Air speed indicator corrections

The speed indicated by the air speed indicator is called the Air Speed Indictor Reading (ASIR). There are several sources of error in this reading. Firstly, the instrument itself may not have been calibrated correctly, or may be suffering from some wear. This error is called instrument error. By recalibrating the instrument it is possible to determine what the correction should be at every indicated speed. The speed corrected for instrument error is called the Indicated Air Speed (IAS). There will also be errors due to the positioning of the pitot and static tubes on the aircraft. It is virtually impossible to find a pos­ition where the static pressure is always exactly the same as the pressure in the free airstream away from the aircraft. To determine the correction for such position errors, the aircraft can be flown in formation with another aircraft with specially calibrated instruments. Once the position error correction has been applied, the speed is known as the Calibrated Air Speed (CAS). Finally, for any aircraft that can fly faster than about 200 mph, it is necessary to apply a correction for compressibility, since Bernoulli’s equation only applies to low – speed effectively incompressible flows.

After all the corrections have been applied, the resulting speed is called the Equivalent Air Speed (EAS). Once the equivalent air speed has been obtained it is quite easy to estimate the True Air Speed (TAS) which is required for navi­gation purposes. For a light piston-engined aircraft, the corrections will be relatively small, and for simple navigational estimates, the pilot can assume that the speed read from his instrument the ASIR is roughly the same as the equiv­alent air speed EAS. The procedure for calculating the true air speed is as follows.

Suppose that at 6000 m the air speed indicator reads 204 knots, i. e. 105 m/s. Ignoring any instrument or position errors, this means that the pressure on the pitot tube is the same as would be produced by a speed of 105 m/s at standard sea-level density of 1.225 kg/m3; but this pressure is kpV2, i. e. 1/2 X 1.225 X 105 X 105.

Now according to the International Standard Atmosphere the air density at 6000 m is 0.66 kg/m3, and if the true air speed is V m/s, then the pressure on the pitot tube will be 1/2 X 0.66 X V2, which must be the same as

1/2 X 1.226 X 105 X 105 so

0.66V2 = 1.226 X 1052 or

V = Vl.226/0.66 X 105 = 1.36 X 105

142.8 m/s.

Thus the indicated air speed is 105 m/s, and the true air speed approximately 143 m/s.

Note that, expressed in symbols, the true air speed (TAS) is the equivalent air speed (EAS) divided by the square root of the relative density (a), or

TAS = EAS/ Vcr

A similar calculation for 12 200 m will reveal the interesting result that at that height the true air speed is slightly more than double the indicated air speed!

We have said that the instrument indicated air speed may sometimes be more useful to the pilot that the true air speed. For purposes of navigation, however, he must estimate his speed over the ground, and with traditional navigation methods, he must first determine the true air speed, then make cor­rections to allow for the speed of the atmospheric wind relative to the ground. The true air speed can be determined using the procedure above, but to do this we need to know the air density or the relative density. This can be obtained by using the altimeter reading, and tables for the variation of relative density in the ISA. In practice, as an alternative to calculations, the pilot can use tables showing the relationship between true air speed and indicated air speed at dif­ferent heights in ISA conditions.

For high-speed aircraft, the indicated air speed has to be corrected for com­pressibility, and to do this we need a further instrument, one that indicates the speed relative to the local speed of sound. This instrument is called a machmeter. However, we are again trying to run before we can walk, and we will leave the treatment of compressible flow to later chapters.

Nowadays navigation has been revolutionised by the introduction of ground-based radio, and satellite navigation systems which can give very accu­rate indications of position and speed relative to the ground. Despite these advances, however, aspiring pilots still have to learn the traditional methods of navigation in order to qualify for their licence. Old instruments like the air speed indicator and the altimeter are simple and reliable, and will not break down in the event of an electrical failure or a violent thunderstorm. Even on the most advanced modem airliners, an old mechanical air speed indicator and pressure altimeter are fitted, and will continue to work even if all the electrical systems have failed.