022 Instrumentation topic guide
IAS, CAS, EAS, TAS and Mach
Every airspeed value on the flight deck is the same physical quantity passed through a chain of corrections, and the exam expects you to know which correction sits between which pair of names. Indicated airspeed becomes calibrated airspeed once instrument and position error are removed, calibrated airspeed becomes equivalent airspeed once compressibility is accounted for, and equivalent airspeed becomes true airspeed once the actual air density is applied.
The chain matters because the corrections do not all switch on together. Some pairs are equal across huge parts of the flight envelope, while others only match at one specific condition, and true airspeed and Mach number then behave differently from each other as altitude and temperature change. Losing track of which correction is 'live' at a given point in a stem is where most marks in this area go missing.
The correction chain, step by step
Indicated airspeed is the raw cockpit reading, distorted by instrument manufacturing tolerances and by position error, the small pressure error caused by the airflow around the airframe at the static source. Removing those two errors, usually with a correction card, gives calibrated airspeed.
Calibrated airspeed still assumes the air behaves as an incompressible fluid, which is a good assumption at low speed but breaks down as speed and altitude rise. Correcting for that compressibility effect gives equivalent airspeed, a value that reflects the true dynamic pressure the airframe is experiencing.
Equivalent airspeed still expresses that dynamic pressure using the standard sea-level density. The final step divides out the actual, lower density found at altitude, and the result is true airspeed, the aircraft's real speed through the air mass.
Where the differences disappear
At low speed the compressibility correction is negligible, so calibrated airspeed and equivalent airspeed are effectively equal; this is why light aircraft performance material can often skip straight from CAS to TAS without mentioning EAS at all. Equivalent airspeed and true airspeed, however, are equal at only one specific density: the ISA sea-level value. Anywhere else, the density correction is doing real work, and the two values pull apart.
How TAS and Mach move during a climb
Hold calibrated airspeed constant through a climb and both true airspeed and Mach number increase, because falling air density means the same dynamic pressure now corresponds to a faster true speed, and falling temperature reduces the local speed of sound, so the same, growing true airspeed represents an even larger fraction of it.
Hold Mach number constant instead, the usual technique in the upper part of the climb and in cruise, and falling temperature works the other way: true airspeed decreases, because true airspeed equals Mach number multiplied by the local speed of sound, and the local speed of sound itself falls as the air gets colder.
Worked example
Worked example: constant CAS through a climb
Not quite. The correct answer is B.
Correct answer: B
- A. This gets the density effect on TAS right but the wrong direction for Mach: falling temperature lowers the local speed of sound, which pushes Mach up, not down, on top of a rising TAS.
- B. Correct: falling density makes a constant CAS correspond to a faster true speed, and falling temperature shrinks the local speed of sound, so the same, growing true airspeed represents a larger Mach number.
- C. This treats a constant CAS as if it meant a constant TAS, skipping the density correction that is the entire point of the CAS to TAS step.
- D. This is the constant-Mach case read backwards; holding CAS constant, not Mach, is what the stem actually describes.
Step by step
- Fix the constant quantity in the stem: calibrated airspeed, not Mach number.
- Apply the CAS to TAS step: air density falls through the climb, so the same dynamic pressure corresponds to an increasing true airspeed.
- Apply the TAS to Mach step: temperature falls through the climb, lowering the local speed of sound, so a rising true airspeed becomes an even larger fraction of that shrinking figure.
- Conclude that both true airspeed and Mach number rise, which is why aircraft flying a constant CAS climb schedule are switched to a constant Mach schedule before they reach a structural or aerodynamic Mach limit.
Common mistakes
Treating calibrated airspeed and true airspeed as interchangeable at any altitude
Limiting speeds such as manoeuvring or flap-limit speeds are set in CAS terms, while range and endurance calculations need TAS. Substituting one for the other at altitude silently corrupts whichever calculation follows.
Skipping the instrument and position error step
Some stems supply a correction card specifically to test whether you apply it. Reading the raw IAS as if it were already CAS throws away the one number the question was built around.
Assuming equivalent airspeed equals true airspeed away from ISA sea level
That equality holds at exactly one density. Anywhere else, skipping the final density correction understates true airspeed, and the error grows with altitude.
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Last reviewed July 2026