Short-Field Ops: Publications?

Actually, In a discussion about being slow (i.e airspeed in relation to stall speed), CAS is irrelevant.

Not true, it's very relevant, and the only way to accurately calculate performance numbers.

Lets look at your 172 example:

Calibrated VS0 (full flaps) with maximum forward CG in a Cessna 172 is 47KCAS.
47 * 1.3 = 61 KCAS = 60 KIAS
POH listed short field approach speed in the same 172 = 60 KIAS
Magical.


For the students on here, CAS (Calibrated Airspeed) is a correction to KIAS published by the manufacturer to allow for a more accurate calculation of True Airspeed (TAS). When operating the aircraft, the only speed you should be using is Indicated. Indicated airspeed is important, as it tells you how fast the wing is moving through the airmass regardless of altitude or temperature. This is why the manufacturer publishes all limitation and operating airspeeds in KIAS.

No they dont, not necessarily. Here's a stall speed chart for a Cessna 150, where do you see anything listed in indicated airspeed?

c150-stall.jpg


You have to go look at the IAS/CAS conversion chart for yourself. Why? Because IAS is what you see in the cockpit but is not an accurate measurement of your actual speed through the air.
I know you know (but for students), this is due to the fact that the pitot tube is cutting through the air somewhat sideways at a high angle and not picking up as much impact air pressure as it does with a lower angle of attack. This is why performance is calculated by CAS, or the actual speed of air flowing over your wing and control surfaces.


Yes, you obviously make the conversion to IAS so you know what to look at as you fly, but that's the last step, not the first.
 
I am glad that you brought up the 172RG, despite the fact that it is an entirely different airplane with different V speeds altogther.

As promised, your 172R model numbers:

From POH 5-12 with flaps 30 and zero degrees of bank the most reward and forward CG stalling speeds are identical.

KIAS: 33
KCAS: 47

Short field speed with full flaps, 62 KIAS. (POH 4-16)

47 * 1.3 ~ 61 KCAS, this converts to ~60 KIAS

The published short field approach speed is in fact 1.34 Vso. Pretty close.


Now let's say you choose to fly 1.3 * 33 equaling approximately 43 knots indicated. Converting that to calibrated puts you flying on the order of 50 knots or 1.06 Vso (50/47). In other words, you'd be flying stupidly close to stall speed. And in many cases, because we fail to teach this, unknowingly so.



What you should realize is that my giving you speeds from another aircraft doesn't matter. You can run these numbers in anything from a home built to a transport jet and the results would parallel those given here. If the aircraft uses a standard fixed mount pitot tube it will be subject to serious errors at slow speeds. And, regardless, when you're dealing with calculations you are to deal with CAS, not IAS.



For what it's worth, the PHAK is a bit misleading in the definition you gave. Specifically the portion you bolded. A better wording for that first bold sentence might be something like: Manufacturers use this speed to tell the pilot what performance they can expect at given indicated speeds, such as Vy, Vx, and best glide speeds.

At least that wording wouldn't lead someone to believe that performance calculations are done using indicated speeds. They aren't. Flight test data is all converted to TAS/EAS as applicable given the following order: KIAS > KCAS > KTAS > KEAS. Indicated speeds are completely useless for performance information. Haven you ever wondered why the first pages of the performance section include the chart for KIAS to KCAS conversion? It's because you're supposed to do that before you convert to TAS if accuracy is of importance to you.

As a final note, realize that at cruise speeds calibrated and indicated speeds are typically on order of one to two knots difference. Because these differences are so small this step is often overlooked, assumed pointless. The point, however, is the knowledge of calibrated speeds purpose. We can both agree bickering over a knot is silly, but I'll never agree that a pilot shouldn't understand the purpose and usefulness of calibrated airspeed.
 
Lets look at your 172 example:

Calibrated VS0 (full flaps) with maximum forward CG in a Cessna 172 is 47KCAS.
47 * 1.3 = 61 KCAS = 60 KIAS

DAMN YOU RFRAME STOP STEALING MY THUNDER!!! :bang:

I'm only messing, good post, well said.
 
it is also important to be familiar with how each airplane reacts during the flare and what kind of float to expect, if any. I seem to recall the hard thing about SF in a 172 is that the airplane likes to float. The reason I emphasize this when teaching is in order to select a good aiming point, and then determine the best power-idle point. if you nail the speeds, aiming point, and when to reduce to idle, you have a sort of "standard" formula. then you just have to adjust it for conditions. I also teach +5/-0 kts and anything else is a go around because the PTS lets us be a little long, but not short. Although for a truly short field, you may want everything +/- 0.
 
One last effort before I give up...

I have never seen, nor do I believe the Airspeed Calibration charts were intended to be used to convert CAS to IAS. (They ARE intended to convert IAS to CAS). Due to the fact that the airspeed error is not linear, by converting IAS to CAS, multiplying (a linear function), and then reconverting, I believe a considerable mathematical error is introduced.

Here's what happens: At Vso 33 KIAS/47KCAS the airspeed error is 47 - 33 = 14. We know that this error decreases at an unknown, but probably exponential rate as airspeed increases. When we multiply by 1.3, we are also multiplying that error instead of reducing it (14 x 1.3 = 18). When we convert back, we take one knot away (61-60) to get a 17 knot mathematically introduced error. Now if we subtract it out... 60 - 17 = 43 KIAS (Hmmm). Obviously, the math is fuzzy here, but my point is that an error is introduced.

For another example, lets apply your math to best glide speed: Using the Airspeed calibration chart (on page 5-9 - flaps up) 65 KIAS converts 66 KCAS. Flaps up, power off stall speed (from page 5-12) is 51 KCAS. 66/51 = 1.29 Vs1. Less than our short field approach speed, which is supposed to be well behind the power curve, according to the AFH, and not anywhere near the peak of the Lift/Drag curve. Once again, never does Cessna, nor the FAA ever mention the aforementioned math.

Finally, once you've spent a significant amount of time in a type of aircraft (I spent over 1,000 hours in the right seat of a Skychicken), you should know by the feel of the aircraft what the approximate angle of attack is. Even down towards the "stupidly slow" 50 kts, the airplane is still very controllable and will accelerate well with the application of power (not a sign that you are near minimum controllable airspeed). Up towards 60 kts, you are near the top of the Lift/Drag curve, and the airplane performs magnificently. (Disclaimer: I am not recommending anything other than an approach at the manufacturer's recommended approach speed, I am merely attempting to explain why late model skyhawks have a tendency to float.)

Humbly yours,
Hammertime

---Let the flaming recommence!
 
I have never seen, nor do I believe the Airspeed Calibration charts were intended to be used to convert CAS to IAS. (They ARE intended to convert IAS to CAS).

What would make you think the conversion cannot go both ways? Just look at one of the tables. Each indicated speed has a corresponding calibrated speed and vice versa.

we are also multiplying that error instead of reducing it

You're not multiplying the error. You've eliminated the error by choosing to use the actual speed through the air instead of the erroneous speed the instrument is displaying.
 
Hammertime I'm sorry if you've taken this as flaming. Like you, I'm just looking for a friendly discussion that can be learned from. This is not an easy topic, and I may be doing a poor job of getting my point across. Perhaps it would be easier to give you a short explanation about the flight testing process and how the numbers got on that calibrated airspeed chart.

Here is a brief excerpt from the USAF Flight Test Engineering Manual. This information is located on page 1-30 and the few pages that follow include methods for how this information is gathered if you're interested in some more background.

USAF Flight Test Engineering Manual said:
Air-speed checks can be made by any method which measures the calibrated air speed (Vo) at the same time indicated air speed (Vi) is recorded.

Edit: Oops forgot to give you a link: http://books.google.com/books?id=uQ...age&q=flight test calibrated airspeed&f=false

The point of this specific quote as it pertains to this discussion is to show that for any given indicated airspeed there is a specific calibrated airspeed. And this data can be read accurately in either direction. In other words, I could say that for any given calibrated air speed there is a specific indicated air speed and it would be no less accurate a statement.

The reason such lengths are taken to find these calibrated speeds is simple: the airplane will perform based on the actual speed of the air going over the body of the aircraft. A speed which is inaccurately measured by the airspeed indicator. Meaning, if you wish to calculate an aircraft performance, to fly at a speed that is 30 percent greater than the speed at which a stall is incurred for instance, you must work with the speed the air is actually passing over the the aircraft; the calibrated speed.

The method to do this can be made simple by work exclusively with the actual (calibrated) speed. In this case, the calibrated stall speed for your 30 percent calculation. Next, since you don't have any way when flying to see that speed, convert it over to the indicated speed. Then and only then will you have a speed that is accurately 30 percent above the speed at which a stall will occur. And this is how your approach speed made it in your POH and explains why 1.3 Vso of indicated speed never seems to accurately match books published speed.

Is that a bit more clear?
 
All that CAS stuff is fine and correct for building a foundation of the short field approach in ground school. -- What really should be emphasized is what it takes to fly the airplane to obtain and maintain those speeds.

I've one more bone to pick here! I agree completely that it's important to emphasize the ability to fly the aircraft. There is no logical reason to argue otherwise, but it shouldn't be used as a crutch to avoid ensuring ground school concepts such as this are clearly understood. Otherwise you wind up with a 25,000 hour airline pilot who I asked today and a 5,000+ hour mountain flying instructor who I straightened out on this topic some months back thinking that 1.3 Vso is in reference to indicated speed.

And for what it's worth, when it comes to the flying concepts and the IFR system, I have not met a more knowledgeable pilot than this airline pilot. His level of understanding are absolutely astounding. But certain basic concepts related to the science behind flight are missing.

Apologies Captain if you happen to read this! I'll buy you an extra beer next qb meeting to make up for it. (WTF?!?! When did JC take out the beer emote thing!? Doug this is so not cool.)
 
You're not multiplying the error. You've eliminated the error by choosing to use the actual speed through the air instead of the erroneous speed the instrument is displaying.

Valid point. (In fact, when I was puzzling that over, something was bothering about that, but I couldn't put my finger on it...). However, I'm curious to read your response to my other points. On the surface, your math is right. I just think there is something you are missing that I can't place my finger on. It has to do with the fact that your derived approach speed is so close to the peak of the of the Lift/drag curve. That's counterintuitive to the whole concept of a short field approach, as it makes a steeper approach, and shorter roll much more difficult.

P.S. No worries, "flaming" was poor word choice on my part, brought on by too much feverish thumbing through FAA publications at a late hour.
 
It has to do with the fact that your derived approach speed is so close to the peak of the of the Lift/drag curve. That's counterintuitive to the whole concept of a short field approach, as it makes a steeper approach, and shorter roll much more difficult.


Short field performance has three important aspects:

First is our climb angle, which is based on excess thrust. So our location on the drag curve is of importance. Typically we approach at a speed that's right at the bottom. Faster is safe, slower..well we've all heard the phrase 'don't be low and slow'.

Second is the speed, or stopping distance, which directly relates to speed via kinetic energy. Or half the square of velocity times mass. This is why flying just a little fast is a really bad thing, since kinetic energy increases with the square of velocity.

Finally, and this is where the balancing act of safety vs utility comes into play, is our safety margin above stalling speed. The shortest short field landing will occur, assuming power idle is optimal (it really isn't), if you flew the approach at stalling speed. This would give your greatest T-D difference for a steep angle and the slowest speed for the shortest stopping distance. I think we can both agree, however, that this is not the safest option.

Which brings us back to our 1.3 Vso, or a safety margin of 30 percent above stall speed. Chosen not because it maximizes utility but because it gives adequate safety and utility. The issue that arose from our discussion here was what margin are we really getting if you take 30 percent of an inaccurate speed. And that's why I made a point to show you that 30 percent of the indicated stalling speed of 33 really only gives you a 6 percent safety margin.

You might be ok with that margen, honestly any professional pilot could probably fly with that kind of margin on a relatively calm day without issue. The problem arises with the student or low time pilot, or even professional pilot, flying that speed under the false notion that they have a 30 percent safety margin. If they based their calculation of indicated spee they will not have a 30 percent margin, their margin will be on the order of 5-20 percent for most aircraft. Not 30 percent.


Hope that helps, it's late and I'm half asleep. I'm not quite sure what other specific portion of your replies you wished to hear my comments on. Have a good night.
 
I've one more bone to pick here! I agree completely that it's important to emphasize the ability to fly the aircraft. There is no logical reason to argue otherwise, but it shouldn't be used as a crutch to avoid ensuring ground school concepts such as this are clearly understood. Otherwise you wind up with a 25,000 hour airline pilot who I asked today and a 5,000+ hour mountain flying instructor who I straightened out on this topic some months back thinking that 1.3 Vso is in reference to indicated speed....

I'm curious what the importance is of that airline pilot understanding/misunderstanding this concept?
 
I'm curious what the importance is of that airline pilot understanding/misunderstanding this concept?

I imagine none when flying any transport aircraft since they will have calculate or published speeds to fly, right? But when he retired and is now teaching primaries and especially cfi's to calculate based on indicated. Well then it's a problem because not every airplane we fly has published speeds so it's important he teaches us to do it correctly.
 
Hope that helps...

Well, that's a great lesson on short field approaches, but I'm sorry to say it didn't really. I was more interested in your comments on why Vg (L/Dmax) has a smaller margin over stall speed than short field approach speed, and the fact that based on the mechanics of slow flight and the "power curve," your margin puts us at the peak of the Lift/Drag curve (the safest place we can be). This also happens to make for a much more difficult short field approach (gee, thanks Cessna).

There are three reasons we carry excess speed when making a short field approach:
1. Margin of error (for that loose nut between the yoke and the headset)
2. Gust factor
3. We need just enough energy to be able to arrest the descent rate.

I have always taught that the extra airspeed is insurance, and anytime you are "purchasing" insurance, you are balancing versus the risk factor. As you get closer to the ground, your risk factor decreases (and your overrun risk factor increases), so you need less insurance.


...Whoops, got to catch my ride to work. More detail later.
 
Interesting read. Someone made a statement earlier in the thread that in the real world you bring it in low and slow with power for short fields. I would disagree. We fly into <2000 runways a lot where I fly and its power to idle and keep the pattern tight aiming for right before the runway edge so that the little bit of float you get takes you onto the runway. But there are many different ways to skin a cat. The big reason we do it this way though is that we are flying into islands so if you loose your engine during that low and slow approach your going into the cold water which during the winter will kill you before you can be reached. With the power to idle tight patterns we know we can make the runways with an engine failure.
 
I was more interested in your comments on why Vg (L/Dmax) has a smaller margin over stall speed than short field approach speed

Approach speeds in the 172R are 60-70 knots with the short field approach procedure calling for 62. Best glide is 65, a higher, not lower margin. Again though, it really doesn't matter. Flying at 62 (1.34 Vso) only matters for achieving the book values for landing distance. You could fly it slower if you're comfortable with a smaller safety margin, or faster if you're comfortable with a longer than book value landing distance.

I'm having trouble seeing why you think L/D max is relavent though. Perhaps your next reply will clear this up for me.
 
So...uh, we didn't get a chance to go to the strip we wanted to...there were some pretty ominous clouds/storms between us and there so we did something different...

Near Summit Point, there is a grass strip with trees on both ends. It's almost 3,000 feet long, to be sure, but since I had exactly ONE grass landing in my logbook (and that was on my checkride) my CFI decided that some soft-field practice would be just as productive, so we flew in there.

I'm fully signed off for my BFR now, but I still want to go up with my CFI and do some night work and some XC work. I'm sloppy on pilotage these days and it's nice to have someone along, too.
 
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