Above a certain speed, the air molecules are hitting the pitot probe too fast to normally flow through the probe. This build-up creates a higher pressure and will result in an erroneous airspeed reading due to a higher dynamic pressure. This is the compressibility error, and it will induce the ASI to over-read. For instance, the IAS (or CAS) will be too high than it is in reality, and a correction needs to be applied to account for the compressibility error to obtain the Equivalent Air Speed (EAS). The higher the airspeed, the higher the compressibility error.
Besides the airspeed, the compressibility error is also influenced by the altitude. In fact, the less dense air at high altitudes is more easily compressed than at lower levels. The ASI’s calibration formulae include a factor addressing the compressibility of the air so that correct dynamic pressure is measured.
Compressibility is becoming significant when the TAS exceeds 300 kt or the IAS is greater than 150 kt.
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