Calibrated airspeed (CAS) is the speed shown on your airspeed indicator after it has been corrected for two known sources of error: position error and instrument error. The FAA defines it formally as “the indicated airspeed of an aircraft, corrected for position and instrument error,” and notes that CAS equals true airspeed under standard atmospheric conditions at sea level. In practice, CAS is the most operationally important airspeed for pilots because it directly reflects the aerodynamic forces acting on the airplane, regardless of altitude or temperature.
How CAS Fits Into the Airspeed Chain
Aircraft don’t measure their speed the way a car does. Instead, a pitot tube captures the pressure of oncoming air while static ports sense the surrounding atmospheric pressure. The difference between those two pressures corresponds to the airplane’s speed through the air. But this raw measurement picks up errors along the way, so aviation uses a chain of increasingly refined airspeed values:
- Indicated airspeed (IAS) is the raw number displayed on the airspeed indicator. It contains all the instrument’s built-in errors.
- Calibrated airspeed (CAS) is IAS with position and instrument errors removed.
- Equivalent airspeed (EAS) is CAS with an additional correction for air compressibility at higher speeds.
- True airspeed (TAS) is EAS adjusted for the actual air density at your altitude, giving you the real speed of the airplane through the air mass.
Each step peels away another layer of measurement error. CAS sits near the beginning of that chain, but it’s the version pilots rely on most in the cockpit because it represents something physically meaningful: the dynamic pressure on the wings and control surfaces.
What Errors CAS Corrects
The two errors that separate IAS from CAS are position error and instrument error.
Position error (sometimes called installation error) comes from the physical placement of the pitot tube and static ports on the airframe. As the airplane changes angle of attack, speed, or configuration (flaps extended, gear down), the airflow around the fuselage distorts the pressure readings at the static ports. A static port mounted on the side of the fuselage, for instance, may sense slightly higher or lower pressure than the true undisturbed atmosphere depending on the flight condition. This is typically the larger of the two errors.
Instrument error is mechanical. The airspeed indicator uses a small, pressure-sensitive diaphragm and gearing to move a needle. Manufacturing tolerances, wear, and temperature changes in the instrument itself introduce small inaccuracies that vary across the speed range. These are sometimes called scale errors.
Aircraft manufacturers measure both errors during flight testing and publish correction tables or charts in the pilot’s operating handbook. Some charts show the correction as a simple table: at a given IAS and flap setting, add or subtract a few knots to get CAS. In many light aircraft at cruise speeds, the difference between IAS and CAS is only 1 to 3 knots, but it can be larger at low speeds or with flaps extended.
Why Pilots Use CAS for Critical Speeds
Nearly all of an airplane’s published performance speeds, known as V-speeds, are expressed in calibrated airspeed. Stall speed, rotation speed, approach speed, and maximum flap-extension speed are all CAS values. The reason is straightforward: CAS describes the dynamic pressure acting on the airplane’s surfaces regardless of altitude, temperature, or wind. At a given weight, an airplane will stall, rotate for takeoff, and fly a stable approach at roughly the same CAS whether it’s at sea level or at a 7,000-foot mountain airport, even though the true airspeed and groundspeed at the higher field will be noticeably faster.
This consistency makes CAS the natural language for aircraft performance. A pilot who knows the airplane stalls at 48 knots CAS can rely on that number at any altitude. True airspeed, by contrast, changes with altitude even when the aerodynamic situation is identical, making it unreliable as a reference for aircraft control.
CAS vs. EAS: When Compressibility Matters
At lower speeds and altitudes, CAS and equivalent airspeed are essentially the same number. Below about 200 knots CAS and under 10,000 feet, the difference is negligible. But as speed or altitude increases, air compressibility starts to affect the pressure readings. Air piling up in front of the pitot tube gets compressed, producing a slightly inflated pressure reading that makes the airspeed appear higher than the true dynamic pressure warrants.
This compressibility effect becomes significant above about Mach 0.3. To remove it, CAS is corrected into equivalent airspeed using thermodynamic relationships between total and static pressure. For most general aviation flying, which happens at relatively low speeds and altitudes, this correction is small enough to ignore. For jets cruising at high altitude and high Mach numbers, the gap between CAS and EAS can be meaningful, and flight crews working with performance data need to account for it.
How Modern Cockpits Calculate CAS
In older aircraft, the airspeed indicator is a purely mechanical instrument. The pilot applies the position and instrument error corrections mentally or by referencing a table. The cockpit gauge shows IAS, and the pilot knows the published corrections for the current configuration.
Modern aircraft use an air data computer (ADC) that takes raw pitot and static pressure inputs and applies all necessary corrections digitally. The ADC uses stored calibration data and algorithms to compute CAS, true airspeed, Mach number, and pressure altitude, then feeds those values to the glass cockpit displays and to the flight control system. This removes the manual step and reduces the chance of a pilot misapplying a correction, though the underlying physics is identical.
CAS vs. TAS: Different Jobs
CAS and true airspeed answer different questions. CAS tells you how the airplane is behaving aerodynamically. TAS tells you how fast you’re actually moving through the air mass, which matters for navigation, fuel planning, and flight plan timing. True airspeed is not directly measurable. It has to be calculated from equivalent airspeed using the ratio of actual air density to sea-level standard density. At sea level on a standard day, CAS, EAS, and TAS are all equal. As you climb, TAS increases relative to CAS because the air is thinner, even though the aerodynamic forces on the airplane (represented by CAS) haven’t changed.
A practical example: an airplane flying at 120 knots CAS at 10,000 feet on a standard day has a true airspeed of roughly 138 knots. The wings “feel” 120 knots of dynamic pressure, but the airplane is covering ground (in still air) at 138 knots. Both numbers are useful, just for different purposes.