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

The family of airspeeds describes the same motion through the air corrected in stages for the errors and physical effects that separate a raw instrument reading from the aircraft's actual speed. The sequence is defined in the FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) and the Instrument Flying Handbook (FAA-H-8083-15). Indicated airspeed (IAS) is the value read directly from the airspeed indicator, driven by the difference between pitot (ram) pressure and static pressure. It is uncorrected, but it is the airspeed pilots actually fly, because the aerodynamic forces on the wing — lift, stall, and structural loads — track dynamic pressure, which is essentially what the indicator senses. That is why V-speeds such as stall, rotation, and never-exceed are published as indicated values: at any altitude, the aircraft stalls at the same indicated airspeed even though its true speed differs.

Calibrated airspeed (CAS) is IAS corrected for position and instrument error — the small inaccuracies caused by where the static port sits on the airframe and by the instrument itself, which vary with configuration and angle of attack and are largest at low speed with flaps and gear extended. The correction is tabulated in the Pilot's Operating Handbook. Equivalent airspeed (EAS) is CAS corrected for compressibility, the effect of air being compressed at the pitot inlet at higher speeds; the compressibility correction is negligible at low speeds and low altitudes but becomes significant above roughly 200 knots and at high altitude, which is why EAS chiefly concerns turbine and high-performance aircraft. True airspeed (TAS) is EAS corrected for air density — that is, for pressure altitude and temperature. Because air thins with altitude, the airspeed indicator under-reads the true speed as the aircraft climbs: for a given indicated airspeed, TAS increases by a useful rule of thumb of about 2 percent per 1,000 feet of altitude. An aircraft indicating 150 knots at 10,000 feet is genuinely moving through the air at roughly 180 knots.

Mach number is the ratio of true airspeed to the local speed of sound. The speed of sound is not constant; it depends only on temperature, falling from about 661 knots in standard sea-level air to around 574 knots near the tropopause as the air grows colder. A given true airspeed therefore represents a higher Mach number the higher and colder the aircraft flies. At high altitude the two speeds are managed together: an aircraft climbing at a constant indicated airspeed eventually reaches a Mach number where compressibility effects — and ultimately the risk of exceeding the maximum operating Mach, MMO — govern the limit, so the crew transitions from holding an indicated-airspeed schedule to holding a Mach schedule. This crossover, and the narrow gap between low-speed buffet and high-speed Mach buffet at very high altitude known as the "coffin corner," is why Mach matters to jet operations and hardly at all to a piston trainer.

The practical division for a pilot is straightforward. Fly indicated airspeed for everything that concerns how the aircraft handles — approach speeds, stall margins, turbulence penetration, and structural limits — because those depend on dynamic pressure and stay tied to IAS regardless of altitude. Use true airspeed for navigation and flight planning, because TAS combined with the wind gives groundspeed, which determines how long the flight actually takes and how much fuel it burns. EAS and Mach enter the picture as speed and altitude rise. The definitions are identical under EASA and ICAO; the same IAS, CAS, EAS, TAS, and Mach relationships apply worldwide, with the only routine variation being local convention on units such as knots versus kilometers per hour.

For flight schools, the airspeed family is a ground-school cornerstone that students often find confusing because the differences are invisible at the speeds and altitudes of primary training. Early in the syllabus, IAS, CAS, and TAS are nearly the same number, so the distinctions can feel academic — until a cross-country flight plan produces a groundspeed and fuel burn that only make sense once the student computes TAS from pressure altitude and temperature. Instructors have to make the sequence concrete: why V-speeds are indicated, why the flight computer converts to true airspeed for navigation, and why the gap between indicated and true speed widens with altitude.

The topic also scales with the career track. A private student needs IAS, CAS, and TAS and the 2-percent-per-1,000-feet rule of thumb; a commercial or airline-track cadet must add EAS, Mach number, and the reasons a jet flies a Mach schedule at altitude. Schools running integrated ATPL or type-rating programs are expected to teach the high-altitude aerodynamics — compressibility, MMO, and coffin corner — to the depth airline employers assume, so a program that stops at TAS leaves cadets short for the theory exams and the simulator.

Aviatize's Training Management module tracks airspeed theory as graded curriculum items, so an instructor can see whether a student has genuinely mastered the IAS-to-TAS chain and the navigation use of true airspeed before signing off cross-country planning. The record shows where the concept was introduced and where it was assessed, rather than leaving mastery to assumption.

Aviatize's Ground Training & Checking module organizes the theoretical-knowledge syllabus so that the deeper high-altitude material — equivalent airspeed, Mach number, and Mach-limited operations — is delivered and examined at the right stage for commercial and type-rating cadets, keeping the ground program aligned with what each course and its examiners require.