question: airliner altitude on approach - ILS vs visual

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alanbreck

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AlanB
Just for fun, I'm calculating the AGL altitude that the airliners would be on final approach as they cross over my house, flying the ILS (or LPV). My house is inside the FAF. So it's easy geometry by using Google earth to get distance from my house to the runway TDP, and the approach plates specify the 3 deg glideslope (and the terrain around is all pretty level).

My question for you airline pilots is this: when cleared for the visual (instead of an instrument approach), is the airliner likely to still be on the same glide path? Or typically lower? or higher? Just curious.
 
On it. Company policy.

Also 300' every mile if ya wanna figure agl. So 5 mile out around 1500' agl. Think that's what it was.
 
mscard88 said:
On it. Company policy.

Also 300' every mile if ya wanna figure agl. So 5 mile out around 1500' agl. Think that's what it was.
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318.something. And don't forget curvature of the earth. It really makes a difference in even just a few miles. I wouldn't have believed it. There was a thread here awhile back all about it. Someone might jump in with "numbers"
 
Even if their flying the visual, they still back it up with the glideslope. This is company policy for SWA and I would assume it's industry wide. So yes, they will still be at the same altitude.
 
Same altitude. If we are not stable at 1000 feet, on speed, fully configured, sink less than 1000 fpm then we do a no fault go around automatically.
 
Ryanb said:
Even if their flying the visual, they still back it up with the glideslope. This is company policy for SWA and I would assume it's industry wide. So yes, they will still be at the same altitude.
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Oddly enough, this industry-wide company policy is regulatory. ;)

See 91.129e, which is also quoted as being required in 91.130 and 91.131.
 
MauleSkinner said:
Oddly enough, this industry-wide company policy is regulatory. ;)

See 91.129e, which is also quoted as being required in 91.130 and 91.131.
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"Industry" as in 121 operations.
 
luvflyin said:
318.something. And don't forget curvature of the earth. It really makes a difference in even just a few miles. I wouldn't have believed it. There was a thread here awhile back all about it. Someone might jump in with "numbers"
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I could be way off, but I think it's only about 8 inches at a mile. It would only be significant at large distances, not an issue for approaches. I'm not smart enough to do the math though, I can only repeat what I've seen. It certainly doesn't seem like it could be a factor inside of 20 miles or so.
 
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Thanks for the answers all, that's helpful.

One more question then regarding airliners shooting the visual approach ...
Let's say the airliner is arriving from the East or West, but it's a North/South runway. Where might the airliner join final, relative to FAF... outside? Inside? If inside FAF, how far inside FAF could they aim before turning final? (i.e. how close to the runway approach end could they perform their base leg?)
 
If I'm cleared for the visual I'll turn outside of the FAF. Inside of it is a little to close for me.
 
alanbreck said:
Thanks for the answers all, that's helpful.

One more question then regarding airliners shooting the visual approach ...
Let's say the airliner is arriving from the East or West, but it's a North/South runway. Where might the airliner join final, relative to FAF... outside? Inside? If inside FAF, how far inside FAF could they aim before turning final? (i.e. how close to the runway approach end could they perform their base leg?)
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If it's an instrument approach, an ILS for example, the controller has to give the final turn to intercept the localizer I think it was 2 nm from the 'approach gate' (ATC term/use), so think of it as roughly 2-3 miles from the FAF.

A visual you can join inside and just outside the FAF, but I'd say the way I did it was a base leg, curving path to join final probably 3-4 miles from the runway.

Now, if it was the "going home" leg, I might aim for the numbers, slowing and configuring as I do so. Hey I might have a tight connection flight home! Commuters fly a little differently. :D
 
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Cooter said:
I could be way off, but I think it's only about 8 inches at a mile. It would only be significant at large distances, not an issue for approaches. I'm not smart enough to do the math though, I can only repeat what I've seen. It certainly doesn't seem like it could be a factor inside of 20 miles or so.
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I'll see if I can find the thread on it. Might be a few days. It surprised the ell out of me, but it's measured in feet and more than just a few of them out where FAF's usually are.
 
luvflyin said:
I'll see if I can find the thread on it. Might be a few days. It surprised the ell out of me, but it's measured in feet and more than just a few of them out where FAF's usually are.
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A little quick math tells me it about 8 feet 5 miles out, but I could be wrong.
 
Croomrider said:
A little quick math tells me it about 8 feet 5 miles out, but I could be wrong.
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I thought I remembered from my Navy days that 6ft above the surface let you see for about 10 miles or so. There is an online calculator that does that math for you if you Google it. But it doesn't give exactly what we're looking for here.

Edit: the Calculator below shows 6 feet at 3 miles and maybe 32 feet at 7 miles?
 
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jordane93 said:
Absolutely! Until I get more comfortable in the plane, I'll continue to fly the base slightly outside or at the FAF.
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Just having a little fun with you, we have all been there. After 300 hrs I'm finally getting comfortable in the bus.
 
Croomrider said:
Sorry, but that seems useless for what is being discussed.
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Doesn't D1 and H0 get you close? I found the formula to calculate it online but didn't plug in number to compare it to the calculator. I'm an admitted math imbecile, but the picture represented in the calculator seems to measure drop at H0.
 
8 inches for one mile

To calculate this, use Pythagorean with a hypoteneuse of 3965 miles (Earth radius) and a short leg of one mile:

(sqrt(3965^2-1^2)-3965)*5280*12 = 8 inches
 
NoHeat said:
8 inches for one mile

To calculate this, use Pythagorean with a hypoteneuse of 3965 miles (Earth radius) and a short leg of one mile:

(sqrt(3965^2-1^2)-3965)*5280*12 = 8 inches
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Yes, this is the correct formula. I had used the wrong earth radius in my first calculation. However, as you probably know it's not a linear drop as you increase the distance from the airport. For instance, 5 miles out would be a drop of 16.65 feet!
 
jordane93 said:
If I'm cleared for the visual I'll turn outside of the FAF. Inside of it is a little to close for me.
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If traffic following you isn't a factor, I will usually vector airliners for a 30 degree intercept around the FAF for visuals. If it's a tight gap and I'm trying to make you number 1/2 vs number 8/9 you may intercept around a 3 mile final. I try not to force it too close to the runway. If I'm doing this, I've already told you to expect a short approach. In the rare opportunity I've had someone balk, they go to the end of the line if that's the next gap.
 
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