Static pressure is the force exerted by still air. The partices in the air (or any other gas) are constantly moving around, bouncing off whatever they come in contact with and this is what causes the force known as static pressure. This pressure is exerted equally in all directions.
This can be thought of as the pressure inside a scuba diving tank – the air inside is still but there is very high static pressure.
Static pressure reduces as altitude increases.
When an object is moving through the atmosphere, it collides with the molecules it passes. This impact creates additional pressure, known as dynamic pressure.
Dynamic pressure increases as velocity increases.
When you put your hand out the window of a moving car, the extra pressure you feel on the front of your hand comes from the dynamic pressure. However since the static pressure is still acting in all directions, the front of your hand is feeling both static and dynamic pressure. This is known as total pressure.
An aircraft is subject to static pressure at all times, both in flight and on the ground. In flight the front facing parts of an aircraft are also subject to dynamic pressure from the air the you’re flying through. This means all the front facing sections (nose, leading edges of the wings and tail etc.) of the aircraft are subjected to total pressure.
Air density describes how much mass of air is contained within a specific volume. The density of the air is proportional to pressure, so an increase in pressure leads to an increase in density and vice versa.
Air density is also affected by temperature. An increase in temperature will decrease air density and a decrease in temperature will increase density. This is why hot air rises – as the air heats up, it becomes less dense and so it will rise while any surrounding, more dense air will sink.
Since air pressure decreases as we go higher in the atmosphere, air density also decreases as altitude increases and it is denoted by the Greek letter ρ (rho).
The airspeed indicator (ASI) has two inputs; one from a forward facing pitot tube that measures total pressure and the other from a static port on the side of the aircraft that measures static pressure. The ASI then subtracts the static pressure from the total pressure, such that it displays only the dynamic pressure. So the ASI is simply measures pressure, but the pressure indication shown is calibrated in knots.
Indicated airspeed is the speed that is shown on the airspeed indicator (ASI). However, the ASI is calibrated to read accurately in stardard conditions (ISA) at sea level – it is not able to compensate for any changes in air density.
So when flying at higher altitudes, the decrease in air density leads to a decrease in the air pressure measured by the airspeed indicator. This causes the airspeed indicator to show a slower speed when the aircraft is in fact still flying at the same speed, it is only the outside air pressure that has reduced.
In this case, we say that the airspeed indicator is underreading.
True airspeed is exactly what you would imagine it to be – the speed at which the aircraft is travelling through the air. In fact, TAS is the only speed – all other types of airspeed are in fact measurements of pressure.
The image below shows two aircraft travelling at the same TAS, but since the air density is lower at a higher altitude, the top aircraft will have a lower indicated airspeed.
You can convert IAS to TAS by using a flight computer (such as the CRP-1 or CRP-5). This is not required in the Principles of Flight exam.