Back to the topic of the relationship of CAS and IAS, in the context of an aircraft with a Vmo slower than 200 knots (in this case, a turbine DC-3). I am qualifying a second pair of pitot tubes and static ports, they are a previously approved alternative to the normal pitot/static tubes, and there is a chart in the flight manual relating IAS to CAS for both normal and alternate pitot/static systems. I have to verify everything by test.
I start with the state of the art pitot/static tester to calibrate the ASI's. This unit states, and the instructions confirm, that the "speed" it simulates by pressurizing the pitot tube is CAS. My further reading produces the statement that "CAS corresponds to impact pressure on the ASI capsule alone" (no atmospheric corrections), so this would validate what the test unit manufacturer is saying.
In flight, CAS is the actual speed (not considering EAS nor TAS corrections) that the airplane is experiencing aerodynamically. IAS may have position errors (static port location/disturbance), or alignment errors (pitot tube angle off streamline), or combination of both. The alignment errors would be more low speed resulting from higher AoA angles, and the position errors at higher speeds, as airflow over the static ports may be affected more. So the possible difference between CAS, and alignment/position affected IAS, is presented to the pilot in a chart in the performance section of the flight manual. But once the pilot considers the difference presented in the chart between IAS and CAS, the two are no longer the same value, contrary to the test unit telling the maintenance technician that CAS = IAS.
Does this simply mean that there are two distinct and possibly different meanings for "CAS"? The first being: "CAS corresponds to impact pressure on the ASI capsule alone" = IAS for a stationary ASI under calibration test..... And, CAS differs from IAS by the combination of position error and alignment error in flight?
And, courtesy of Genghis, I remind myself that the differences between IAS and CAS, as per 25.1323 shall not exceed 3% or 5 knots, whichever is the greater, throughout the speed range.
Can anyone offer insight into what I perceive to be two different meanings for "CAS" - on the ground during test, CAS = IAS, but in flight, the two differ by position and alignment errors, which do not exist on the ground? A technician can confirm calibration on the ground, ans assert that the CAS is correct, but really the technician has only confirmed that there is no instrument error, the technician cannot quantify position and alignment errors - CAS to IAS from the pilot perspective.