IAS vs Mach: what's the difference and when is each used?

IAS is the speed shown on the airspeed indicator, normally in knots, and is used for take-off, landing, handling and configuration limits. Mach is speed divided by the local speed of sound, so fast aircraft use it in the upper climb and cruise, where compressibility and Mach limits govern.

In Aviation & Real-World Flying, the same logic applies in a simulator as in a real cockpit. IAS is not simply a slower form of Mach, nor can one be converted to the other without additional information about the aircraft and atmospheric pressure.

IAS vs Mach: what does each number mean?

Indicated airspeed (IAS) is the reading presented on the airspeed indicator or primary flight display. It comes from the pitot-static system and is normally shown in knots as KIAS.

Mach number is the aircraft's true speed divided by the local speed of sound. Mach 0.78 means 78% of the speed of sound in the surrounding air. It has no unit and is normally displayed as a decimal, such as M 0.78.

Strictly, equivalent airspeed (EAS) is the airspeed most directly related to dynamic pressure and aerodynamic load. IAS remains the practical cockpit reference because aircraft limitations, speed markings and operating procedures account for the relevant corrections. Calibrated airspeed (CAS) is IAS corrected for instrument and position error.

How do you convert IAS to Mach?

There is no fixed IAS-to-Mach conversion: the same indicated speed produces a higher Mach number as static pressure falls during a climb.

Dividing IAS by 661 knots is a common mistake. About 661 knots is the speed of sound under standard sea-level conditions, but IAS is not true airspeed and the local speed of sound changes with temperature.

1. Correct IAS to CAS using the aircraft's correction data if an accurate result is required. For a rough simulator estimate, IAS is often treated as CAS.
2. Obtain static pressure, normally from pressure altitude or the aircraft's air-data system. Geometric altitude alone is not enough.
3. Calculate Mach from pressure ratio, or read the Mach value already calculated by the aircraft. Modern air-data computers perform this continuously.

For subsonic flight, CAS can be converted through impact pressure using these relationships:

qc = p0 * ((1 + 0.2 * (CAS / a0)^2)^3.5 - 1)

M = sqrt(5 * ((1 + qc / p)^(2/7) - 1))

Here, qc is pitot impact pressure, p is local static pressure, p0 is standard sea-level pressure and a0 is the standard sea-level speed of sound, approximately 661.5 knots. CAS and a0 must use the same speed unit, while p and p0 must use the same pressure unit. This subsonic formula is not the correct pitot relationship for supersonic flight.

The following figures show how dramatically altitude changes the answer. They assume 250 KIAS is approximately 250 KCAS and use standard pressure levels:

These are illustrations, not target speeds. At FL400, 250 KIAS would exceed the Mach limit of many aircraft.

If true airspeed and outside-air temperature are available, Mach can instead be found from Mach = TAS / local speed of sound. In knots, the local speed of sound is approximately 38.94 * sqrt(temperature in kelvin). Temperature is needed for this TAS method; when Mach is calculated directly from pitot and static pressure, the relevant temperature effects are already represented by the pressure relationship.

Why do pilots use IAS for take-off and landing?

Pilots use IAS at lower levels because it provides the practical cockpit reference for lift, stall margin, handling and aerodynamic loading.

• Take-off: V-speeds, rotation and initial clean-up speeds are presented as indicated speeds.
• Approach and landing: target approach speed and stall margin are managed in IAS, with any required wind correction added according to the aircraft procedure.
• Flaps and landing gear: extension and operating limits are specified as indicated-speed limitations in normal cockpit use.
• Lower-level speed control: published restrictions and many ATC speed instructions are given in knots rather than Mach.

Groundspeed cannot replace IAS. A strong headwind may produce a low groundspeed while the aircraft has a safe approach IAS; a tailwind can produce an impressive groundspeed while the wing is still close to its stall angle of attack. Groundspeed is useful for navigation, timing and runway calculations, not for judging stall margin.

When do pilots switch from IAS to Mach?

Fast aircraft normally climb at a scheduled IAS or CAS until reaching the crossover altitude, then continue at a scheduled Mach number. The sequence is reversed during descent.

At a constant indicated climb speed, Mach rises as static pressure falls. Crossover occurs where the scheduled knot value and scheduled Mach value describe the same flight condition. A schedule such as 290 knots followed by Mach 0.78 is an example, not a universal target.

For a fixed CAS/Mach pair, crossover corresponds to a particular pressure altitude. Outside-air temperature changes the associated true airspeed and geometric altitude, but it does not independently move the pressure-based crossover point. Changing the selected knot or Mach target does move it.

Many piston aircraft and lower-flying turboprops never make this change because they do not operate high or fast enough for Mach to become the controlling reference.

Crossover altitude must not be confused with transition altitude. Crossover concerns the speed reference; transition altitude concerns the change between a local altimeter setting and standard pressure.

Why did the autopilot change from knots to Mach?

An automatic change from knots to Mach in the climb is usually intentional and follows the aircraft's programmed speed schedule.

Some autoflight systems perform the change automatically, while others require manual selection. During descent, the display normally changes back from Mach to knots near crossover. The precise behaviour depends on the aircraft and whether managed or selected speed is being used.

Check the flight mode annunciation or equivalent status display, not just the number in the speed window. A Mach target shown on the panel does not by itself prove that the autoflight system is actively controlling Mach; another pitch or thrust mode may be governing the aircraft.

Why does Mach matter at high altitude?

Mach matters at altitude because compressibility effects can become limiting even when the indicated speed appears modest.

Airflow accelerates over the wing, so parts of it can reach local sonic speed while the aircraft as a whole remains below Mach 1. The resulting shock waves can cause drag rise, buffet and changes in control response.

Fast aircraft therefore have both VMO, a maximum operating speed expressed in knots, and MMO, a maximum operating Mach number. VMO usually governs lower down; MMO becomes the active limit higher up. On many glass-cockpit speed tapes, the overspeed cue moves automatically as the controlling limit changes.

The precise limits are aircraft-specific. Our explanation of the A320's operating speed and altitude limits shows how VMO, MMO and maximum altitude fit together in a familiar airliner.

At very high altitude, the low-speed buffet boundary and high-speed Mach buffet boundary can move closer together. Holding an appropriate cruise Mach preserves the intended margin from the high-speed boundary while the aircraft's weight and altitude determine the low-speed margin.

Is Mach the same as true airspeed or groundspeed?

No. Mach, true airspeed and groundspeed describe different things and cannot be substituted for one another.

• True airspeed (TAS) is the aircraft's actual speed through the surrounding air mass.
• Mach is TAS divided by the local speed of sound, which varies mainly with air temperature.
• Groundspeed is speed across the earth and equals TAS adjusted for the wind component along the flight path.

During a constant-IAS climb, TAS and Mach generally increase while groundspeed also responds to changing wind. During a constant-Mach climb, IAS normally decreases as pressure falls. This is why a jet can cruise at Mach 0.78 while showing an IAS that would look unremarkable near sea level.

The practical rule is to use IAS when lift, handling, configuration or lower-level speed restrictions matter, and Mach when compressibility, buffet margin and MMO govern. Definitions of CAS, EAS, TAS and related cockpit terms are also covered in our real-world aviation answers for simmers.