172 Approach Speed

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Gray Ghost

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Gray Ghost
I am a student pilot, so please excuse my ignorance.

The Airplane Flying Handbook repeatedly says that you should fly final approach at 1.3 x Vs0. Vs0 in the 172R I am mostly flying is 33 kts. 1.3 times that is 43 kts. Nobody flies the approach that slowly in the 172, and I have been instructed to fly it at 65 kts, which works well.

Can someone explain the discrepancy to me? Does the 1.3 x Vs0 formula just not apply to a 172?

Thanks in advance.
 
There seems to be a cushion built into the approach speed on the 172s and the Warriors /Archers I trained in.

Maybe that 1.3 is on short final . . .


In my POH for the 172 R I’m renting now, it indicates Vs0 at 40 KIAS - 1.3 is a slightly more reasonable 52 knots (I’ve never purposefully been that slow on approach unless over the runway).

Great question. I look forward to learning the answer.
 
I think there is an issue with your VSO. The manual I found online for a 172R says 47 kts.
 
I see Terry posted as I was typing. Interesting we have a spread from 33 kts to 47 kts depending on who's manual we are referencing. I can't imagine there is that much variation in 172R. Anyone with better information?
 
midwestpa24 said:
I see Terry posted as I was typing. Interesting we have a spread from 33 kts to 47 kts depending on who's manual we are referencing. I can't imagine there is that much variation in 172R. Anyone with better information?
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33KT was Vs0, not the speed he was flying final at.
 
Perhaps there is some knots / mph confusion making things worse? It’s been awhile, but I seem to recall 65 mph to be what I was taught.
 
Do not fly an approach at 33 kts in a 172, unless you inches above the runway. See your instructor. Something is wrong with the calculation.
 
My Airplane Flying Handbook says "In the absence of the manufacturer's recommended airspeed, a speed equal to 1.3Vso should be used. The example in here has 60kt, so speed should be 78kts on final. 43kts is maybe just the slowest you could flying at before getting too close to stalling.
 
Flying is part art, it is not all science. Birds don't have these types of calculations is strictly follow, they feel the air and conditions and modify accordingly

1.3*X
2100 RPM @ X
18" @ X

These are all just general rules but shouldn't replace actual feel for flight. Every flight and every plane is going to be a little different.. get a ton of landings under your belt experimenting with various approach speeds (all over 60 knots) and see what feels best.. that "1.3 times Vso" is not a god-given edict that is a fact of flying to hold true across every single airplane, it's just a general benchmark and the absence of other data
 
The 172R's Vso is not 33 knots. That stall speed is typical of a J3 Cub, maybe. Two different online versions of the 172R AFM, probably different serial number ranges---note the max takeoff weights:

upload_2022-3-1_21-28-43.png


upload_2022-3-1_21-30-22.png


You have to start with correct data to get correct answers. Garbage in, garbage out.
 
Two things (in addition to whether your Vso is actually 33.

The 1.3 Vso calculation is based on calibrated airspeed. It's 1.3 times stall speed in CAS, then converting that number to IAS with the tables.

As already mentioned, some POH recommendations are not 1.3 Vso.
 
I am not allowed to post links here, but reference this manual:

https://www.se.edu/aviation/wp-content/uploads/sites/4/2020/10/172R-POH.pdf

Pages 2-5 and 5-12 both lists Vs0 for the 172R as 33 kts. If I am reading it wrong, someone please tell me.

Also, as I indicated in the OP, I am flying stable approaches at 65 kts just fine. The point of this thread is to understand the theory and why the 1.3 formula doesn't seem to apply.
 
@midwestpa24 i see 48 knots in my POH too. The difference between our quotes (in hindsight) is I quoted 40 knots INDICATED vs 48 knots CALIBRATED.

Both are in my 172R POH (in different spots).

48x1.3 gets pretty close to the 65 knots I was taught for final.

ETA: I have to convert my 62.5 KCAS back to KIAS to compare to the 65 knots on approach. I’ll check that out later.
 
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Terry M - 3CK (Chicago) said:
@midwestpa24 i see 48 knots in my POH too. The difference between our quotes (in hindsight) is I quoted 40 knots INDICATED vs 48 knots CALIBRATED.

Both are in my 172R POH (in different spots).

48x1.3 gets pretty close to the 65 knots I was taught for final.

ETA: I have to convert my 62.5 KCAS back to KIAS to compare to the 65 knots on approach. I’ll check that out later.
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I just pulled up a 172R manual. Consistent with what you see. In that one, Vso is 47. So...
47 X 1.3 = 61.1 KCAS.
Table converts it to 60 KIAS.
Recommended final approach speed in checklists:
60-70 KIAS for normal landing
62 KIAS for short field landing​

("I was taught" may be irrelevant since, keeping in mind those numbers are for max gross weight, many instructors pad them even more.)
 
Gray Ghost said:
I am not allowed to post links here, but reference this manual:

https://www.se.edu/aviation/wp-content/uploads/sites/4/2020/10/172R-POH.pdf

Pages 2-5 and 5-12 both lists Vs0 for the 172R as 33 kts. If I am reading it wrong, someone please tell me.

Also, as I indicated in the OP, I am flying stable approaches at 65 kts just fine. The point of this thread is to understand the theory and why the 1.3 formula doesn't seem to apply.
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Read @midlifeflyer ’s post again…33 knots is IAS. CAS is used for the calculation, and that’s 47 knots. Multiply that by 1.3, then convert that number back to IAS and report back. ;)
 
Gray Ghost said:
I am not allowed to post links here, but reference this manual:

https://www.se.edu/aviation/wp-content/uploads/sites/4/2020/10/172R-POH.pdf

Pages 2-5 and 5-12 both lists Vs0 for the 172R as 33 kts. If I am reading it wrong, someone please tell me.

Also, as I indicated in the OP, I am flying stable approaches at 65 kts just fine. The point of this thread is to understand the theory and why the 1.3 formula doesn't seem to apply.
Click to expand...
These are from the link you posted. The 1.3 formula applies just fine.


upload_2022-3-2_8-5-59.png

upload_2022-3-2_8-10-54.png
 
Gray Ghost said:
I am a student pilot, so please excuse my ignorance.

The Airplane Flying Handbook repeatedly says that you should fly final approach at 1.3 x Vs0. Vs0 in the 172R I am mostly flying is 33 kts. 1.3 times that is 43 kts. Nobody flies the approach that slowly in the 172, and I have been instructed to fly it at 65 kts, which works well.

Can someone explain the discrepancy to me? Does the 1.3 x Vs0 formula just not apply to a 172?

Thanks in advance.
Click to expand...

65 IAS is OK for a full flap landing and is best to train with, gives you a little safety cushion. Now a days I am flying closer to 52-55 IAS when I am over the numbers with 40° flaps.
 
Gary Ward said:
65 IAS is OK for a full flap landing and is best to train with, gives you a little safety cushion. Now a days I am flying closer to 52-55 IAS when I am over the numbers with 40° flaps.
Click to expand...
Does the R model have 40 degrees of flaps? ;)

What does Cessna publish for a short field approach speed? How close to max weight are you when you’re doing this?
 
MauleSkinner said:
Does the R model have 40 degrees of flaps? ;)

What does Cessna publish for a short field approach speed? How close to max weight are you when you’re doing this?
Click to expand...
If the R has 160 motor than it will have 40 flaps. My late N POH says 59 IAS for short field approach with 40° flaps. I am 150# under 2300 normally.
 
If you look at the airspeed calibration chart in Section 5 with 30° flap configuration, calibrated = indicated airspeed in the 60-70 KIAS range.. From the stall speed page in Section 5, the stall CAS is 47. 47x1.3= 61 calibrated and indicated airspeed.

The short field landing KIAS is recommended at 62. So Cessna is recommending approach speed greater than 1.3 Vso. In a high wing design ground effect is not as great, thus the higher speed reccomendation.
 
Get the accurate figure and fly it, adjust only with experience. Too slow is obviously no bueno but often pilots I watch out some “padding on the padding” even in non gusty times. That too is not wise. Floating halfway down a runway may not be as treacherous as stalling short of a runway but it’s not as safe as landing where you want to and should.

Get the accurate figure then learn to fly pegging thy airspeed! You will be glad you did when you go to land on 1800’ instead of 4-5k… or in an emergency…. I can land my plane in a smaller field in a pinch than someone who’s become reliant on adding some padding to the padding.
 
if you compare the stall speed chart vs the Airspeed calibration chart you will note that the 30 degree flap stall speed is different.
Stall speed (power off) = 33kias = 47kcas
Airspeed calibration (power on) = 40kias = 47kcas

so at least some of your Indicated airspeed discrepancy is due to power setting
but as others have indicated the 1.3VSO comes from KCAS. I suspect Cessna added a few knots to the approach speed due to the higher than typical drag from those big flaps.



Gary Ward said:
Now a days I am flying closer to 52-55 IAS when I am over the numbers with 40° flaps.
Click to expand...
I am betting you are NOT 52kts, Power off. and gross weight at least until within 10 feet of the ground. Unless you like hard landings.

Brian
 
Clip4 said:
If you look at the airspeed calibration chart in Section 5 with 30° flap configuration, calibrated = indicated airspeed in the 60-70 KIAS range.. From the stall speed page in Section 5, the stall CAS is 47. 47x1.3= 61 calibrated and indicated airspeed.

The short field landing KIAS is recommended at 62. So Cessna is recommending approach speed greater than 1.3 Vso. In a high wing design ground effect is not as great, thus the higher speed reccomendation.
Click to expand...

It's 1 knot of difference. I doubt Cessna said, "Since it's a high wing we'll add one knot." More likely, the company has access to more accurate data for the calculation and the difference is a rounding error.
 
brcase said:
if you compare the stall speed chart vs the Airspeed calibration chart you will note that the 30 degree flap stall speed is different.
Stall speed (power off) = 33kias = 47kcas
Airspeed calibration (power on) = 40kias = 47kcas

so at least some of your Indicated airspeed discrepancy is due to power setting
but as others have indicated the 1.3VSO comes from KCAS. I suspect Cessna added a few knots to the approach speed due to the higher than typical drag from those big flaps.




I am betting you are NOT 52kts, Power off. and gross weight at least until within 10 feet of the ground. Unless you like hard landings.

Brian
Click to expand...


Maybe not? I will look closer next time. I am only 10' above the numbers most times though. Been landing at a turf air strip that is 1600' long and use less than half of it.
 
dmspilot said:
It's 1 knot of difference. I doubt Cessna said, "Since it's a high wing we'll add one knot." More likely, the company has access to more accurate data for the calculation and the difference is a rounding error.
Click to expand...

1.3 Vso +1 on a short field when the value on a short field is usually 1.2.
 
Gary Ward said:
Maybe not? I will look closer next time. I am only 10' above the numbers most times though. Been landing at a turf air strip that is 1600' long and use less than half of it.
Click to expand...

Cool will be interested to see what you report...

1600 foot at my altitude (2500ft MSL) I would normally do a short field approach just to increase my margins but probably wouldn't need to.
Even with my normal 65-70kt, 10-20 degree flap approach (power off) I usually stop in under 1000ft on pavement.

What you have probably done is figured how to use your factory AOA indicator, AKA the windshield and are adjusting your approach speed based on angle of attack or how high the nose is.
Most pilot do this the last 50 feet of the approach and many don't realize they are doing it, they just make the nose look about right for landing and touch down at what ever speed that is. This automatically corrects for Density Altitude and Weight of the aircraft.

I did fly with a guy in his cub last year that apparently had forget this. He only fly's out of his 900ft private strip by himself. For his flight review we were returning to his strip after fueling the airplane, on final I hear him say 50mph out loud which I could tell by how high the nose was (I was in the back and couldn't see the ASI well) was slower than I would normally approach, The stall warning squeaking was also a clue, but was acceptable and some guys prefer the slow, power on approaches for extreme short fields.
At about 10 feet above the runway he says it doesn't look right and goes around, 2nd approach was a repeat of the 1st. I think either approach would have been fine but would have been a steep approach with almost no arresting of the descent, due to the low speed, other than power. On the 3rd approach he asks what he is doing wrong and I advise I would come in about 10mph faster to make the attitude look more like I would usually do the approach. As soon as we turned final at 60mph he say it looks much better and proceeded to make a very nice landing. In reviewing it we determined he always fly's the approach at 50mph when by himself, but with the full fuel and a passenger this resulted in a much higher AOA and steeper, more nose high approach than he was used to.

Brian
CFIIG/ASEL
 
Gray Ghost said:
I am not allowed to post links here, but reference this manual:

https://www.se.edu/aviation/wp-content/uploads/sites/4/2020/10/172R-POH.pdf

Pages 2-5 and 5-12 both lists Vs0 for the 172R as 33 kts. If I am reading it wrong, someone please tell me.

Also, as I indicated in the OP, I am flying stable approaches at 65 kts just fine. The point of this thread is to understand the theory and why the 1.3 formula doesn't seem to apply.
Click to expand...
This is from Page 2-5:

upload_2022-3-2_12-41-15.png

The white arc has nothing to do with stall speed. That's the flap operating range, and one might see 33 kts on touchdown when light, in ground effect, in a minimum-speed landing. The high AOA makes the ASI underread, and that chart is INDICATED airspeed.

Page 5-12 is the chart for indicated versus calibrated airspeeds.

upload_2022-3-2_12-44-32.png

Note that 33 indicated is 47 calibrated. If you start using 1.3 x 33 knots you will kill yourself shortly. It's at such a high Angle of Attack, far beyond the approach AoA, that it is irrelevant for figuring approach speeds.

The correct stall speed is found on that page I posted earlier.
 
RoscoeT said:
The 172 rental pilots at the flight school at my airport like to fly final 80KTS power on, 20 flaps...and float out of sight down a 6K' runway.
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That describes most nosewheel pilots! :)
 
This kinda stuff doesn’t happen in a mooney ya know
 
MauleSkinner said:
Last I checked, 1.2 was “not more than 1.3”, but with the new math, who knows.
Click to expand...

Please follow along. The sources clearly disprove Clips's argument that 1.3 x Vso is "higher than usual".

Also, he literally said "Cessna is recommending approach speed greater than 1.3 Vso". It is not greater than 1.3 Vso on account of a 1-knot rounding error, which was my original point.

When I pointed this out, he exercised the fallacy known as "moving the goalposts". Oh, he didn't mean 1.3, he meant 1.2. Whatever.
 
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