How Airplanes Measure Speed and Altitude

Safe flying requires that the pilot knows the speed and altitude that they are flying at. Air speed is measured using small tubes called Pitot tubes (pee-to). These cylindrical tubes are positioned on the aircraft and air naturally flows into them during flight. The tube is expanded or compressed by the differing pressures caused by the speed of air entering the tube, and from this the air speed is calculated and displayed in the cockpit, using a comparison to static air in a different tube on the aircraft.

The airspeed calculated in this way will have a built in error depending on the positioning of the Pitot tubes. The active Pitot tube has an opening facing forward and is usually found towards the front of the plane. The static tubes may be incorporated into the active tube or placed separately.

The airspeed collected this way is known as indicated airspeed, and is shown in knots per hour in the cockpit. Some smaller aeroplanes have the speed in miles per hour, and so do some soviet built planes.

Variations in the design of different aircraft mean that the actual airspeed of different aircraft may be different even when it shows as the same on the dial. Variations for high speed, and the compressibility of air need to be factored in. This calibrated air speed is calculated using a variation of the Bernoulli equation. The calibration figure should be the same for all aircraft of a particular model.

A third measure is sometimes quoted, that is the equivalent airspeed. This is the speed that would produce the same dynamic disturbance at sea level as is being produced at the altitude the aircraft is at.

In reality the indicated airspeed is the one that matters for the safe flight of a particular aircraft and it is this that the pilot uses. In some aircraft coloured bands are often marked on the dial indicating the desirable speeds for take-off, landing and the maximum airspeed permitted.

None of these speeds is speed relative to the ground, which is calculated from positional information transmitted from way stations. The means of measuring airspeed available on board an aeroplane calculate the aircraft’s movement relative to the air mass outside the aeroplane. Groundspeed can only be calculated from the ground using radar or way station information. Groundspeed is important for air traffic control and scheduling so it is usually tracked.

The measurement of altitude is much simpler, but much more important in many respects. As altitude increases atmospheric pressure decreases and this is measured using an aneroid barometer that transfers the information as altitude to the cockpit. The important factor is what base line is used.

Most aviation uses feet above sea level as the measure of true altitude, but it is the absolute altitude that is important. The absolute altitude is the height above the ground. It can be seen that it is vital to know which altitude is being referred to.

At the start of a flight an aircraft uses the barometer measure of height up to an altitude known as the transition level. At low altitudes the distance from the ground is what is important. In the United States this height is 18,000 feet, but can be as low as 3,000 feet in parts of Europe. At the transition point all altimeters are set to an agreed standard point so that all aeroplanes flying in a given region have the same relative settings to each other. At higher altitudes the positions of planes relative to each other are more important than where the ground is, relatively speaking.

Flight levels are the altitude divided by one hundred, for instance an aircraft flying at 25,000 feet is at flight level 250. When the plane descends to land the altimeter is reset to local pressure at the transition point for that area, known as the Q point as once again the distance relative to the ground is more important.

Airspeed and altitude are two of the most important factors for safe flight and the measurement of both is largely mechanical