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Approach Speed in GA Aircraft

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Approach Speed in GA Aircraft

Old 6th Sep 2020, 21:36
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Approach Speed in GA Aircraft

I thought I might transfer some information from a post in another forum(with a few modifications for clarity)......If the aircraft is lighter than gross weight, the actual Vso is much lower than the book figure. In that case, on very short final, I will be bleeding off airspeed as I start to round out, and if Iím alone or with only one other person in a typical 4 or 6 passenger GA single, our airspeed should be significantly lower than the published 1.3 Vso.

If an angle of attack instrument is installed and used, which in effect compensates for weights lower than gross, the approach airspeed, both on long final and on short final, will be lower than book figures, sometimes dramatically so, especially in airplanes with a large W&B envelope. Thatís because Vso is much lower.

I found the underlined part interesting. Do you agree with what was said?
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Old 6th Sep 2020, 21:39
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Sounds logical enough to me.
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Old 6th Sep 2020, 22:45
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Very nice topic. I'd like to talk about light twins in this situation. We should consider speeds like Vmca during all phases, including approach and landing. Don't you guys think that can be a dangerous approach with less speed than published?
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Old 7th Sep 2020, 01:01
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If the aircraft is lighter than gross weight, the actual Vso is much lower than the book figure. In that case, on very short final, I will be bleeding off airspeed as I start to round out, and if I’m alone or with only one other person in a typical 4 or 6 passenger GA single, our airspeed should be significantly lower than the published 1.3 Vso.

Some thoughts.

(a) stall speed ratio will vary near enough with the square root of gross weight ratio

(b) before playing with numbers, make sure you are working with CAS rather than IAS to avoid any problems with near stall PEC variation which can present a problem on occasion.

(c) I wouldn't necessarily use terms such as "significantly" lower - just do the sums.

(d) be wary of power (thrust) effects when comparing what you might be playing with in the aircraft with what the AFM/POH might have to say. The certification flight test requirements are fairly specific and thrust can play a significant role with the numbers

(e) Vso doesn't vary as it is a specific certification thing.

If an angle of attack instrument is installed and used, which in effect compensates for weights lower than gross, the approach airspeed, both on long final and on short final, will be lower than book figures, sometimes dramatically so, especially in airplanes with a large W&B envelope. That’s because Vso is much lower.

And ..

(a) keep in mind that you may be playing with varying the design rule book for your jurisdiction should you intentionally come aboard at speeds lower than what the AFM/POH suggests. Potentially, that could present a legal concern in the event of a mishap.

(b) I presume that the envelope reference is to CG range ? I can't immediately think of any reason why the GW range might have a material effect. For the CG range, the AFM/POH stall figures (and the usual operational figures based on the stall data) will be for forward CG as the stall speed varies somewhat with CG, due to tailplane forces coming in to play. Vso, however, is a certification animal and doesn't vary with CG, while the real world figures you might see on any given day may well show a measurable variation. Again, do be circumspect about playing outside the AFM/POH ambit due to the potential for legal censure in the event of a mishap.
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Old 7th Sep 2020, 03:17
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Yes, the C of G position will affect the stall speed a small amount, Some flight manuals do present this data. That said, for the one or two knot difference, it's a nice target for approach speed precision, but rarely do I see a GA plane flown to within one knot of the target airspeed on approach. If you can, certainly do!

As my colleague mentioned, unless otherwise specified, stall speeds will be presented in calibrated airspeed, rather than indicated. Some flight manuals offer the courtesy of presenting it in both. Otherwise, this is a common error, a pilot believing that a stall is immanent based upon knowing Vso, but not as a CAS value, and watching the airspeed indicator to see it.

If you're using an after-market AoA system for very precise approach speeds, okay, but there are a few things to be certain of before you believe it 100%. You must determine if it was set up correctly. Two important factors really affect this: Does the plane have any modifications which affect lift (Gross weight increase, STOL kit, VG's, Wing extensions, for example)? If the plane does, the stall speed has probably been affected, and further, the relationship between IAS and CAS may be different, or, if the plane stalls more slowly, there will be a need to have IAS to CAS conversion for the slower speeds, which may not have been provided. This is extra important if there are multiple "lift" modifications, as the resulting stall speed will be a combination of the effects of the mods, and it will be very unlikely that there's an applicable IAS to CAS correction table. And, when the set up was done with reference to the airspeed indicator, was it recently calibrated? Kinda pointless if it was out!

Then, once you have determined the configuration of the plane (in terms of mods), and the availability (or not) of IAS to CAS correction for that exact configuration, you need to verify that the AoA was set up correctly. I have set up a few planes with an after-market AoA, and it was not trouble free. The manufacturer's instructions were unclear about the use of IAS vs CAS for the set up procedure. When I chose to make the required calculations based upon CAS, it was not possible, as each of the planes had wing mods, so the stall speed was slower, and there were no POH, nor flight manual supplement IAS to CAS correction tables for those slower speeds. So I set the planes up to the POH values, as it was all I legally could use, but it made the AoA system [conservatively] not as accurate as it could be. And, even the AoA system manufacturer states that the system may not be used for primary speed reference, in place of the original stall warning, nor for increased capability (slow approaches, shorter landings) - so it's really for entertainment only once you turn final.

My best (and very experienced) advice is to use the values in the POH, and expect the performance stated there. If the plane has mods, look for flight manual supplements. If there are multiple mods, and no one flight manual supplement provides information for that combination, you have been left with inadequate data to fly the plane (don't worry, you're not the first). When I approve airplanes (by STC) with combinations of mods, I do the testing and provide the flight manual supplement to support exactly that configuration.

A last word of wisdom, if you're flying a plane with wing mods which reduce the stall speed, continue to fly Vy climbs, and approaches at the POH speeds, to prevent being too slow in the event of an engine failure. Even if you're flying a glide approach, the fact that the wing has been modified to give you more lift (or you're seeing that on an AoA) will not give you any more reserve of energy in the flare. Roberston STOL Cessnas are famous (and the Roberston FMS warns) for pilots getting too slow on final, and not having enough reserve of energy to flare. If you're between 500 feet AGL, and he flare, you are most safe at either VY or "best glide" speed, at the slowest.
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Old 7th Sep 2020, 07:11
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Originally Posted by tcasblue View Post
I thought I might transfer some information from a post in another forum(with a few modifications for clarity)......If the aircraft is lighter than gross weight, the actual Vso is much lower than the book figure. In that case, on very short final, I will be bleeding off airspeed as I start to round out, and if Iím alone or with only one other person in a typical 4 or 6 passenger GA single, our airspeed should be significantly lower than the published 1.3 Vso.

If an angle of attack instrument is installed and used, which in effect compensates for weights lower than gross, the approach airspeed, both on long final and on short final, will be lower than book figures, sometimes dramatically so, especially in airplanes with a large W&B envelope. Thatís because Vso is much lower.

I found the underlined part interesting. Do you agree with what was said?
The question is: does it matter?

I.e. almost any landing I see in the US is far from slow or anything near Vso. I'd estimate they fly approaches 5 to 10 knots faster compared to the continent. Leaves us here. Who ever comes close to Vso in landings? Even the Cessna does honk far before any Vso and most pilots stay right at the edge of the horn even at short fields - not making use of the knots below horn and above Vso. I guess we all learned to fly on buttery controls in flight school, but suspect almost nobody goes slow flight afterwards. So back to my first question, the way we fly, where does it matter even though the physics may be correct?
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Old 7th Sep 2020, 10:33
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Originally Posted by ChickenHouse View Post
The question is: does it matter?
I agree with you. And for a second reason.
When flying slow the effect of turbulence on AoA will be higher. To compensate for this the safety margin needs to be higher. The end result is almost no benefit of the lower mass.
From a practical point of view the speed in the POH will take all these effects into account. Best not to overthink it.
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Old 7th Sep 2020, 10:56
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The question is: does it matter?
I too concur. If you're too slow on approach, you feel it in your pants, right? If you are dependent on those precious instruments, what're you going to do the day they fail? This is what we train for - and take a refresher with an instructor, from time to time.
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Old 7th Sep 2020, 11:08
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Originally Posted by Pilot DAR View Post
As my colleague mentioned, unless otherwise specified, stall speeds will be presented in calibrated airpseed, rather than indicated. Some flight manuals offer the courtesy of presenting it in both. Otherwise, this is a common error, a pilot believing that a stall is immanent based upon knowing Vso, but not as a CAS value, and watching the airpseed indicator to see it.
If anything it is the other way round. The IAS must be specified. You might see CAS or simply 'airpseed' in preamble such as specifications but in the part of the AFM where it counts the airspeeds must be indcated airspeed (even though one may see CAS alongside). I refer below to CS 23 as it appears to be identical to the old Part 23.
CS 23.1545 Airspeed indicator.
(a) Each airspeed indicator must be marked as specified in paragraph (b) of this section, with the marks located at the corresponding indicated airspeeds...
...a white arc with the lower limit at VS0 at the maximum weight,

CS 23.1581
(c) The units used in the Airplane Flight Manual must be the same as those marked on the appropriate instruments and placards.
(d) All Airplane Flight Manual operational airspeeds, unless otherwise specified, must be presented as indicated airspeeds.




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Old 7th Sep 2020, 11:45
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Originally Posted by Jan Olieslagers View Post
I too concur. If you're too slow on approach, you feel it in your pants, right? If you are dependent on those precious instruments, what're you going to do the day they fail? This is what we train for - and take a refresher with an instructor, from time to time.
Well it does matter when you are flying commercially into short strips. The stopping performance is predicated on the correct approach speed for the weight. If you fly any faster you are inviting an overshoot. Flying the correct airspeed becomes very important. Should the ASI fail you fly powers and attitudes and take it somewhere with a healthy stopping margin.
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Old 7th Sep 2020, 12:29
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The IAS must be specified. You might see CAS or simply 'airpseed' in preamble such as specifications but in the part of the AFM where it counts the airspeeds must be indicated airspeed
The design requirement states that Vso is defined in CAS

Sec. 23.49

Stalling speed.

(a) is the stalling speed, if obtainable, or the minimum steady speed, in miles per hours (CAS), at which the airplane is controllable,................
Therefore, if a pilot is factoring stall speed, the speed being factored should be the CAS speed, and the factor should also be CAS. So, if you want to fly a 1.3Vso approach, a pilot would have to know Vso in IAS. Perhaps the flight manual conveniently provides this. If not, the pilot will have to find it them self by observing IAS at the point of the stall, correcting to CAS with the flight manual table, multiplying by the factor in CAS, obtaining the result, and correcting back to IAS for reference to the airspeed indicator reading. For some types, there can be a 10 knot difference between IAS and CAS at the stall.

I agree that that the use of IAS and CAS can be inconsistent among manufacturer's and types, particularly in the earlier years. And, it would appear that values required to be presented in CAS are being marked on the airspeed indicator in IAS. Once random example of this would be for a 1974, Cessna 185; the POH Airspeed correction table (table 6-1) states "Maximum flap speed 110 MPH, CAS", though in the line above states flaps down IAS is 110, where the CAS is 112. So one would infer that the white line should begin at about 108 MPH to assure that the flaps speed limit is actually observed in CAS. But, on page 4-2, Airspeed limitations, which does state the limits are in CAS, states that the white arc (which is what the in indicating (IAS) pointer shows) is 110 MPH.

Now the 2 MPH difference is not going to create a risk to safety of slight, but it serves to demonstrate the the interchanging of IAS and CAS, even in a POH has been done such that a pilot should really pay attention.

I agree with Chickenhouse that approaches tend to be flown fast, and too fast, more often that at 1.3Vso. Note that 1.3 Vso is a minimum speed for certification (23.73), the manufacturer may recommend a faster speed. Again referring to the 1974 C 185 (just 'cause I have the book open), full flap stall speed is 56 MPH CAS, X 1.3 = 73 MPH CAS = 69 MPH IAS, so to fly a full flap 1.3 Vso approach, the math would have you at 69 MPH IAS. The POH (page 2-20) says: "For short field landings, make a power off approach at 78 MPH with 40 flaps....". This corresponds to page 1-7, which states landing airspeed of 75 - 85 MPH flaps extended. We have to presume that the POH is presenting these speeds as IAS, though it does not state. Now if that plane has a Horton STOL cuff on the wing, all those speeds will be different, and there is no flight manual supplement to tell the pilot the different speeds (or even a placard saying that they might be different)!
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Old 7th Sep 2020, 12:44
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Well it does matter when you are flying commercially into short strips.
You did observe that we are discussing "private flying", here? Not that I am unwilling to learn from the professionals, though.

agree with Chickhouse
I love it! - sorry @DAR, that was irresistible.
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Old 7th Sep 2020, 12:47
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To follow on from oggers we are in the danger of fiddling with detail but with little gain. I'm not a fan of the various AofA gizmos either and to be frank I find them a bloody irritating distraction. The fans of these things become infatuated by them. Power plus attitude is equally of value whether during the climb out or on the approach just as much as it is in the cruise. If you are too slow the controls will be sloppy - take heed. Adding airspeed arbitrarily should be avoided always because at some point you will need to land. Using the formal standard 1.3 the stall is already a safe speed to use for a particular weight but controllability is also important and using the POH approach figures, which will normally be well above that, is more than enough. There can be no case for adding to the POH speeds for turbulence, this suggestion is wrong and foolish in my view.

Using the 1978 172N POH figures, the Performance Specifications figures give the stall: zero flaps 50kts CAS and with 40 degrees of flap 44 kts CAS. At these speeds the IAS is 9 kts higher according to the airspeed calibration table. However between 60 - 90 kts the CAS/IAS is mostly the same or varying to a maximum of just 2 kts. In line with most light aircraft the variation in CAS/IAS for approach speeds is inconsequential.

The Cessna 172N recommended approach speeds are: flaps up 60-70 KIAS, 40 degrees flap 55-65 KIAS. For Short field they recommend 60 KIAS. At these speeds the CAS and IAS are the same both on the approach and of course the climbout. It is worth noting, as its been mentioned, that the stall speed can vary by 5 KIAS between the most forward CG and the most rearward CG permitted for this aeroplane.
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Old 7th Sep 2020, 13:08
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agree with Chickhouse
I love it! - sorry @DAR, that was irresistible.
Pardon me! Typo corrected!
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Old 7th Sep 2020, 13:33
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Originally Posted by john_tullamarine View Post
(a) keep in mind that you may be playing with varying the design rule book for your jurisdiction should you intentionally come aboard at speeds lower than what the AFM/POH suggests. Potentially, that could present a legal concern in the event of a mishap.
Some manuals will take into account different approach speeds for different weights. For example the TB20 gives you performances with two different weights leading to two different speeds.

Another problem I would like to point out on this topic is that some light aircraft operators tend to recommend speeds higher than 1.3Vs and higher than the manual.
If the manual tells you to approach at 1.4Vs, for example, then OK. But if the manuals recommends 1.3 and your airclub 1.6, what then ?
This will lead to a habit of very long flares.
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Old 7th Sep 2020, 13:36
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Originally Posted by Pilot DAR View Post
The design requirement states that Vso is defined in CAS
Yes I have seen that reg. Design airspeeds are CAS but operational airpseeds are IAS. The information has to be made available to the pilot in IAS as stated in the regulation 23.1581 that I quoted. Instrumentation during the flight test will determine the CAS, but the production airspeed system is also used in order to determine the IAS, which is what the pilot needs to know.

Therefore, if a pilot is factoring stall speed, the speed being factored should be the CAS speed, and the factor should also be CAS
Yes I agree. If you wish to factor the stalll speed the process is [CAS x factor] and then convert to IAS using the airspeed corrections table. No argument. The point I was responding to was:
stall speeds will be presented in calibrated airpseed, rather than indicated. Some flight manuals offer the courtesy of presenting it in both
...this is not a courtesy. The requirement for operational airpseeds in the AFM is 'indicated':
(c) The units used in the Airplane Flight Manual must be the same as those marked on the appropriate instruments and placards.
(d) All Airplane Flight Manual operational airspeeds, unless otherwise specified, must be presented as indicated airspeeds.

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Old 7th Sep 2020, 14:05
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Originally Posted by Pilot DAR View Post
I agree that that the use of IAS and CAS can be inconsistent among manufacturer's and types, particularly in the earlier years.
I remember the Aeronca Champ having a placard that listed some speeds as "true indicated airspeed" - go figure!
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Old 7th Sep 2020, 14:23
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The requirement for operational airpseeds in the AFM is 'indicated':
(c) The units used in the Airplane Flight Manual must be the same as those marked on the appropriate instruments and placards.
(d) All Airplane Flight Manual operational airspeeds, unless otherwise specified, must be presented as indicated airspeeds.
Yes, I agree, noting that this requirement is for a latest FAR Par 23 certified plane, but is not a requirement for a CAR 3 or older 23.1581. CAR 3.777 makes no mention of IAS/CAS. CAR 3.778(a) also makes no mention, though points you to 3.757, which refers to TAS (just add to the confusion, I suppose!). 23.1581 does not seem to mention IAS until revision 23-50 of 1996. So any plane certified prior to that would not have to comply.

So the complication comes in a later manufactured CAR 3 plane (like a 172/182/206) where the requirement in the certification basis (CAR 3, and earlier 23.1581), did not specify presentation in IAS/CAS, so it's not required, but Cessna presents it anyway, as they would be required to for a latest 23-1581 cert basis plane. Using the C 206H as an example, its a FAR 23, amendment 6 cert basis plane, so it's not required to comply with 23.1581 at amendment 50, but Cessna can voluntarily do so, and it's wise. I find that generally, Cessna go the extra distance for POH information (probably at the legal team's advice!)

These details are well beyond what we would expect the pilot to be aware of, and it's probably annoyingly confusing to most pilots. But, as the planes and certification basis have evolved, and flight manuals/POHs have been refined, pilots need to be reminded to read and interpret what's presented in the POH in context. IAS/CAS awareness is important if you're actually using an airspeed indicator during flight near the stall speed. It's extra important if you're setting up and AoA system, and then has an additional layer of complexity for each lift mod installed on the plane.
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Old 7th Sep 2020, 14:52
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Yes, I agree, noting that this requirement is for a latest FAR Par 23 certified plane, but is not a requirement for a CAR 3 or older 23.1581. CAR 3.777 makes no mention of IAS/CAS. CAR 3.778(a) also makes no mention, though points you to 3.757, which refers to TAS (just add to the confusion, I suppose!). 23.1581 does not seem to mention IAS until revision 23-50 of 1996. So any plane certified prior to that would not have to comply
Your ability to track down the various iterations of the regs is impressive. Incidentally, those extracts from POHs I pasted above are both from the '70s. As regs do change over time, the general advice, as ever, has to be 'consult your AFM'.
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Old 7th Sep 2020, 15:16
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If the manual tells you to approach at 1.4Vs, for example, then OK. But if the manuals recommends 1.3 and your airclub 1.6, what then ?
Simple, you fly the aeroplane in accordance with the POH/AFM. No one including flying/schools/clubs or any other "operator" have the right to demand anything different. When differences are demanded they are usually based on ignorance, hearsay and the thoughtless but well honed " that's what we've always done here".
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