that comes to just under 60 feet back from where you would be if you were right on the GS.

Even at 90 kts ground speed, 60 feet go by in less than a half second.

I can't see any significant advantage there.
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No matter how many 1/16ths of a dot result in being 6 feet low it still takes 120 feet longer horizontally to reach DH on the glide path. I only commented on the 6 feet value, not the fraction of a dot. Would seeing one more flashing light help? Who knows? I've made approaches where I didn't see any at all and some were only one or two. If my gear were hanging down a block behind my seat, though, I'd probably fly a quarter dot high.

dtuuri
 
Well I think his point is though that if you keep the needles centered and keep on coming, you will come down to a point beyond the numbers and some distance down the runway. Whereas if you use this technique and keep on coming as you say the approach end with the more intense lighting will be what you are heading towards.
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This is true, and may be the reason he practices it. But, I am staying on or above centered instead of below. Obstacle clearance gets tighter and tighter the closer you get Not good to hit something hard :).
 
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