IAS or CAS
 

Was reading up on performance recently and found a passage that says aircraft which have an air data computer (that interprets raw data and process it in some way before displaying it as flight parameters) will automatically display CAS on the PFD rather than IAS. I.e. the computers do the calculation from IAS to CAS. So basically that speed we're looking at is CAS not IAS .

At the same time I can't find any proof either way in my aircraft manuals. Can anyone clarify this?

Because at that point the speed will be shown as a mach number instead of in knots.

What do you mean by CAS?
calibrated or computed airspeed?

The difference between CAS (Calibrated airspeed) and IAS (Indicated airspeed) is: IAS is CAS as corrected by the correction table in the flight manual, which accounts for the effects of alignment error of the pitot tube [to free stream air] at higher angles of attack (slower speeds), and static port position error (faster speeds). If the aircraft has an airspeed indicator, that is a pressure gauge which reacts to impact pressure on the pitot tube, and pure static pressure, and does not know of alignment or position errors, hence the need to present a correction table in the FM. If there is instrument error, that is not accounted for in the IAS to CAS table, as the error would be unique to that particular instrument, and might be corrected with calibration or replacement of that particular airspeed indicator. An air data computer may be programmed to account for the difference between IAS and CAS, and thus present the airspeed to the pilot as CAS. This is possible if the air data computer is also receiving AoA data. If this is the case, the flight manual should state which speed is being presented to the pilot.

When I did a system calibration last week using a very advanced calibration unit, I noticed that the calibration unit presented the "airspeed" as being "CAS". However, as we were using it on a regular airspeed indicator. Therefore the "CAS" of the calibration unit simulated airspeed is really "IAS" as seen by the indicator, because the test unit cannot simulate position or alignment errors for that type of aircraft. In the case of the calibrations we were doing, the difference between the test unit simulated value, and the airspeed indicator value was instrument error, and yes, was a little different left and right in the cockpit.

I do find it to be a disconnect when some speeds and speed limitations are actually, and correctly presented as CAS, and that speed is marked on the airspeed indicator, which is presenting in IAS - thus ignoring a possible difference.

EAS and TAS were not considered in the calibration we were doing, as it was a 170 KIAS Vmo airplane, so they are not factors for this type.

You are probably right for EAS. TAS is a factor for any aircraft.

I would think of it like this: in this case the IAS is CAS. The instrument and pitot static error have been removed, the total system error is 0 (within some tolerance), and CAS = IAS.

Most modern aircraft I flew had less than one knot difference between IAS and CAS, hence you could assume IAS to be CAS without much of an error.

Which is determined from the IAS to CAS chart in the flight manual? Or a different means? Did the difference increase as speed reduced toward minimum flying speed?

Might I suggest that you check the front of the manual where acronyms are defined? I suspect that since you have an air data computer, the manual uses CAS to mean “computed” airspeed and not calibrated airspeed. Regardless, any references to airspeeds (limits, procedures, etc) unless otherwise stated will refer to the speed read off of the PFD whether that is really IAS, CAS, EAS or whatever. The only time it will likely make any difference is if you have to revert to an uncorrected (standby) airspeed gauge, but as others have pointed out the difference will only be a knot or so.