Originally Posted by

**jsbc**
There's one thing that bothers me. Let's say my IAS at stall is 38mph, and my CAS at stall is 44 mph. Given that the big error in IAS is very close to the stall, which number - IAS or CAS - should I be using for 1.3Vs, 1.2Vs and other approach speeds?

I have been using 1.3Vs IAS on base and 1.2 Vs IAS for final. That gives me approx 50mph IAS for base and 45mph IAS on final.

But if the IAS/CAS error at 50mph is much less than at stall, my 50 mph IAS may actually be close to 50mph CAS which is only 1.15Vs.

Here's what Imeson says: "If the pilot goes out and stalls the airplane, noting the indicated airspeed and multiplies this value by 1.3 (for approach) there is an error introduced into the equation that will result in the approach being made at too slow an approach speed."

He then goes on to suggest, though he is not totally clear on the subject, that the IAS should be converted to CAS and then multiplied by 1.3. In my case that would give more like 55mph on base and 50mph on final which is significantly different.

First of all, the plane I'm flying is a Citabria, instead of a Cub, so my airspeeds are probably considerably higher than yours...

That said, I think you left out something that Imeson probably said in that explanation about converting IAS to CAS and then multiplying by 1.3. At that point, you've got a target **Calibrated Airspeed** for your approach, but to be useful, you'll need to** convert your "1.3 Vso" back to IAS** for it to be of ANY use to you. Most of us don't have "Calibrated Airspeed" instruments – we only have our standard ASI, which reads "Indicated Airspeed" (IAS) only.

Let's create a hypothetical example, based on the POH for a Citabria 7ECA. Here's the relevant portion of the Airspeed Correction chart from that POH:

I.A.S. (MPH) |
C.A.S. (MPH) |

50 |
58 |

60 |
66 |

70 |
75 |

80 |
83 |

Let's say I go flying, and determine that the stall occurs at 41 mph IAS. Let's assume I'm flying at max gross weight. Calculating the base leg airspeed (1.3 Vso) using only that IAS, I would determine a target speed of **54 mph IAS. **

But Imeson says that speed is not accurate, and he's right. Very much so, in the case of the Citabria! Since how airplanes fly (and stall) is based upon their "real" airspeed (which is Calibrated Airspeed), we need to determine our target airspeeds in CAS. The chart above says we need to add ~10 mph to our 41 MPH IAS observed stall speed to obtain the actual CAS stall speed. Making the conversion, my **real **stall speed is** 51 MPH CAS**. (Sanity check: The Citabria POH shows stall speed at gross weight, power off , 0º of bank is **51 MPH CAS**. So my "observed" stall speed is right on target. So far, so good.)

Now let's calculate our **1.3 Vso target speed **for the base leg using the two different methods:

First, using **Indicated Airspeed only:**

**41 mph IAS** stall speed x 1.3 = **53 mph IAS** target speed for base leg

Now using Imeson's **Calibrated Airspeed **method:

**51 mph CAS** stall speed x 1.3 = 66 mph CAS

66 mph CAS converted to IAS = **60 mph IAS** target speed for base leg

And for our **1.2 Vso target speed **for final approach using those same two methods:

First, using **Indicated Airspeed only:**

**41 mph IAS** stall speed x 1.2 = **49 mph IAS** target speed for final approach

Now using Imeson's **Calibrated Airspeed **method:
**51 mph IAS**stall speed x 1.2 = 61 mph CAS

61 mph CAS converted to IAS = **53 mph IAS** target speed

Using Calibrated Airspeed (which is the "real" airspeed), we would fly our base leg at **60 mph IAS**, and our final approach at **53 mph IAS**. Those speeds give us the actual 30% and 20% margin over the actual stall speeds.

If we incorrectly used IAS only in our calculations, we would fly base leg at **53 mph IAS**, and final approach at **49 mph IAS**. Our margin above the actual stall speed would be substantially smaller than we believe it to be.

Using a 30º bank for our base-to-final turn, our stall speed would go up 8% to 55 CAS (~47 IAS). A 53 mph IAS, my margin above the stall speed is somewhat smaller than I was expecting... (22% for base, and 12% on final)

But let's say I increase the bank angle to 45º in that turn... Now the stall speed increases by 19% to 61 CAS (~53 IAS), and at 53 mph IAS, I have no safety margin at all... The slightest increase in back pressure or bank angle will stall the airplane. Here I thought I had plenty of safety margin (30% above stall speed), but I actually have none.

As I said, your airplane is different than mine, so your calculations could be dramatically different. I know of one airplane that stalls at 34 mph CAS, but the airspeed indicator shows "zero" airspeed well before stalling. If you're trying to fly 1.3 VSO based purely on the ASI indication in that airplane, you're going to be in a world of hurt... As many others have said before me, you have to develop a "feel" for your airplane at MCA, and that may require covering the ASI up completely for a few hours while you practice slow flight and then progress to pattern work. I found CC Pocock's book **Bush and Mountain Flying **was pretty helpful in getting comfortable doing that, and heartily recommend it. He also teaches bush and mountain flying

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