CAS, IAS, POH

[Martin S.]

Evening everybody,

the M20J POH (section 2-3) shows: Vs0 = 61 MPH CAS & Vs1 = 68 MPH CAS

The speed tape of my airspeed indicator (original analogue one) in my cockpit also shows these values, i.e.: Vs0 = 61 MPH & Vs1 = 68 MPH

I understand an airspeed indicator shows the indicated airspeed (IAS), not the calibrated airspeed (CAS). Accordingly, there should be a small difference between the the indicated and the calibrated airspeed. Why then are the POH CAS values the same as the speed-tape marked IAS values?

Greets and thanks for your thoughts,

 

Martin

Edited October 31, 2021 by Martin S.

[carusoam]

Great question Martin!

Depending on the year the plane and instruments were built…

The more modern Mooneys… the CAS / IAS differences are pretty small… compared to the older Mooneys…

This is for analog instruments from the 90s…

 

Look closer at the data you are referencing…

Something is amiss…

We know CAS and IAS have variations and very few times over the range are they identical…

it is highly unlikely that your IAS  matches the CAS exactly…

If it does…

Post your CAS table and a pic of your ASI… 

I would have to be staring at my ASI… to read it with enough accuracy to be able to adjust it to CAS… by then, the speed will change…

 

Better question… with all the digital velocity tapes available… are they ever going to use CAS because they can?

PP thoughts only, not an instrument guru…

Best regards,

-a-

 

[PT20J]

FAA regulations originally required the colored arcs on airspeed indicators to be calibrated airspeeds. At some point in the 1970s, the FAA realized this was not very useful and changed the requirement so that the colored arcs now reference indicated airspeeds.

[Martin S.]

Thanks for your thoughts so far. I added fotos of both the ASI and the POH. Take V_FE for an example. It is 125 mph according to the ASI. This corresponds to the POH CAS value for V_FE, but not to the V_FE IAS value of 132 mph that one would expect. I don't understand the FAAs logic from back in those days. If nobody flies according to the IAS, what was the purpose of mentioning it in the POH?

Greets, Martin

Edited November 1, 2021 by Martin S.

[carusoam]

Martin,

Good news… your logic is solid…!

Bad news… does your POH have the column names reversed?

 

The FAA has done a really good job of matching everything to IAS…

So the pilot doesn’t make a mistake due to memory or calculations during important busy times of the flight…

Let’s ask @midlifeflyer (M20J question, POH / ASI funky challenge… could be a printing error?)

 

PP thoughts only, not a CFI…

Best regards,

-a-

[Martin S.]

16 hours ago, carusoam said:

Martin,

Good news… your logic is solid…!

Bad news… does your POH have the column names reversed?

 

The FAA has done a really good job of matching everything to IAS…

So the pilot doesn’t make a mistake due to memory or calculations during important busy times of the flight…

Let’s ask @midlifeflyer (M20J question, POH / ASI funky challenge… could be a printing error?)

 

PP thoughts only, not a CFI…

Best regards,

-a-

Clarence, I have the POH of a 83J for comparison. It lists the same IAS values as the ones above (originating from a seventies J model). Accordingly, it should be safe to say that there is no mistake in the CAS/IAS table.

 

Greets,

 

Martin

Edited November 2, 2021 by Martin S.

[carusoam]

7 hours ago, Martin S. said:

Clarence, I have the POH of a 83J for comparison. It lists the same IAS values as the ones above (originating from a seventies J model). Accordingly, it should be safe to say that there is no mistake in the CAS/IAS table.

 

Greets,

 

Martin

Martin,

See the post from Skip PT20J above…

He references a change that occurred that predates my aviation knowledge…

Best regards,

-a-