Airspeed is a measurement of the plane's speed relative to the air around it. The pitot static tube system is an ingenious device used by airplanes for measuring forward speed. The device is really a differential pressure gauge and was invented by Henri Pitot in 1732.
Indicated Air Speed (IAS) is read directly off the air speed indicator and has several factors that must be corrected in order to determine the actual speed of an aircraft over the ground. To determine the aircraft’s Indicated Air Speed, two pressures are measured:
- A pitot tube is positioned on the exterior of the aircraft so that the air molecules of the atmosphere “ram” into it. The faster the aircraft is traveling, the greater the ram pressure. As an aircraft climbs, the atmospheric air pressure decreases, as does the ram pressure.
- To account for this, the aircraft has a static air pressure port that is also connected to the air speed indicator.
The greater the difference between the ram and static pressures, the greater the indicated air speed.
As an aircraft changes its air speed and configuration, such as occurs when slowing down and lowering flaps and landing gear, the airflow pattern over the fuselage changes. This change of airflow affects the pressure in the pitot tube and static port. To account for this, the pilot refers to an air speed calibration chart to read the calibrated air speed (CAS). Each type of aircraft has its own calibration chart because the airflow pattern depends on the aircraft itself.
When flying faster than 200 knots, the air ahead of the aircraft becomes compressed. This air compression increases the air density and the pressure in the pitot tube. To account for compressibility, the pilot refers to an air speed compressibility chart. The greater the CAS and the higher the altitude, the more the pilot must subtract to attain the equivalent air speed (EAS).
The air will be more compressed the faster the aircraft is traveling and the higher the pressure altitude. So for the same CAS as the pressure altitude increases, so does the amount that must be subtracted from the CAS to determine the EAS. Equivalency charts are used to make this correction. The pilot enters the CAS and pressure altitude into the chart and determines how much to subtract. Equivalency charts are not airplane-specific.
It is the EAS that the aircraft feels. EAS is a measure of the dynamic pressure exerted on the aircraft. This dynamic pressure plays a key role in the lift and drag created by the aircraft. For a given EAS the aircraft feels the same dynamic pressure, and therefore lift and drag, regardless of altitude. The higher the density altitude, the thinner the air, and the faster an aircraft must travel through the air mass to obtain the same EAS. The actual speed of the aircraft through the air mass is called the True Air Speed (TAS).
By knowing the air density, the pilot can calculate the actual speed through the air mass, or true air speed. The only time that EAS is equal to true air speed is when an aircraft is flying at standard sea level (SSL) conditions. It is to the TAS that the velocity of the wind is applied, to determine the speed over the ground. The presence of a tailwind or headwind will increase or decrease the ground speed.