Descent Rate on Glide Slope

Keystoner

Keystoner

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Keystoner
I made a spreadsheet.

Given:

0.5 mi pattern. TPA = 1,000’ AGL

Downwind 85 mph. Base 75 mph. Final 70 mph.

Assume start of descent abeam numbers.

Assume 3 degree glide slope starting on base.

Working backwards:

0.5 mi final @ 70 mph takes 0.43 minutes. Descent is 138.4’ at 3 degrees. This gives 323 fpm descent.

0.5 mi base @ 75 mph takes 0.40 minutes. Descent is 138.4’ at 3 degrees. This gives 346 fpm descent.

0.5 mi downwind @ 85 mph takes 0.35 minutes. Descent is 723.3’ at 15.3 degrees. This gives 2,049 fpm descent.

Is this right? 2,049 fpm descent at 15.3 degrees just to get on the glide slope? What happened to 500 fpm controlled descent?
 
I don’t understand what you’re doing but 2000fpm / 15 degrees is not right.
 
Take your groundspeed and divide it by 2 and add a zero. (or multiply by 5)


100kts 500fpm
70kts 350pm

etc

That's the VS needed for a 3deg path.
 
I'm doing basic math. Junk in/junk out. Could have converted wrong, but I don't see where.
 
A traffic pattern of 1/2 statute mile wide is super tight. Unless you're talking about Cubs or Champs the typical traffic patterns is probably 1/2 nautical mile at a minimum...but that's more for an 800' AGL pattern which used to be the standard. And probably closer to a 4.5° slope. By the way, trying to calculate a precise vertical speed for a VFR traffic pattern is a pointless futile exercise and I can't think of a more misguided way to spend your time and effort. Did you know a vertical speed indicator is not even required for IFR flight, let alone VFR?
 
I worked out the math in the OP. Base and final took up 276.8' of 1000', leaving 723.2' to descend in 0.5 miles on downwind traveling at 85 mph.
 
Keystoner said:
I worked out the math in the OP. Base and final took up 276.8' of 1000', leaving 723.2' to descend in 0.5 miles on downwind traveling at 85 mph.
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Ok, then your numbers are right. If you want to stay that close to the runway you’re going to have to drop quick. 1/2 mile is VERY close. When you’re at pattern altitude the runway is going to appear to be practically underneath you if you’re that close.
 
Is this reasonable then:

For a 1 mile pattern: the downwind 1 mile at 85 mph becomes 633 fpm descent at 4.8 degrees.
 
Keystoner said:
Is this reasonable then:

For a 1 mile pattern: the downwind 1 mile at 85 mph becomes 633 fpm descent at 4.8 degrees.
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Yes, from a theoretical perspective, I think there's value in doing this to understand better. In reality, I agree with the others that say "look out the window".

This will be important when you start IFR, but you won't be flying a pattern so it makes more sense.
 
As a practical matter, 3° final using papi lights is really for multi-engine approaches, or else a true instrument glide slope approach on a very long final. If I see the “correct” 3 degree glide slope papi light configuration, I know I’d likely not make the runway if the engine quits on a single.

In a standard pattern, you’d have to fly a really long downwind to turn base & intercept a 3° approach on final at an acceptable speed & rate of sink.
 
rhkennerly said:
As a practical matter, 3° final using papi lights is really for multi-engine approaches, or else a true instrument glide slope approach on a very long final. If I see the “correct” 3 degree glide slope papi light configuration, I know I’d likely not make the runway if the engine quits on a single.

In a standard pattern, you’d have to fly a really long downwind to turn base & intercept a 3° approach on final at an acceptable speed & rate of sink.
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Yes, I'm seeing this too with my spreadsheet. Where I'm learning, unless I'm doing strictly pattern work, we typically come in on a straight final. A descent of 1000' at 3 degrees would start 3.6 miles away. I'm always trying to use power to stay on the PAPI glide slope, but your point makes sense--it's too low and far to be able to glide and reach the numbers. I'll talk to my CFI about this but what would you recommend as more practical?
 
What's your purpose for this thought experiment?

Regardless, my thoughts
I agree with rhkennerly...I think a vfr pattern is typically much steeper on average than 3
and it's not a fixed angle all the way around
Where I learned, and the way I learned, downwind was roughly 1/2 mile out. Keep it close
and when I'm furthest from the runway, at the turn to base, and when I turn base to final, I like to be high....so not a fixed angle through the pattern
this is supported by speeds and flap settings too..... 1 notch of flaps abeam the numbers, 2nd notch on base, so steeper angle, etc...
also, probably loose more in the turns...I never paid attention to measure how much more though.

Not knowing what this is all about so just spit ballin' here.... maybe calculate your altitude at the point you turn final so that that point is on the 3 degree slope or whatever angle you're shooting for, then figure out the altitude you need to loose from TPA to slope intercept point...maybe figure out a different angle for the downwind and base legs based on that....
 
GeorgeC said:
Sure, that works in practice, but does it work in theory?

(I went through a similar exercise when I was a student pilot...)
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My response was specific to this question

Keystoner said:
…I'll talk to my CFI about this but what would you recommend as more practical?
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I think it is more practical to learn how to enter the traffic pattern and fly it visually. The extra .1 isn’t going to matter over the course of training. What is gained is energy management, practical application of ground reference maneuvers with wind correction, and slow flight, off the top of my head. I think the CFI preferring a straight in entry is a disservice to the student, especially when it comes to the XC and night flying experience requirements.
 
Keystoner said:
I worked out the math in the OP. Base and final took up 276.8' of 1000', leaving 723.2' to descend in 0.5 miles on downwind traveling at 85 mph.
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The g-loading on those square corners must be a *****! ;)

don’t forget to figure in the circumference of your turns.
 
Keystoner said:
I'll talk to my CFI about this but what would you recommend as more practical?
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Well, I dunno. It’s really variable. But I’ve never thought about an angle as an actual number to consider. instead, I think about energy management (kinetic energy, not power plant).

I strive to fly a standard pattern (assume zero wind) in a standard way every time: downwind alt & speed, base alt & speed, final alt & speed. But that’s usually only possible at uncontrolled airports, where you can chop the power abeam the numbers on downwind…and with an airplane that has a lot of flaps. I loved steep, draggy, full-flap landings in Cessnas. There’s magic in arresting descent just as the wheels touch.

My flapless champ is very lighter & really needs a bit of power to stabilize the descent (pull the power off & it decelerates too fast & increases the rate of decent while retarding speed over the ground. It’s a deceptive combo. If you to nose way down to keep the airspeed up, you’ll likely end up way short or if in close, way too fast over the runway, depending on how you play it.) the approach isn’t stable. A squirrely approach to concrete is a prelude to a ground loop.

At controlled airports or with mixed speed traffic in the pattern, finals are usually longer. It’s easy to get sucked into the papi slope, but i hold onto some altitude, stay above the papi, & slip off the altitude once I have the runway made.

at this stage of your training, look & feel & are what you want to develop through experience. After a while, you’ll feel the right speed without looking. The sight picture will tell you how to adjust.
 
I don't get the numbers, either. If you're flying a cub type approach, you might be at those distances, but the descent rate is going to be relatively constant, and the speed is going from 65 to 50mph. So maybe something like 500fpm and maybe 6 or 7 degrees, guessing.

Simulated engine out in a pa-28? A bit faster, probably steeper, but no way you're going to have 3 degrees on final doing that. That would be kind of cool to see in a flight tracking app. I'm sure it's not as steep as a sewing machine falling out of the sky, but it does kind of feel a bit like it.
 
I'm definitely lost in the numbers world of airline flying, but I don't get the desire to breakdown this... just look out the window.
 
I like to simplfy my normal approach. For a 172 I set power and trim for about 90mph (subtract 10 if you are in knots will get close on the numbers) Abeam my touch down point I reduce power to less than 1500rpm and set flaps between 10-20 degrees. The combination of power and flap adjustment will offset each other and slow you about 10mph to 80mph. I keep my approach very stable from this point and fly the rest of the approach at 80mph until the round out, if I fly the pattern correctly I should not have to change anything until I start my round out at about 25-50 feet where I will bring my power back to idle.

For the math using your .5 mile legs using rough estimates. 90mph =.5 miles in about 23 seconds (1/2 of 45 seconds) so lets say 80mph takes roughly 30 seconds . You have 1000 feet to lose and you will be going 30 seconds per mile it will take you 1.5 minutes to get to the runway. 500ft/min descent would get you down 750 feet so looks like you are going need about 625ft/min descent. I agree that this will be well above the 3 degree glide slope which is normal for most single engine airplanes on VFR approaches.



Brian
CFIIIG/ASEL
 
The whole 3 degree slope thing is an IFR oriented thing IMHO. Training in a 172 you might flight the 5 mile final at 90 knots and a 400-500 FPM descent to be on slope (slow) or in something a bit faster you can come down at 120 knots and 600-800fpm. At that rate and a mile or two out there’s still time to slow it down a bit more and hit the landing zone (which is some distance from the threshold.

I’m pulling all those numbers from memory but I’ve never thought that’s an appropriate way to make VFR landings going around the pattern.

TLAR with a few checkpoints should do trick, always keeping in mind that the pattern may need to be reshaped for other traffic or whatever with actual power settings and descent rates modified for wind.

I try to hit an IP opposite the threshold at a distance that looks about right. Drop some flaps and hit a certain airspeed flying downwind until my stabilizer passes the threshold as I look back. Then I just keep looking at the threshold or touchdown point holding the desired airspeed and adjusting power to keeping TLAR. More flaps and slower speed somewhere on final to keep the touchdown point on target and land.

But checking descent rates and trying to hit the Papi as you turn final shouldn’t even be considered. Just kept the picture looking right and check the Papi on short final. If you end up on a 5 mile final due to traffic, things are different but that’s probably a good day to fly somewhere else if you are doing pattern work, no?


Sent from my iPad using Tapatalk Pro
 
Thanks fellas. The answers received made my question worthwhile. I've been looking out the window, but I'm an engineer and I want to know the numbers. Just looked at Google maps satellite view and I realize that my actual pattern is about a mile away. I had it in my head to shoot for 1/2 to 1 mile from runway downwind, and that's why I asked the original question. I now know not to fixate on the PAPI lights, or at least not be concerned about being a bit high on long finals, especially when doing straight-in landings (looking forward to discussing this with my CFI).
 
Keystoner said:
Thanks fellas. The answers received made my question worthwhile. I've been looking out the window, but I'm an engineer and I want to know the numbers. Just looked at Google maps satellite view and I realize that my actual pattern is about a mile away. I had it in my head to shoot for 1/2 to 1 mile from runway downwind, and that's why I asked the original question. I now know not to fixate on the PAPI lights, or at least not be concerned about being a bit high on long finals, especially when doing straight-in landings (looking forward to discussing this with my CFI).
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I'm an engineer too. From one engineer to another: Look out the friggin window with your hand firmly on the throttle (At least initially, as you get more comfortable, you will know your power settings and have to make very few adjustments). That's the right way to do this. You start to learn a site picture that can be applied in a number of situations. How are your numbers going to change with a high tailwind, or headwind, or crosswind, etc.? You gonna take out your E6B on base? (try it and then count the expletives out of your CFI)

Fly the numbers of your plane (power wise) and then adjust *based on your site picture*. I can't stress that enough.
 
I like to understand the basis for gouge and rules of thumb, so my hat's off to you for working it out. I'm also no stranger to wrong assumptions and math errors ;)

Nauga,
dropping like a turd off a tall moose
 
GeorgeC said:
Sure, that works in practice, but does it work in theory?
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It's your job to go tell all the bees they can't really fly.*




*Actually based on a 1930's misconception. The Insectarium at the Saint Louis Zoo used to have a wonderful exhibit with slow motion animation.
 
Rgbeard said:
We say he's overthinking it. But what's that REALLY mean?

It's something to ponder.
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It means someone is getting into the weeds into areas that don't really need an acronym or a formula. Flying is sometimes like a field where acronyms and formulas, and in many cases, multiples for the same item, are like the trees insomuch that we can't see the forest because of them. Flying is feeling and sight picture for me. Adding a math problem on top of it just complicates things.
 
Timbeck2 said:
It means someone is getting into the weeds into areas that don't really need an acronym or a formula. Flying is sometimes like a field where acronyms and formulas, and in many cases, multiples for the same item, are like the trees insomuch that we can't see the forest because of them. Flying is feeling and sight picture for me. Adding a math problem on top of it just complicates things.
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When you fly IFR, numbers become more relevant (since by definition you can't see).

But for VFR, look at the darn window. Period.
 
Timbeck2 said:
It means someone is getting into the weeds into areas that don't really need an acronym or a formula. Flying is sometimes like a field where acronyms and formulas, and in many cases, multiples for the same item, are like the trees insomuch that we can't see the forest because of them. Flying is feeling and sight picture for me. Adding a math problem on top of it just complicates things.
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I’m excessively monosyllabic…can you rephrase that?
 
Timbeck2 said:
It means someone is getting into the weeds into areas that don't really need an acronym or a formula. Flying is sometimes like a field where acronyms and formulas, and in many cases, multiples for the same item, are like the trees insomuch that we can't see the forest because of them. Flying is feeling and sight picture for me. Adding a math problem on top of it just complicates things.
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You've given me a lot to think about.

Can you expand on this?
 
I don't understand why people are giving him such a hard time on this. There's no harm in understanding the math if that's what floats your boat. If he's fixating on VS numbers trying to land, that will be a problem, but making fun of him isn't likely to teach him that lesson.
 
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