Speed Measurement

Aircraft depend for a great deal on measuring the correct outside air pressure for the indication of airspeed, altitude and vertical speed. A job for the pitot static system which senses air pressures around the aircraft.

In the cockpit we see instruments known as pressure instruments and they operate by sensing changes in the pressure of the air surrounding the aircraft. With an EFIS the pressure sensors are connected to the air data computer for processing.

We will investigate the concept of aircraft speeds: indicated airspeed, true airspeed and how it is used in navigation and keeping the airplane flying without stalling.

Best glide speed is discussed together with the forced landing procedure as these two are intimately related. The use of flaps and influence of aircraft weight and wind is also shown.

Maximum climb speeds and climb performance is important for clearing obstacles in the flight path and last but not least we take a look at maneuvering speed and why that is crucial for the pilot.

Pitot Static system

In the pitot static system three instruments are connected to the static port(s) and the pitot tube via tubing, in some cases there is an alternate static port provided in the cockpit.

These three pressure instruments are:

• The altimeter showing static pressure as altitude
• The airspeed indicator indicating the difference between the pitot pressure and static pressure
• The vertical speed indicator giving an indication of the rate of static pressure change relating to the rate of climb or descent

Image to the right shows how the components are connected to the system.

Pitot tube

To be able to measure the dynamic pressure the pitot tube must face forward in the undisturbed airflow. The movement of the aircraft creates a pressure over the static pressure and this is fed through tubing (pitot line) to the airspeed indicator. Some pitot tubes are fitted with a connection for static pressure, these are called pitot head.

Pitot heat

It is common practice that the pitot tube is heated to protect against ice blockage in low temperature and high humidity conditions.

Static port

Usually taken at the fuselage and even more common at both sides to equalize pressure when the aircraft is flown out of balance or the nature of airflow around the aircraft (turbulence).

Some light or ultralight aircraft have no outside static port but take static pressure inside the cockpit where it is slight lower than outside the aircraft. A bit less accuracy is the result and the opening or closing of windows or air vents will influence the static pressure.

Alternate static port

Other aircraft (unpressurized) have an alternate static port to be used when the outside ports fail or the static line is blocked for some reason. Usable in unpressurized cockpits and the indication will have an error as described above.