Airspeed Definitions: IAS, CAS, EAS, TAS and Mach

An airplane has one speed through the air, but the cockpit shows it in several forms. The airspeed indicator measures a pressure, not a speed, and its reading has to be corrected step by step before it gives the real speed through the air mass: indicated airspeed (IAS) becomes calibrated airspeed (CAS), then equivalent airspeed (EAS), then true airspeed (TAS). At high altitude a fifth measure, Mach number, becomes the limiting speed.

Each answers a different question. The wing and the structure respond to dynamic pressure, so stall speeds and flap limits are indicated or calibrated airspeeds, and ATC gives its speed instructions in IAS. Navigation, fuel planning and the flight plan need the true speed, and the high-speed limits of a jet depend on the speed of sound. How the instrument itself works is covered in airspeed indicator and Machmeter; this article follows the conversions and shows where each speed is used.

Why there are several airspeeds

The airspeed indicator, or the air data computer behind a speed tape, compares pitot pressure with static pressure. At low speed the difference is the dynamic pressure, ½ρV², which contains the air density ρ as well as the speed V. The same reading can therefore mean very different true speeds. In the International Standard Atmosphere (ISA) the air at FL200 has about half its sea-level density and at FL400 about a quarter, so the same dynamic pressure needs about 1.4 times the speed at FL200 and twice the speed at FL400.

Indicated airspeed

Indicated airspeed is the reading of the airspeed indicator, written KIAS when expressed in knots. The instrument shows the true speed only at ISA mean sea level density, 1.225 kg/m³. The FAA's definition in 14 CFR 1.1 specifies that the calibration reflects compressible flow in the standard atmosphere at sea level and that IAS is uncorrected for airspeed system errors.

IAS is the speed the pilot flies, because the airplane responds to it. At a given angle of attack a wing produces the same lift at the same dynamic pressure whatever the altitude, so a light airplane stalls, rotates and approaches at the same indicated airspeed at a sea-level airport and at a high one. What changes is the true airspeed and groundspeed behind the reading, which is why takeoff and landing rolls grow with density altitude.

Calibrated airspeed and position error

Calibrated airspeed is IAS corrected for instrument error and position error; older British texts call it rectified airspeed. Instrument error comes from the instrument itself: capsule tolerances, friction and backlash in the linkage. Position error, also called pressure error, arises because the static source, and to a lesser degree the pitot probe, sits in airflow that the airframe has disturbed (see pitot-static system). It varies with airspeed, angle of attack and configuration and is greatest at high angles of attack, at low speed with flap extended. A sideslip upsets it too, so a light airplane with a single static port can show a noticeable airspeed error in a forward slip.

Light airplane handbooks publish the combined correction as an airspeed calibration table or graph, sometimes with a separate one for the alternate static source, and a PPL navigation log starts by applying it to the planned IAS. Air data computers apply the correction automatically, from coefficients stored for the particular aircraft. For transport airplanes, CS 25.1323 and 14 CFR 25.1323 limit the installation error, excluding the indicator's own calibration error, to 3 percent or 5 kt, whichever is greater.

CAS is the speed of performance and operating manuals, and certification speeds are defined in it: under CS-25 the reference stall speed VSR, for example, is a calibrated airspeed. In the cruise the position error is small; it matters most at the low speeds of takeoff and landing.

Equivalent airspeed and compressibility

Air brought to rest in the pitot probe is compressed, so at speed the pitot senses more than ½ρV². The calibration allows for this compression only as it occurs at sea level in ISA. Higher up, the same dynamic pressure corresponds to a higher Mach number and more compression, so the instrument over-reads. Equivalent airspeed is CAS corrected for this compressibility error. The correction is always subtracted, and at ISA sea level EAS equals CAS.

The error is small at low speed. ATPL texts treat it as negligible below about 300 kt TAS, so for light airplanes CAS and EAS are taken as equal. It grows with speed and altitude and can exceed 20 kt close to the speed of sound. Air data computers apply it automatically; on paper it comes from a correction graph or the navigation computer.

EAS is the truest measure of dynamic pressure, which is why designers use it. Gust loads are specified in EAS: CS 25.341 and 14 CFR 25.341 use a reference gust velocity of 56 ft/s EAS at sea level at speeds between VB and the design cruising speed VC. An airplane of a given weight lifts off and stalls at the same EAS at any altitude until compressibility lowers its maximum lift coefficient. On a jet transport the 1 g stall speed in EAS starts to rise once the Mach number at the stall reaches about M 0.4, at around 30,000 ft.

True airspeed and density

True airspeed is the speed of the airplane relative to the undisturbed air mass, the only one of these speeds that is a real velocity. It is EAS corrected for density error: TAS = EAS ÷ √σ, where σ, the relative density, is the actual air density divided by the ISA sea-level value. With σ about 0.5 at FL200, TAS is about 1.4 times EAS; with σ about 0.25 at FL400, it is about twice EAS.

Whenever the air is less dense than ISA sea-level air, at height or on a warm day, the airspeed indicator under-reads TAS. The correction needs the pressure altitude and the static, or outside, air temperature, not the total air temperature, which includes ram heating (see air data computer and air temperature). On a flight computer the temperature is set against the pressure altitude and TAS is read opposite CAS; an air data computer works TAS out from Mach number and static air temperature.

The mental estimate adds about 2 percent of the CAS per 1,000 ft. It is good enough as a check below about 10,000 ft but errs slightly high: 100 kt CAS at 6,000 ft in ISA gives 112 kt by the rule and about 109 kt on the computer.

TAS is the speed of navigation. It enters the triangle of velocities, the flight plan and the fuel log, and it sets the geometry of a turn. At a given bank angle the turn radius grows with the square of TAS, so doubling TAS quadruples the radius. The bank angle for a rate-one turn of 3° per second is roughly 15 percent of the TAS, about 18° at 120 kt; ATPL texts use TAS ÷ 10 + 7.

Mach number

Mach number is TAS divided by the local speed of sound (LSS), and the LSS depends only on absolute temperature: about 38.95 × √T kt with T in kelvin. That gives about 661 kt at ISA mean sea level (288 K), 588 kt at FL300 (228 K) and 573 kt at and above the tropopause (216.5 K). An airplane flying at 480 kt TAS where the LSS is 600 kt is at M 0.80.

A Machmeter, or the air data computer, finds Mach number from the ratio of the pitot-static pressure difference to static pressure, so it needs no temperature input, while CAS depends on the pressure difference alone. At a given pressure altitude each CAS therefore corresponds to one Mach number whatever the temperature; temperature changes only the TAS that goes with them. That is why the crossover altitude, where a chosen climb IAS and Mach number coincide, is fixed by the speed pair alone: a 300 kt and M 0.78 schedule crosses near FL290, on a cold day or a hot one. Choosing a higher IAS lowers the crossover altitude.

The Machmeter reading corrected for instrument error is the indicated Mach number; corrected also for position error it is the true Mach number. On the North Atlantic the Mach number technique, which keeps successive aircraft spaced by assigning each a Mach number, uses true Mach number, and a jet's flight plan gives its cruising speed either as a true Mach number, M082 for M 0.82, or as a TAS in knots, such as N0450. See high-speed flight for what happens as Mach number rises.

Air France 447 shows why the measures must be kept apart. Cruising at FL350 at about M 0.80, the A330 was indicating about 275 kt. Yet the BEA found that the margin to the stall warning threshold was only about 1.5° of angle of attack, with one degree worth about 25 kt in cruise against about 5 kt at takeoff or on approach. When ice crystals most likely obstructed the pitot probes, the left displayed speed fell to about 60 kt; the stall warning, by design, stopped whenever all three measured airspeeds were below 60 kt.

Climbing and descending at constant CAS or Mach

Because CAS, TAS and Mach number respond differently to density and temperature, holding one constant makes the others change:

Jets therefore climb at an indicated airspeed and change to Mach at the crossover altitude, and descend the other way: at Mach until the IAS has risen to the planned value, for example M 0.78 then 300 kt, and at that IAS below. In a constant-Mach descent the IAS keeps rising, so a speed-mode or autothrottle failure can carry the airplane past VMO below the crossover altitude. Some US arrival charts carry a note for the change from Mach to IAS: the crew holds the cruise Mach in the descent until reaching the published speed in knots, then holds that speed.

Conversions and worked examples

Light airplane cruise. A navigation log plans 100 kt CAS at 7,000 ft pressure altitude in roughly standard conditions. The 2 percent rule gives 100 × 1.14 = 114 kt TAS; the flight computer, set with the forecast temperature, gives the exact figure, a few knots less. If the air at the cruising level turns out 10 °C warmer than forecast, the same IAS gives a higher TAS than planned.

Jet climb. Climbing from sea level to FL360 at 330 kt CAS in ISA, TAS rises from 330 to about 555 kt and Mach number from 0.50 to about 0.97, which is why a jet changes to Mach long before. Near the top of such a climb the compressibility correction exceeds 20 kt: the EAS is only about 304 kt.

Jet cruise. At FL380 in ISA the temperature is −56.5 °C (216.5 K), the LSS 38.95 × √216.5, about 573 kt, and M 0.85 a TAS of about 487 kt. On an ISA +10 day the temperature is −46.5 °C (226.5 K), the LSS about 586 kt, and the same M 0.85 gives about 498 kt. With a 60 kt headwind component the groundspeed is about 438 kt.

Airspeeds in the rules

• Speed limits and ATC: IAS. SERA and ICAO apply 250 kt IAS below 10,000 ft AMSL to all flights in airspace Classes D to G and to VFR flights in Class C, using FL100 where the transition altitude is lower; some states, France among them, extend it. The FAA's 14 CFR 91.117 applies 250 kt below 10,000 ft MSL to all aircraft, with 200 kt within 4 NM of a Class C or D primary airport at or below 2,500 ft AGL and beneath Class B airspace. ATC assigns speeds as IAS or Mach number, and the AIM expects pilots to hold an assigned speed within 10 kt or M 0.02. Holding speeds and the approach category speeds, including VAT, are indicated airspeeds.
• Flight plan and reports: TAS and true Mach number. Under SERA, as amended from 1 May 2025, ATC must be told if the sustained TAS or Mach number at cruising level differs from the flight plan by 10 kt TAS or M 0.02 or more; the FAA's AIM asks for a report when the average TAS varies by 5 percent or 10 kt, whichever is greater. In the United States, circling areas designed since late 2012 are sized for the TAS at the circling altitude.
• High altitude: Mach number. Above the altitude at which VMO and MMO coincide, the limit is MMO rather than VMO, and cruise is flown at a Mach number. On transport airplanes the overspeed warning must sound at the latest when the speed exceeds VMO + 6 kt or MMO + 0.01 (see structural and operating speed limits).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.