11.01 Definitions of speed
The Air Speed window is for calculating True Air Speed (TAS). Four types of flight speeds are used:
IAS (Indicated Air Speed).
This is the speed read from the instrument, calibrated to MSL and ISA. This value, corrected by the values taken from the flight manual gives the
CAS (Calibrated Air Speed).
For speeds < 200 kt and heights < 10,000 ft, this value must be corrected by the compressibility of air in the pitot tube to give the
EAS (Equivalent Air Speed).
This value corrected by the density gives the
DAS (Density Air Speed),
which has to be corrected by the temperature deviation of IAS. After which this gives the
TAS (True Air Speed).
The scales in the Air Speed window can be used to correct this.
In the following examples it is assumed that the OAT (Outside Air Temperature) is known for the flight altitude (e.g. from the weather report). If the TAS is calculated from the outside air temperature measured during the flight allowance should be made for the fact that the indicated temperature is increased through the air friction on the outer surface of the aircraft fuselage.
Note on ‘‘TEMPERATURE RISE‘‘ scale:
The upper scale for the temperature correction value of Δt °C (Ct = 1.0) is based on a temperature recovery coefficient of Ct = 1.0. This corresponds to the calibration of modern sensors. The scale below Δt °C (Ct = 0.7) shows the equivalent values for Ct = 0.7. This corresponds to the calibration of older types of sensor. If in doubt for older aircraft the Ct values can be taken from the flight manual. For examination questions it is recommended to give the Ctvalue used for calculating.
11.03 Mach number
The Mach number M can be calculated on the FLIGHT CALCULATOR side of the Aviation Calculator.
This is the TAS of the aircraft relative to the local sound velocity. E.g. at a TAS of 200 kt and local sound velocity
of LSS 600 kt, the Mach number would be TAS 200/LSS 600 = 0.33 M.
Local sound velocity depends on temperature. In the AIR SPEED window there is an M mark opposite the temperature scale. This can be used to calculate the Mach number.