The air speed indicator

As stated above, the pitot and static tube combination provides a means of measuring the dynamic pressure. It does not tell us the speed directly, but we can work out the speed if we know the density. Until recently, there was no simple method of measuring density, so all that could be done was to use the dynamic pressure-measuring device described above, and mark on the dial the speed that this dynamic pressure would correspond to at standard sea-level air density. This instrument is called the Air Speed Indicator (ASI). You will see that since the instrument is calibrated assuming the standard sea-level value of air density, it does not give the true speed, unless the aircraft is flying at a height where the density just happens to be equal to the standard sea-level value.

Nowadays, there are devices which can measure the true air speed, but the air speed indicator described above is still an important item on any instru­ment panel. This is because the lift and other forces on the aircraft are dependent on dynamic pressure, and the air speed indicator gives a reading which is directly related to dynamic pressure. For example, if the dynamic pressure is too small, the wings will not be able to generate enough lift to keep the aircraft in steady level flight. The value of the dynamic pressure and hence the indicated speed at which this occurs will always be the same whatever the height. The pilot just has to remember to keep above this minimum indicated speed. If the pilot had only a true speed indicator, he would have to know what the minimum speed was at every height.