7.4 Different types of Airspeed

7.4.1 In aviation, Speed is categorised and divided into 5 groups:

Indicated Air Speed (IAS)

Calibrated Air Speed (CAS)

Equivalent Air Speed (EAS)

True Air Speed (TAS)

Ground Speed (GS)

Study the “ICE Tea is a Pretty Cool Drink” diagram:

speeds_ice_t

7.4.2 Before you read more, it is important for you to understand that an Air Speed Indicator relies on two pieces of information:

  1. Dynamic Pressure (Pitot Tube) i.e. forward movement of air into pitot tube.
  2. Static Pressure (Static Port) i.e. still atmospheric pressure surrounding the aircraft.

IAS less Position error = CAS

CAS less compressibility error = EAS

EAS less density (pressure and temperature) error = TAS

7.4.3 CAS/RAS:

Because the static port and pitot head are at fixed positions, they may display slightly different readings at different angles of attack/speed/configuration settings. CAS can be found using the aircraft manual. Once IAS has been compensated for position error, it is known as CAS.

7.4.4 EAS:

Not really applicable to slow moving aircraft (below 300kts). If the aircraft flies really fast, the air becomes compressible hence causing errors to read on the Air Speed Indicator. Once the CAS has been compensated for compressibility, it is called Equivalent Air Speed.

7.4.5 TAS:

The effect of altitude and temperature plays a key role in altimeter error. At high altitudes of around 35000ft your Indicated airspeed can nearly be half your true airspeed. This is because the air is so much thinner at high altitude; therefore a lesser volume of air is being monitored, showing a lesser airspeed. Once your EAS has been calibrated for temperature and altitude, the speed is called TAS.

7.4.6 GS:

Groundspeed is simply TAS compensated for wind effect. So flying at a TAS of 100kts, and a Ground Speed of 120kts, means we have a tailwind of 20kts. Ground speed can be measured by using GPS or else taking distance flown and dividing it by the time taken to fly that distance (GS = Dist / Time). TAS is what the aircraft would be flying in ZERO wind conditions.

Conventional radar can only determine GS.