IAS, TAS, GS, Mach: What All These Airspeeds Actually Mean
Did you ever stare at your ND or PFD and wondered why there are four different speed readouts? You are not alone. Airspeed in aviation is confusing, because there is no single answer to the question “how fast are you going?” It depends on what you measure.
This guide explains each speed type, what your instruments actually show, and why it matters when you are flying in MSFS 2024.
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Why There Are So Many Airspeed Types
The core problem is that air is not a fixed medium. It changes density with altitude and temperature. A wing does not care how fast it is moving over the ground. It rather cares how many air molecules are hitting it per second. That quantity changes with altitude even if your actual speed does not. This is why aviation uses multiple speed definitions: each one answers a different question.
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IAS – Indicated Airspeed
What it is: The raw speed reading from your pitot-static system, uncorrected for anything.
What it tells you: How much aerodynamic pressure the aircraft is experiencing, basically how the wings and control surfaces are behaving.
Why it matters: Every aircraft limitation and performance figure, such as Vne, Vmo, flap speeds, approach speed, is given in IAS. Your stall speed at a given configuration is always the same IAS, regardless of altitude. This is why IAS is the speed you fly by during takeoff, approach, and landing.
In MSFS 2024: IAS is what your airspeed indicator shows by default in every aircraft. The speed tape on your PFD is always IAS.
The catch: IAS contains small errors from instrument position and installation. Correcting for those gives you CAS. Also do not forget to turn on pitot heat that measures IAS. If it is blocked by ice, your readings will be false.
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CAS – Calibrated Airspeed
What it is: IAS corrected for instrument and position errors.
Why it matters: At low speeds and high angles of attack, the pitot probe is not perfectly aligned with airflow, introducing errors. CAS corrects for this. In practice, the difference is small, often less than 2–3 knots in normal cruise, which is why most simmers never think about it.
In MSFS 2024: High-fidelity add-ons like the Fenix A320 model this distinction accurately. For default aircraft, treat your airspeed indicator as IAS.
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TAS – True Airspeed
What it is: Your actual speed through the air mass, corrected for altitude and temperature.
What it tells you: How fast you are moving through the air. This is useful for navigation, fuel planning, and time-distance calculations.
Why it matters: As you climb, air gets thinner. Your IAS drops relative to TAS, because fewer air molecules hit the pitot probe. At cruise altitude (FL350–FL390), TAS is typically 80–100 knots higher than IAS for an airliner. A rough rule of thumb: IAS reads approximately 2% lower than TAS per 1,000 ft of altitude — so at FL350, TAS will be roughly 70% higher than your indicated speed.
In MSFS 2024: TAS is shown on the ND alongside wind and GS. Your wind readout including direction and speed in the top corner of the ND is relative to TAS, not GS.
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GS – Ground Speed
What it is: Your actual speed over the ground.
What it tells you: How fast you are covering distance. This is used for ETA calculations and fuel burn over distance.
Why it matters: GS is TAS modified by the wind. Tailwind adds to GS, headwind reduces it. The relationship is simple: GS = TAS ± wind component (tailwind positive, headwind negative).
In MSFS 2024: GS is shown on the ND next to TAS. Your FMS uses GS for all ETA and fuel predictions. If the wind shown on your ND does not match the actual wind being applied by the sim, your GS and TAS will appear inconsistent. This is.a known MSFS 2024 live weather issue.
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Mach Number
What it is: Your speed expressed as a fraction of the local speed of sound.
Why it matters: At high altitudes, the speed of sound drops — because temperature decreases with altitude, and the speed of sound is a function of temperature, not pressure. Airliners switch from IAS to Mach as their primary speed reference at around FL280–FL300, because above that point Mach limits become the binding constraint. Fly too fast in Mach terms and you get compressibility buffet. Structural limits are expressed as Mmo, Maximum operating Mach.
In MSFS 2024: The autopilot in most airliners switches from IAS hold to Mach hold automatically during climb. Watch for your Mmo: it is the top of the barber pole on the speed tape.
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EAS – Equivalent Airspeed
What it is: CAS corrected for air compressibility at high speeds. Mainly relevant at high subsonic speeds and almost never referenced directly in the cockpit. You can safely ignore it in the sim unless you are reading performance manuals.
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The Number That Confuses Everyone
If GS is lower than TAS, you have a headwind component. If GS is higher than TAS, you have a tailwind. The wind display on your ND shows wind FROM a direction. A wind from 246° at 102 kt while flying a heading of 120° is almost directly in your face, not at your back.
The confusion usually comes from misreading the wind direction convention. In aviation, wind is always reported as the direction it is blowing from, not toward. A westerly wind (270°) pushes you east. If you are flying east, that is a tailwind.
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Speed Accuracy
MSFS 2024 models IAS, TAS, GS, and Mach reasonably well in most aircraft. The area where problems tend to appear is the relationship between the displayed wind and the actual wind being applied by the weather engine. If your GS/TAS delta does not match what the ND wind readout predicts, the most likely cause is a live weather data desync — the sim is applying one wind while displaying another. This tends to be worse near jetstream boundaries and during live weather updates mid-flight.
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Conclusion
IAS keeps you safe. TAS tells you how fast you are really flying. GS tells you how fast you are getting there. Mach keeps you structural. Once you know which question each speed answers, the cockpit makes a lot more sense — and so do the moments when the numbers seem to contradict each other.