Definition
An error in airspeed indication caused by the compression of air against the pitot tube at high speeds. Standard airspeed indicators are calibrated assuming air behaves as an incompressible fluid, but at higher airspeeds and altitudes, ram air actually compresses as it enters the pitot system, producing a higher indicated airspeed than the true value. The error becomes significant above roughly 200 knots and at higher altitudes, and must be corrected for to obtain true airspeed.
Plain English
When you fly fast, the air piling into the airspeed sensor gets squeezed, and that squeeze makes the airspeed read higher than you're really going. Compressibility error is the size of that overshoot.
Context Anchor
Seen in airspeed indicator error discussions, high-speed performance charts, and conversions between indicated airspeed, calibrated airspeed, and equivalent airspeed.
Derivation
From 'compress' (Latin comprimere, 'to press together') plus 'ability.' The name points directly to the cause: at higher speeds, air can no longer be treated as if it stays the same density when it hits the pitot tube — it actually compresses, and that compression skews the reading.
Why Pilots Care
Uncorrected readings can cause pilots to exceed structural limits or fly too close to stall speed without realizing it.
Grounding Statement
As an airplane moves faster, the air at the pitot tube is squeezed more tightly before its pressure is measured.
Intuition Check
Compressibility error does not mean the instrument is broken. It means the air itself is being compressed enough to affect the pressure reading.
Example Sentence 1
After correcting for instrument and position error, the pilot applied a compressibility correction before reading true airspeed off the flight computer.
Example Sentence 2
At 35,000 feet the flight manual listed a 4-knot compressibility error that had to be applied to the airspeed indicator reading.