The 5 Types of Airspeed: IAS, CAS, TAS, GS, and Mach Explained for Pilots

Nathan Hodell
Aug 27, 2025
10 min read

Updated: 5 days ago

There's no single "airspeed" in aviation. Pilots reference different airspeeds depending on context — what the airspeed indicator shows, what the aircraft is actually doing through the air, what the GPS shows over the ground, and at higher altitudes, what fraction of the speed of sound you're flying. Understanding the differences isn't just academic — using the wrong airspeed reference can affect everything from approach speed selection to fuel planning to whether ATC's groundspeed instructions make sense.

This post covers the five types of airspeed every pilot should know — indicated (IAS), calibrated (CAS), true (TAS), ground (GS), and Mach number — plus the airspeed indicator color arcs and V-speeds that put it all together.

Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >

The Pitot-Static System: Where Airspeed Comes From

Before getting into airspeed types, understanding the system that produces airspeed indications matters.

The pitot-static system has two pressure sources:

Pitot tube (ram air pressure):

Forward-facing tube
Captures air being rammed into it as the aircraft moves
Pressure is dynamic + static + impact pressure
Located on the wing, fuselage, or nose

Static port (static pressure):

Side-facing flush opening
Captures undisturbed atmospheric pressure
Located on the side of the fuselage (typically two — one each side)
Some aircraft use a static port within the pitot tube assembly

The airspeed indicator measures the difference:

Pitot pressure − static pressure = dynamic pressure
Dynamic pressure is converted to airspeed indication through a pressure-sensitive mechanism
The indicator shows airspeed in knots (or sometimes miles per hour in older aircraft)

System errors:

Position error: The static port position affects the pressure reading
Instrument error: Mechanical inaccuracies in the indicator itself
Compressibility error: At high speeds (Mach 0.4+), air compresses around the pitot tube, affecting accuracy
Density error: At altitude, lower air density reduces dynamic pressure for the same true airspeed

These errors are why the various airspeed types exist — corrections for each type of error.

Indicated Airspeed (IAS): What You Actually See

Indicated airspeed is exactly what the airspeed indicator displays.

Why it's the most-referenced airspeed:

Almost everything operationally important is referenced to IAS:

All V-speeds in your POH (Vne, Vno, Vfe, Va, Vs, Vso, Vy, Vx, etc.)
Stall speeds (clean and with flaps)
Maximum flap extension speed (Vfe)
Maximum gear extension speed (Vle)
Maneuvering speed (Va)
Best rate and angle of climb speeds (Vy and Vx)
Approach speeds
Pattern entry speeds

This is the practical airspeed pilots use moment to moment. When your CFI says "fly at 80 knots on final," they mean 80 KIAS (knots indicated airspeed).

Why IAS works at any altitude:

Despite all the corrections that produce TAS, IAS provides one critical advantage: stall speed remains constant in IAS regardless of altitude. A Cessna 172 stalls at approximately 50 KIAS at sea level and approximately 50 KIAS at 10,000 feet — even though the actual TAS at the higher altitude is significantly faster. This is because stall is a function of dynamic pressure (which IAS measures), not actual speed through the air.

The result: All performance-critical operations are referenced to IAS because IAS captures the dynamic pressure on the wings — what actually matters for lift, controllability, and structural loading.

Calibrated Airspeed (CAS): IAS Corrected

Calibrated airspeed is IAS corrected for instrument error and position error.

The corrections:

Instrument error: Manufacturing tolerances in the airspeed indicator itself
Position error: Inaccuracies caused by static port location and airflow patterns

Why CAS exists:

The airspeed indicator measures static pressure through the static port, but the static pressure measured isn't always exactly true atmospheric pressure. At certain speeds and configurations:

Slow flight at high angles of attack — airflow over the static port is disturbed
High-speed flight — compressibility effects
Banked attitudes — airflow direction changes
With flaps extended — flow patterns are disrupted

Each of these can cause a slight discrepancy between IAS and the true dynamic pressure.

Where to find the IAS-CAS correction:

Every POH has an IAS-CAS correction table or chart. It typically shows the correction at various speeds and configurations:

Cruise (clean configuration): typically 0-3 knots
Slow flight or with flaps: typically 3-8 knots
Near stall: can be 5-15+ knots

When CAS matters:

Performance calculations (more accurate than IAS for V-speed determination)
Flight testing (precise speed measurements)
Navigation calculations (more accurate baseline for TAS calculations)

For most pilots:

The IAS-CAS correction is small enough at typical operating speeds that pilots don't usually convert. The exception: any time you're flying near published V-speeds in unusual configurations, or in very precise flight planning.

True Airspeed (TAS): The Actual Speed Through the Air

True airspeed is the actual speed of the aircraft through the air mass it's flying through.

Why TAS differs from IAS:

The fundamental issue is air density. At sea level in standard conditions, the air is dense — each unit of dynamic pressure corresponds to a specific airspeed. At altitude, the air is less dense, but the airspeed indicator still measures dynamic pressure as if the air were sea-level dense. The result: at altitude, you're actually moving faster through the air than the indicator shows.

The relationship:

TAS at sea level ≈ IAS (close, with small CAS corrections)
TAS increases approximately 2% per 1,000 feet of altitude above sea level
The exact relationship depends on temperature

The pilot's rule of thumb:

For quick mental math:

Add approximately 2% to IAS for each 1,000 feet of altitude
Or: TAS ≈ IAS × (1 + 0.02 × altitude in thousands)

Examples:

At 5,000 feet, IAS 100 KIAS → TAS ≈ 110 KTAS
At 10,000 feet, IAS 110 KIAS → TAS ≈ 132 KTAS
At 15,000 feet, IAS 120 KIAS → TAS ≈ 156 KTAS
At 20,000 feet, IAS 100 KIAS → TAS ≈ 140 KTAS

This rule of thumb is approximate — temperature variations affect the actual conversion. The E6B flight computer or modern avionics provide exact calculations.

When TAS matters:

Flight planning: Calculating en route times based on cruise TAS
Fuel planning: Knowing actual airspeed for fuel calculations
Navigation: Wind correction calculations require TAS
Performance: POH cruise speeds are typically published in TAS
Filing flight plans: TAS is what you file in the flight plan

Avionics that show TAS:

Modern glass cockpit systems display TAS automatically based on air data computer inputs:

Pressure altitude
Outside air temperature
Indicated airspeed

The display updates continuously with current conditions. Some older aircraft have a separate TAS indicator or use the E6B flight computer for calculations.

Groundspeed (GS): Speed Over the Ground

Groundspeed is the actual speed of the aircraft over the ground, factoring in wind effects.

The relationship to TAS:

GS = TAS + tailwind component (or − headwind component)
A 100 KTAS aircraft with 20 knots of tailwind has a GS of 120 knots
A 100 KTAS aircraft with 20 knots of headwind has a GS of 80 knots
A 100 KTAS aircraft with a 90° crosswind has a GS very close to 100 knots (only slightly less)

Why groundspeed matters:

Time en route: Distance to destination ÷ groundspeed = time
Fuel planning: Combined with fuel burn, determines fuel required
ETE accuracy: GPS time-to-destination calculations
ATC instructions: Sometimes ATC issues groundspeed instructions
Mountain flying: Groundspeed and tailwind matter for terrain clearance

Sources of groundspeed:

GPS: Most accurate, available in modern aircraft
Flight planning calculations: TAS combined with forecast winds
DME: Time-and-distance calculations from radio navigation aids
VOR-based: Less common in modern operations

The wind triangle:

In flight planning, calculating groundspeed and required heading for a planned course involves:

Planned course (over the ground)
Forecast winds aloft (direction and speed)
Aircraft TAS

The wind triangle calculation gives:

Required heading to maintain course
Estimated groundspeed
Wind correction angle

This is why pilots use E6B flight computers, electronic equivalents in EFBs, or onboard computers that integrate GPS and TAS data.

Mach Number: Speed Relative to Sound

Mach number is the ratio of an aircraft's true airspeed to the speed of sound in the surrounding air.

Why Mach matters:

At higher altitudes and speeds, the speed of sound becomes critical because aircraft performance changes dramatically near it:

Mach 0.7-0.8: Compressibility effects begin to be significant
Mach 0.8-0.9: Critical Mach number for most jet transports
Mach 1.0: Speed of sound (sonic)
Mach > 1.0: Supersonic

For most piston GA aircraft, Mach number is irrelevant — these aircraft fly well below Mach 0.4. For jet aircraft and turboprops at high altitudes, Mach becomes essential for staying within structural limits.

Speed of sound varies with temperature:

Sea level standard temperature (15°C): ~661 knots
Higher temperature: faster sound speed
Lower temperature: slower sound speed
At 36,000 feet (typical cruise altitude): ~573 knots

Mach indicator:

Aircraft that operate near or above Mach 0.5 typically have a Mach indicator (often called a Machmeter) that displays Mach number based on:

True airspeed
Outside air temperature

For airliners:

Above approximately FL280, airliners typically fly Mach numbers rather than knots:

Cruise: typically Mach 0.78-0.85 for medium-range
Speed of sound at altitude: ~573 knots
A Mach 0.80 cruise = approximately 458 KTAS
This corresponds to approximately 280 KIAS at FL360 (typical cruise altitude)

For GA pilots:

Most piston GA aircraft don't have Machmeters. Cessna 172s, Cirrus SR22s, and similar aircraft cruise at Mach 0.2-0.3 — well below where Mach effects matter. Turboprops and jets in GA approach Mach territory (King Air at FL280 cruises at approximately Mach 0.4-0.5).

The Airspeed Indicator: Color Arcs and V-Speeds

The airspeed indicator displays critical information beyond just current speed. The color-coded arcs and markings tell pilots:

White arc (low end):

Bottom: Vso — stall speed in landing configuration (full flaps, gear down)
Top: Vfe — maximum flap extension speed
Operating range: with flaps extended

Green arc:

Bottom: Vs1 — stall speed in clean configuration
Top: Vno — maximum structural cruising speed
Normal operating range

Yellow arc (caution range):

Range: Vno to Vne
Operations only in smooth air
Avoid during turbulence

Red line:

At Vne — never exceed speed
Structural damage possible if exceeded

Other markings:

Vmo (Maximum operating limit speed) — on some aircraft
Vmc (Minimum control speed for multi-engine aircraft) — red radial line
Vyse, Vmca — various V-speeds depending on aircraft type

Common V-speeds:

V-speed	Meaning
Vso	Stall speed, landing configuration
Vs1	Stall speed, clean configuration
Vfe	Maximum flap extension speed
Vle	Maximum gear extended speed
Vlo	Maximum gear operating speed
Va	Maneuvering speed
Vno	Maximum structural cruising speed
Vne	Never exceed speed
Vy	Best rate of climb
Vx	Best angle of climb
Vyse	Best single-engine rate of climb (multi-engine)
Vmc	Minimum control speed (multi-engine)

Important: All V-speeds are in IAS (or sometimes CAS).

Putting It All Together: A Real Flight Example

Let's trace through the airspeed types during a flight:

Departure: Salt Lake City (KSLC), elevation 4,227 feet

On takeoff at 70 KIAS:

IAS: 70 knots (what the indicator shows)
CAS: ≈ 71 knots (small position error correction)
TAS: ≈ 78 KTAS (CAS + altitude correction)
GS: ≈ 78 + tailwind/− headwind component

Climbing to 10,000 feet, IAS 120 knots:

IAS: 120 knots
CAS: ≈ 121 knots
TAS: ≈ 145 KTAS (about 2% per 1,000 feet × 6 = 12% increase)
GS: depending on wind, could be 165 (tailwind) to 125 (headwind)

Cruise at 10,000 feet, IAS 105 knots:

IAS: 105 knots
CAS: 105 knots (clean configuration, minimal correction)
TAS: ≈ 127 KTAS
GS: 127 + wind component

Descending into Boise (KBOI), elevation 2,871 feet, IAS 100:

IAS: 100 knots
CAS: 100 knots
TAS: ≈ 105 KTAS at lower altitude
GS: depending on approach winds

Pattern altitude (3,800 feet), IAS 80:

IAS: 80 knots
CAS: 81 knots
TAS: ≈ 86 KTAS
GS: depending on pattern winds

The takeaway: throughout the flight, IAS is what you reference for safe flying, but TAS and GS matter for navigation and timing.

Common Pitfalls and Misconceptions

"At cruise, IAS should match my POH cruise speed." The POH typically lists cruise speed in TAS for marketing purposes. Compare your IAS to the POH expected IAS at your altitude, then verify TAS matches the listed cruise.
"Higher airspeed always means faster groundspeed." Not in headwinds. A 120 TAS aircraft with 60 knots of headwind has 60 GS — slower than walking pace through the air, but only 60 knots over the ground.
"My GPS shows 95 knots, the airspeed shows 110. Something's wrong." Probably nothing's wrong — you have a 15-knot headwind. GPS shows GS, airspeed shows IAS. They're different things.
"The airspeed indicator broke; I'll just use GPS." Not safe — GPS provides groundspeed, not the airspeed needed for stall, V-speed, or structural protection. If the airspeed indicator fails, treat it as an emergency. There are POH procedures for this.
"Approach speed at high-altitude airports should be the same as at sea level." True for IAS — approach speed is the same KIAS regardless of airport elevation. But TAS and GS will be higher. Plan for longer ground roll because of higher TAS.

On the Written Test and Checkride

Airspeed types appear consistently on tests. The most commonly tested topics:

The four/five airspeed types and their definitions
IAS vs. TAS at altitude
Why stall speeds are referenced to IAS
Calculating TAS from IAS (rule of thumb)
The relationship of CAS to IAS
Groundspeed = TAS + wind component

Quick conversions:

IAS to CAS: Use POH correction table (typically small)
CAS to TAS: Add ~2% per 1,000 feet altitude
TAS to GS: TAS ± wind component
TAS to Mach: TAS ÷ speed of sound at altitude

V-speeds (always in IAS):

Vso/Vs1 — Stall speeds
Vfe — Maximum flap extension
Vle/Vlo — Maximum gear speeds
Va — Maneuvering speed
Vno — Maximum cruising speed
Vne — Never exceed
Vy — Best rate of climb
Vx — Best angle of climb

Why each type matters:

IAS: Aircraft limits and operations
CAS: Precision performance work
TAS: Flight planning and navigation
GS: Time and fuel
Mach: High-altitude/high-speed flight

Memory aids:

Stall speed is in IAS — reliable at any altitude
Cruise speed in POH is usually TAS — for marketing/comparison
Headwinds reduce GS, tailwinds increase it
TAS increases with altitude; IAS doesn't change much

Wind triangle:

Course (over ground) + wind = heading + GS
Required for accurate flight planning
Calculated with E6B, EFB, or onboard computer

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Author: Nathan Hodell

CFI, CFII, MEI, ATP, Creator and CEO

Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.