/Planning & Performance
/Lift, Stall & Speed
/Defining Speeds

Defining Aircraft Speeds

One of the most important factors with navigation is speed. Without it the aircraft would not get anywhere. Aircraft speed measured in relation to the air mass it flies in, is called airspeed. The movement over ground is influenced by the wind and called ground speed and this is what you need for distance, time and fuel calculations.

Even airspeed is not what it looks like, as aircraft performance is related to changes in the atmosphere (temperature, pressure and humidity) which have their influence on performance.

Airspeed is measured by the pitot static system, but as air density varies with temperature and pressure influenced by local weather, this has noticeable effects on the aircraft and engine.

Aircraft performance is measured and compared to standard atmospheric (ISA) settings, so there is a need to convert indicated airspeed to obtain true airspeed (ISA speed) for our calculations.

The pilot can use this converted speed for performance calculations, navigation and fuel consumption. And on this page we explain the different speeds the pilot is dealing with and what factors he or she need to take into account for his navigation plan and takeoff / landing performance.

Indicated airspeed, TAS & GS

Aircraft can only fly by virtue of their speed through the air and the resulting flow of air molecules over and under the wings (as explained by Bernoulli's principle and Newton's third law). For obvious reasons this speed must be indicated to the pilot and it is measured by a pitot tube in the undisturbed air stream.

Pressure in the pitot tube is a combination of airspeed (dynamic pressure in fluid dynamics q = 1/2 ρ V2 based on kinetic energy for rigid bodies Ek = 1/2 m V2) and altitude (static pressure or potential energy) and the pitot tube is part of the pitot static system
Pitot static system, click to read more
.

Indicated airspeed, IAS

The airspeed indicator can be seen as a pressure chamber divided in two sections by a diaphragm connected to the pointer mechanism. Each of the sections has an air connection, one to the pitot tube and the other to the static port. And as the speed of the aircraft increases, the pitot pressure (dynamic + static) increases and is countered by the static pressure on the other side of the diaphragm. So the result is: pitot pressure - static pressure, which gives us dynamic pressure and this is indicated airspeed which is shown on the instrument to the left. More detailed information can be found on the following page: airspeed indicators.

When the aircraft moves through an air mass, dynamic air pressure is build up in the pitot tube and the diaphragm expands moving a pointer on the instrument. airspeed is indicated in knots (or nautical mile per hour) but mph and kph can be found on some instruments too. Some even have dual scales, for example: mph and knots. The scale usually has colored bands indicating flap speeds (white arc), normal operating (green arc) and caution speeds (yellow arc) and a radial red line indicating never exceed speed (VNE).

Calibrated airspeed, CAS

Indicated airspeed corrected for position error is called calibrated airspeed, CAS. These errors are caused by the actual installation and position of the pitot tube on the aircraft and this will result in the incorrect sensing of the dynamic and static pressures. This error is usually small, only noticeable with high angles of attack (during level stall exercises for example). The same occurs when the airflow is disturbed by turbulence, and as this is short lived, it is of no factor to the pilot.

Equivalent airspeed, EAS

For practical purposes, air is assumed to be incompressible up to about 200 KIAS and in the lower atmosphere (below 5000ft). Flying faster above that and the altimeter will over-read (ie. air is starting to compress in the pitot tube) by 1/2 knot. At 10000 ft the error starts at 160 KIAS with a 1/2 knot over-read. airspeed corrected for these compressibility errors is called equivalent airspeed, EAS. For most low level experimental pilots this error is of no concern.

True airspeed, TAS

Airspeed indicators are calibrated under ISA conditions, and only if these conditions exist in the atmosphere the instrument will indicate true airspeed (which is rare). Some airspeed indicators have an outside air temperature (OAT) set knob.

With this subscale the pilot sets OAT against pressure altitude (to obtain that: set the altimeter to 1013 hPa or 29.92 inHg temporarily) so that the airspeed indicator reads indicated airspeed (IAS) and below that true airspeed (TAS) in a movable subscale. These indicators are somewhat more expensive but so very handy. The correction for temperature and pressure is called density error.

The formula to the left is explained as follows: TAS = EAS √ (ρ0 / ρ), where ρ0 = 1,225 kg/m3 at sea level and ρ is the actual air density at your current altitude.
This shows the conversion to True airspeed with limitations mentioned above for EAS.

Check out our E6B app where you can calculate TAS yourself, even during the flight: E6B True Air Speed.

IAS → CAS → EAS → TAS

The above basically says that IAS corrected for instrument and position errors is CAS. CAS corrected for compressibility errors is EAS and EAS corrected for density errors is TAS.

IAS is important because that is what the pilot is working with, and during maneuvering the aerodynamic forces are the result of this indicated airspeed. TAS is important because it gives an idea of the actual speed of the aircraft in a standard atmosphere air mass, and this is what we need to compute our ground speed.

Ground Speed, GS

For navigation, i.e. movement over the ground, we need the speed of the aircraft over the ground. And as True airspeed (TAS) is the airspeed corrected for all mechanical, position, compressibility and density errors this is the speed we need to use when calculating ground speed for navigation.

Wind

When there is no wind at all, GS equals TAS. If there is any wind we need to use the circular side of the E6B or any other navigational computer to calculate the resulting ground speed (wind triangle). Have a look at our own calculator to solve this problem: E6B Application.

Finally, both ground speed and distance relates into time so that the fuel needed for the trip can be calculated. We can see that it is therefore important that the pilot has a good understanding of these speed concepts.

Written by EAI.

Enjoyed our Website?

Greetings.
If you enjoyed and found value in our site, consider becoming a member. With your help this website can keep growing as a source of information for all aviation enthusiasts!

Thanks!