What is the difference between IAS and Mach number, and when do pilots use each?
IAS, or indicated airspeed, is the speed shown on the airspeed indicator in knots and is used mainly at lower altitudes, during take-off, climb, approach and landing. Mach number is your speed as a fraction of the speed of sound and becomes the better reference higher up, especially in fast turboprops and jets.
IAS vs Mach: the simple difference
IAS tells us how fast the aircraft is moving through the air from the pitot-static system’s point of view. In day-to-day flying, it is the number pilots use for most handling limits because the wing, flaps and landing gear respond to aerodynamic pressure, not to groundspeed.
Mach number is different. It is a ratio: your true speed through the air divided by the local speed of sound. Mach 0.78 means 78% of the speed of sound in the air mass around you.
That distinction matters because the speed of sound changes mainly with air temperature. At high altitude the air is colder, so the speed of sound is lower. An aircraft can therefore reach a high Mach number without an especially high indicated airspeed.
| Measure | What it represents | How pilots usually see it | Most useful for |
|---|---|---|---|
| IAS | Pressure-based airspeed shown on the indicator | Knots | Take-off, climb, descent, approach, landing, flap and gear limits |
| Mach | Speed as a fraction of the local speed of sound | Mach 0.xx or 1.xx | High-altitude cruise and high-speed jet operations |
Why do pilots use IAS at lower altitude?
At lower altitude, IAS is the practical number because it tracks the aerodynamic loads that matter most to handling. Stall speed, rotation speed, climb speed, flap extension speed and gear extension speed are all tied to indicated airspeed.
If we are too slow in IAS terms, the wing may stall even if the true airspeed sounds impressive. If we are too fast in IAS terms, we may exceed structural limits for flaps, gear or the airframe itself.
That is why training aircraft, general aviation aeroplanes and most low-level operations are flown almost entirely by knots indicated. Even in airliners, departure and arrival speeds are normally briefed and flown in IAS.
- Take-off and climb: V-speeds and initial climb speeds are given in knots IAS.
- Approach and landing: final approach speed is flown in IAS.
- Aircraft limits: flap, slat and gear speeds are based on IAS.
- Low-level ATC restrictions: speed control is often issued in knots.
When do pilots switch from IAS to Mach?
In a fast jet, there is usually a crossover altitude. Below it, the aircraft is flown to a target IAS. Above it, the same climb or cruise is flown to a target Mach number. The exact crossover point depends on the aircraft, the selected speed schedule and the outside air temperature.
There is no single universal altitude where every aircraft changes over. A short-haul airliner climbing on one day might swap from a knots target to a Mach target at one flight level, and at a different one on another day because the temperature and speed schedule have changed.
In real operations and in flight simulators, the autoflight system often handles this automatically. The flight mode display may show a speed target in knots first, then change to a Mach target as the aircraft climbs. During descent, the reverse usually happens: the aircraft starts down at a Mach target and later transitions back to IAS.
What does a typical jet speed schedule look like?
- After take-off, the aircraft accelerates to climb speeds referenced in IAS.
- In the lower climb, pilots or the autoflight system hold a selected IAS target.
- At the crossover altitude, the reference changes from knots to Mach.
- In cruise, the aircraft stays on a planned Mach number, often for efficiency and buffet margin.
- In descent, pilots may hold Mach first, then transition back to IAS as the air gets denser.
Why Mach matters at high altitude
At high altitude, fast aircraft can run into compressibility effects long before IAS looks alarming. As airflow over parts of the wing accelerates, it can locally reach the speed of sound even while the aircraft as a whole is still below Mach 1. That is where drag rises sharply and handling can start to change.
This is why high-altitude jets have a published MMO, or maximum operating Mach number. Below the high flight levels, the limiting speed may instead be VMO, the maximum operating speed in knots. Many aircraft effectively fly to whichever limit is lower at the time: VMO down low, MMO up high.
On modern displays, the overspeed cue or barber pole often reflects this change automatically. As you climb, the limiting reference gradually shifts from a knots limit to a Mach limit.
For pilots, that means Mach is not just a cruise convenience. It is a protection against getting too close to high-speed aerodynamic problems such as Mach buffet, excessive drag rise and reduced margin to the aircraft’s certified limits.
IAS and Mach are related, but they are not interchangeable
This is where many people get tripped up. IAS, true airspeed and Mach may all change differently in the same climb.
If an aircraft climbs while holding a constant IAS, its true airspeed usually increases and its Mach number usually increases as well. The air is thinner higher up, so the aircraft must move faster through it in true-speed terms to produce the same indicated reading.
If the same aircraft climbs while holding a constant Mach, its IAS will usually fall. That is normal. The aircraft is keeping the same fraction of the speed of sound, while the indicated dynamic pressure drops as the air gets thinner.
This is exactly why a jet may climb initially at a fixed IAS, then later switch to a fixed Mach. The first phase protects low-speed handling and ordinary speed limits. The second phase protects high-speed aerodynamic margin.
Do pilots really fly by IAS, or by CAS, TAS and other airspeed types?
In the cockpit, the operational answer is usually IAS. That is the number shown to the pilot and the number used for most normal flying.
Strictly speaking, aircraft performance work can involve CAS (calibrated airspeed), EAS (equivalent airspeed) and TAS (true airspeed). Those are useful for engineering, performance calculations and some manual references. But for the question most people mean, pilots control the aeroplane in knots indicated down low and by Mach at altitude.
So if you hear a crew say, “climb at 290 knots, then Mach 0.78”, that is completely normal. They are not changing how fast the aircraft feels to the wing at random; they are changing to the speed reference that best matches the aerodynamic problem at that part of the flight.
When do pilots use IAS and when do they use Mach?
| Phase of flight | Usual reference | Why |
|---|---|---|
| Take-off | IAS | V-speeds, rotation and safety margins are based on indicated airspeed |
| Initial climb | IAS | Best for stall margin, clean-up and low-level speed control |
| Mid climb | IAS, then Mach | Transition occurs at the crossover altitude |
| Cruise at higher altitude | Mach | Better for efficiency and staying below MMO |
| High-altitude descent | Mach, then IAS | Reverse of climb as the air gets denser |
| Approach and landing | IAS | Approach speeds and aircraft configuration limits use indicated airspeed |
The short practical answer
Use IAS when the main concern is how the aircraft flies on the wing: take-off, climb, approach, landing and most speed limits. Use Mach when flying high and fast enough that compressibility and MMO matter more than raw indicated knots.
That is why pilots do not choose one or the other permanently. They use the reference that best matches the aeroplane’s real aerodynamic limits at that moment.