CAS, IAS and white/green arcs

sarangan

sarangan

Pattern Altitude
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Andrew, CFI-I
Most of the airplanes I have flown specify V speeds in IAS, and the green & white arcs are also in IAS. But I have come across a few older airplanes which specify these in CAS. That in itself is odd, but to add to the confusion, the published V speeds and the markings on the instrument don't always agree.

Here is one example (from a 1975 M model C-172):

white arc: 54-100 mph (CAS)
green arc: 61-145 mph (CAS)
Vs1 = 57 mph (CAS)
Vso = 49 mph (CAS)

From correction table:
flaps up, 58 mph CAS = 50 mph IAS
flaps down, 49 mph CAS = 40 mph IAS

I am scratching my head to make sense of this. The bottom of the white arc (54 mph) is not the stated full flap stall speed in CAS (which is 49 mph), nor the equivalent IAS (which is 40 mph). Similarly, the bottom of the green arc (61 mph) is not the clean stall speed in CAS (which is 57 mph) nor the equivalent IAS (which is 50 mph).
 
Now you know why the industry was getting their hat handed to them in civil court and they agreed on a standardized arrangement and data published in a POH.
 
sarangan said:
Most of the airplanes I have flown specify V speeds in IAS, and the green & white arcs are also in IAS. But I have come across a few older airplanes which specify these in CAS. That in itself is odd, but to add to the confusion, the published V speeds and the markings on the instrument don't always agree.

Here is one example (from a 1975 M model C-172):

white arc: 54-100 mph (CAS)
green arc: 61-145 mph (CAS)
Vs1 = 57 mph (CAS)
Vso = 49 mph (CAS)

From correction table:
flaps up, 58 mph CAS = 50 mph IAS
flaps down, 49 mph CAS = 40 mph IAS

I am scratching my head to make sense of this. The bottom of the white arc (54 mph) is not the stated full flap stall speed in CAS (which is 49 mph), nor the equivalent IAS (which is 40 mph). Similarly, the bottom of the green arc (61 mph) is not the clean stall speed in CAS (which is 57 mph) nor the equivalent IAS (which is 50 mph).
Click to expand...

The airspeed indicator is marked in indicated airspeed (IAS) but the color codes signify calibrated airspeed (CAS). You are correct that it doesn't make sense. As far as the calibration table not agreeing, I belive that is because the calibration table is for power-on steady-state flight, while the stall speeds are power-off.
 
dmspilot said:
The airspeed indicator is marked in indicated airspeed (IAS) but the color codes signify calibrated airspeed (CAS). You are correct that it doesn't make sense. As far as the calibration table not agreeing, I belive that is because the calibration table is for power-on steady-state flight, while the stall speeds are power-off.
Click to expand...

Yea, I was trying to explain that to a new student pilot, and ended up confusing the poor guy. To make matters worse, the pre-solo written exam by the FBO wants these answers down to the last digit.
 
Early versions of FAR 23 required airspeed markings and limitations to be in CAS.

For that 1975 M model C-172 the POH should state the required airspeed markings - how does that compare with what is on the ASI?
 
At what speed does an airplane stall?

There is a reason I don't spend much time splitting hairs about whether stall speed is 54 or 55 mph.
 
Any time I fly a plane I haven’t flown before I go out and see where THAT plane stalls.
 
Sorry for waking up an old thread :)

I noticed this inconsistency on multiple aircraft and am confused as well.

From looking at POH's, looks like this was fixed at some point since the 172M has the mismatch, but the 172N (c.1977) appears to by in sync between published stall speeds and the ASI markings. My 1964 172E and a C185 have the CAS/IAS mismatch and neither CAS or IAS matches the markings.

Anyone have any more thoughts other than they weren't so good with documentation back then?


1979 172N
Vs0 - 47 KCAS / 40 KIAS (power off, forward CG, GW)
Bottom of white arc - 41 knots (should be Vs0)
1 knot-- probably extrapolation error

Vs1 - 53 KCAS / 48 KIAS
Bottom of green arc - 47 knots (should be Vs1)
1 knot-- probably extrapolation error


172M
Vs0 - 49 MCAS / 40 MIAS
Bottom of white arc - 54 MPH
14 MPH difference IAS to white arc

Vs1 - 57 MCAS / 49 MIAS
Bottom of green arc - 61 MPH
12 MPH difference IAS to white arc


1964 172E
Vs0 - 49 MCAS / 37 MIAS
Bottom of white arc - 52 MPH
15 MPH difference IAS to white arc

Vs1 - 57 MCAS / 42 MIAS
Bottom of green arc - 59 MPH
17 MPH difference IAS to white arc


185F
Vs0 - 56 MCAS / 48 MIAS
Bottom of white arc - 62 MPH
14 MPH difference IAS to white arc

Vs1 - 65 MCAS / 61 MIAS
Bottom of green arc - 68 MPH
7 MPH difference IAS to white arc
 
My manuals refer to the speeds as "True Indicated Air Speed."
 
flyingron said:
Speed doesn't make airplanes stall.
Click to expand...
Not entirely, but I have yet to see an airplane not in the powered lift category fly with 0 airspeed. More accurately a lack of speed which will cause a lack of lift will cause an airplane to stall.
 
Paul Yoes said:
Sorry for waking up an old thread :)

I noticed this inconsistency on multiple aircraft and am confused as well.

From looking at POH's, looks like this was fixed at some point since the 172M has the mismatch, but the 172N (c.1977) appears to by in sync between published stall speeds and the ASI markings. My 1964 172E and a C185 have the CAS/IAS mismatch and neither CAS or IAS matches the markings.

Anyone have any more thoughts other than they weren't so good with documentation back then?


1979 172N
Vs0 - 47 KCAS / 40 KIAS (power off, forward CG, GW)
Bottom of white arc - 41 knots (should be Vs0)
1 knot-- probably extrapolation error

Vs1 - 53 KCAS / 48 KIAS
Bottom of green arc - 47 knots (should be Vs1)
1 knot-- probably extrapolation error


172M
Vs0 - 49 MCAS / 40 MIAS
Bottom of white arc - 54 MPH
14 MPH difference IAS to white arc

Vs1 - 57 MCAS / 49 MIAS
Bottom of green arc - 61 MPH
12 MPH difference IAS to white arc


1964 172E
Vs0 - 49 MCAS / 37 MIAS
Bottom of white arc - 52 MPH
15 MPH difference IAS to white arc

Vs1 - 57 MCAS / 42 MIAS
Bottom of green arc - 59 MPH
17 MPH difference IAS to white arc


185F
Vs0 - 56 MCAS / 48 MIAS
Bottom of white arc - 62 MPH
14 MPH difference IAS to white arc

Vs1 - 65 MCAS / 61 MIAS
Bottom of green arc - 68 MPH
7 MPH difference IAS to white arc
Click to expand...

Funny thing is, if you change the book speeds to knots, the AS bottom of white arc, in MPH, is closer to the book. :D
 
flyingron said:
Speed doesn't make airplanes stall.
Click to expand...
Partially true. All of the airspeeds we're talking about are steady state speeds where G-loading would be 1 G. In that scenario, the expected stall speed would be predictable. When markings are off by double digits, it's misleading.

I've always pondered why an angle of attack indicator isn't standard equipment in light aircraft. That will always tell you how close your are to stalling in any attitude and at any airspeed. Consider also that when load factor goes up, AOA increases which will widen the spread between CAS and IAS. So as load factor goes up and AOA goes up, the spread widens where a single IAS can't possibly be correct (which might explain the use of CAS in early docs). Of course that's really going down the rabbit hole....
 
A required placard on my Aeronca 7AC.
s-l500.jpg
 
Paul Yoes said:
Partially true. All of the airspeeds we're talking about are steady state speeds where G-loading would be 1 G. In that scenario, the expected stall speed would be predictable. When markings are off by double digits, it's misleading. .
Click to expand...

That's what the markings are based on, but it lulls pilots into a false sense of security because flight isn't always 1G.
 
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