I found this in the report: Air speed estimation. Another way of adding redundancy to a pitot system, or even dispensing with it altogether, is to estimate CAS

I found this in the report: Air speed estimation. Another way of adding redundancy to a pitot system, or even dispensing with it altogether, is to estimate CAS using other 'non-air' data. WK uses a CAS estimation algorithm, which estimates CAS as a function of throttle position, pitch, engine speed and air density. Presently its output is not used as a redundancy to the pitots, but to calculate the dynamic pressure reference value, which was used in combination with dynamic pressure readings to identify and disqualify erroneous dynamic pressure readings. Several academic papers on CAS estimation exist and the Panel were able to develop a CAS estimation algorithm and prove the concept using WK flight data. WK = Watchkeeper drone. CAS = Calibrated air speed. Note that general aviation airplanes are operated perfectly safely with CAS (or actually just indicated air speed) being used as a proxy for angle of attack. Instead of using this estimate, the on-board computer averaged the values from the two pitot tubes, despite having already ruled them both as giving invalid data. While this is unfortunate, it is also unfortunate that there were only two pitot tubes, and that both of them are prone to being blocked by moisture in clouds or rain, let alone the ice that they were trying to find (the report attributed the blockage to moisture, not ice.)