Airspeed corrections
 

Looking for a bit of advice as to airspeed error corrections. I've read varying explanations as to the error corrections applied to the IAS to reach CAS.

One source describes how the very first reading is the ASIR (Airspeed Indicator Reading), which is corrected for instrument error, giving the IAS. The IAS is then corrected for pressure (position) error to give the CAS. I had accepted that this was the case, however, on completing some practice ATPL questions, they appear to suggest that IAS has not been corrected for instrument error, and that to get from IAS to CAS, corrections need to be made for both Instrument and Pressure errors.

This may just be a terminology issue, as I'm not entirely sure what each error is actually caused by, although I'm not looking for explanations of those here.

Which is the correct version?

Thanks

PT

That's what I was taught too. IAS is basically the reading you get from pressure instrument. CAS requires an airdata computer to apply the necessary and predictable and quantifiable corrections.