PPL(A) EN: At higher altitudes, true airspeed (TAS) tends to be higher than calibrated airspeed (CAS). A rough estimate of the TAS can be obtained by...
Question 90, Aircraft General Knowledge - PPL(A) English - Private Pilot License
Subtracting 2 % of the CAS for every 1000 ft altitude.
Adding 10 % of the CAS for every 1000 ft altitude.
Subtracting 10 % of the CAS for every 1000 m altitude.
EN 