Airspeed
Use the Indicated Airspeed (IAS) calculator below to convert Indicated Airspeed (IAS) to Calibrated Airspeed (CAS), apply position error corrections, calculate weight-corrected maneuvering speed (Va) and stall speed, and compare your speed with published aircraft V-speed limits. The calculator also includes a complete V-speed reference, an airspeed indicator colour arc guide, and a stall speed at weight calculator.
Enter your IAS and the position error correction from your POH to get the Calibrated Airspeed (CAS). Position error varies with speed and configuration — use the correct table column for your flap setting.
Usage Guide
To convert Indicated Airspeed (IAS) to Calibrated Airspeed (CAS) using position error correction, follow the steps below:
To check your current Indicated Airspeed (IAS) against aircraft V-speed limits, follow the steps below:
To calculate weight-corrected stall speed (Vs1) and maneuvering speed (Va) for your actual aircraft weight, follow the steps below:
Definition
Indicated Airspeed (IAS) is the airspeed displayed directly on the aircraft's airspeed indicator using pressure measured by the pitot-static system.
IAS represents the aircraft's dynamic pressure rather than its actual speed through the air or over the ground. It is the primary airspeed reference used by pilots during all phases of flight because aerodynamic performance, stall behaviour, and structural loads depend on dynamic pressure.
The airspeed indicator calculates Indicated Airspeed (IAS) by comparing ram air pressure from the pitot tube with static air pressure from the static port. The instrument displays the resulting speed in knots (kt), miles per hour (mph), or kilometres per hour (km/h), depending on the aircraft.
IAS does not account for instrument error, position error, compressibility effects, air density, altitude, or temperature. Pilots apply additional corrections to obtain Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and True Airspeed (TAS) when greater accuracy is required for aircraft performance or navigation.
Because Indicated Airspeed (IAS) directly reflects the aerodynamic forces acting on the aircraft, manufacturers publish stall speeds, maneuvering speed (Va), flap operating speed (Vfe), maximum structural cruising speed (Vno), and never-exceed speed (Vne) using IAS.
Calculation
Indicated Airspeed (IAS) is not calculated manually because the aircraft's airspeed indicator continuously measures and displays it using the pitot-static system.
The pitot tube measures total pressure created by the aircraft's forward motion, while the static port measures ambient atmospheric pressure. The airspeed indicator compares these two pressures to determine the aircraft's dynamic pressure and converts the result into Indicated Airspeed (IAS).
The relationship between dynamic pressure and Indicated Airspeed (IAS) is based on the following equation:
q = Pt − Ps
Where:
The airspeed indicator is calibrated assuming International Standard Atmosphere (ISA) sea-level air density, which is why Indicated Airspeed (IAS) diverges from True Airspeed (TAS) at higher altitudes where actual air density is lower. Pilots normally read Indicated Airspeed (IAS) directly from the instrument rather than calculating it during flight.
If greater accuracy is required, Indicated Airspeed (IAS) becomes the starting point for additional airspeed corrections. Pilots first correct IAS for instrument and position errors to obtain Calibrated Airspeed (CAS). Further corrections for compressibility and air density produce Equivalent Airspeed (EAS) and True Airspeed (TAS) when needed for aircraft performance or navigation.
Why Use IAS
Pilots use Indicated Airspeed (IAS) because it directly represents the aerodynamic forces acting on the aircraft through dynamic pressure.
The airspeed indicator provides Indicated Airspeed (IAS) in real time using the pitot-static system, giving immediate feedback during all phases of flight. This makes Indicated Airspeed (IAS) the most reliable reference for controlling the aircraft's behaviour in the air.
Safe operating limits such as stall speed, maneuvering speed (Va), flap operating speed (Vfe), maximum structural cruising speed (Vno), and never-exceed speed (Vne) are all defined in Indicated Airspeed (IAS). These limits ensure structural protection and predictable handling characteristics regardless of altitude or temperature.
During takeoff and landing, Indicated Airspeed (IAS) provides a direct indication of lift generation because aerodynamic lift depends on dynamic pressure rather than true speed or groundspeed. Maintaining the correct Indicated Airspeed (IAS) helps prevent aerodynamic stall and ensures stable approach profiles.
Climb, cruise, and descent performance also rely on Indicated Airspeed (IAS) because it remains consistent across changing atmospheric conditions. Unlike True Airspeed (TAS), Indicated Airspeed (IAS) does not vary with altitude or temperature, making it a stable reference for aircraft control.
For flight control purposes, Indicated Airspeed (IAS) is preferred over True Airspeed (TAS) because it reflects the actual aerodynamic loading on the aircraft, which determines how the aircraft responds to control inputs.
Four Airspeed Types
Indicated Airspeed (IAS), Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and True Airspeed (TAS) describe the same aircraft speed after successive aerodynamic and atmospheric corrections.
Indicated Airspeed (IAS) is the speed displayed directly on the aircraft's airspeed indicator. It is measured by the pitot-static system without corrections for instrument error, position error, compressibility, or air density. Pilots use Indicated Airspeed (IAS) for aircraft control, structural limits, and published V-speeds.
Calibrated Airspeed (CAS) is Indicated Airspeed (IAS) corrected for instrument and position errors. Calibrated Airspeed (CAS) provides a more accurate representation of the aircraft's dynamic pressure and is used in aircraft performance charts and operating procedures. For most light aircraft in normal cruise, Indicated Airspeed (IAS) and Calibrated Airspeed (CAS) differ only slightly.
Equivalent Airspeed (EAS) is Calibrated Airspeed (CAS) corrected for compressibility effects caused by higher airspeeds. Equivalent Airspeed (EAS) represents the airspeed that produces the same aerodynamic forces at sea-level standard conditions. Compressibility corrections become important primarily in high-performance aircraft operating at higher speeds.
True Airspeed (TAS) is Equivalent Airspeed (EAS) corrected for air density. True Airspeed (TAS) represents the aircraft's actual speed through the surrounding air mass and is used for navigation, wind triangle calculations, groundspeed, Estimated Time En Route (ETE), and fuel planning.
The four airspeed types build on one another through a sequence of corrections:
IAS → correct instrument and position errors → CAS
CAS → correct compressibility → EAS
EAS → correct for air density → TAS
Each correction removes a specific source of error or atmospheric effect. Indicated Airspeed (IAS) is used for flying the aircraft safely, while True Airspeed (TAS) is used for navigation and flight planning. Calibrated Airspeed (CAS) and Equivalent Airspeed (EAS) provide the intermediate corrections required to transform the instrument reading into the aircraft's actual speed through the air.
V-Speeds
Aircraft V-speeds are standardized reference airspeeds that define structural limits, performance targets, and operational safety margins. V-speeds are published in the Pilot Operating Handbook (POH) and expressed in Indicated Airspeed (IAS). Each V-speed corresponds to a specific aerodynamic or structural condition that applies across all altitudes and temperatures.
V-speeds are defined by the Federal Aviation Administration (FAA) in 14 CFR Part 1 and aircraft certification standards in 14 CFR Part 23 and Part 25. Manufacturer-specific speeds that do not appear in these regulations are not official FAA-defined V-speeds, even if they use the V-speed format.
Vs is the stall speed or minimum steady flight speed at which the aircraft is controllable in the clean configuration (flaps retracted, gear up). It represents the lowest Indicated Airspeed (IAS) at which aerodynamic lift can support the aircraft's weight. All other stall-related V-speeds are derived from Vs.
Vs0 is the stall speed with full flaps extended and landing gear down. It is always lower than Vs1 because full flap extension increases lift coefficient and reduces the speed at which the wing stalls. Vs0 appears as the lower end of the white arc on the Airspeed Indicator (ASI). Approach and landing speeds are calculated relative to Vs0.
Vs1 is the stall speed in a specified configuration, typically the clean configuration at maximum gross weight with power off. It marks the lower end of the green arc on the ASI and defines the minimum speed for safe flight in the cruise configuration. Maneuvering speed (Va) and other performance speeds are calculated as multiples of Vs1.
VR is the speed at which the pilot applies back pressure to rotate the nose upward during the takeoff roll. VR must be at or above Vs1 to ensure the aircraft has sufficient aerodynamic lift to sustain flight after liftoff. In most general aviation aircraft, VR is a fixed value. In commercial aircraft, VR varies with weight, configuration, and environmental conditions.
Vx is the airspeed that produces the greatest altitude gain over the shortest horizontal distance. It is used during obstacle departure when maximum climb angle is required to clear terrain or obstacles after takeoff. Vx is lower than Vy. Operating below Vx reduces angle of climb; operating above it reduces the angle by shifting toward Vy performance.
Vy is the airspeed that produces the greatest altitude gain per unit of time. It is the standard climb speed used after obstacle clearance to reach cruise altitude in the minimum time. Vy is higher than Vx. At higher altitudes, Vx and Vy converge and eventually meet at the aircraft's absolute ceiling.
Va is the maximum speed at which full or abrupt deflection of a single flight control will not overstress the airframe. At or below Va, the wing will stall before structural loads reach the design limit. Above Va, full control deflection can generate aerodynamic forces that exceed structural design limits and cause damage. Va decreases as aircraft weight decreases because the wing stalls at a lower airspeed when lift requirements are reduced.
Vfe is the maximum speed at which the aircraft may be flown with flaps extended. Exceeding Vfe risks structural damage to the flap system, hinge mechanisms, and wing structure. Many aircraft have different Vfe values for different flap settings — a higher speed limit for partial flap and a lower limit for full flap. Vfe marks the upper end of the white arc on the ASI.
Vno is the maximum speed for normal operations in smooth air. Flight above Vno should only be conducted in calm conditions and with caution. Above Vno, the aircraft may experience structural damage from gusts or turbulence because gust loads can push total aerodynamic force beyond design limits. Vno marks the upper end of the green arc and the lower end of the yellow arc on the ASI.
Vne is the absolute maximum speed the aircraft may fly under any circumstances. Above Vne, structural failure, flutter, or loss of control may occur. Vne is displayed as a red radial line on the ASI. It is set at approximately 90% of the demonstrated structural dive speed (Vd) to provide a safety margin. Vne must never be exceeded regardless of flight conditions.
Vle is the maximum speed at which the aircraft may be flown with the landing gear extended in the down and locked position. Exceeding Vle risks structural damage to the gear doors, struts, and attachment points. Vle applies to retractable gear aircraft only and is typically higher than Vlo (the maximum speed for operating the gear).
V1 is the speed by which the takeoff decision must be made. Above V1, the pilot is committed to completing the takeoff even following an engine failure. Below V1, the pilot can safely reject the takeoff and stop within the remaining runway. V1 applies primarily to multi-engine transport category aircraft and is a critical element of commercial takeoff performance calculations.
V2 is the minimum safe airspeed that must be achieved by 35 ft above the runway after becoming airborne. It ensures adequate climb performance with one engine inoperative (OEI) in twin-engine or multi-engine aircraft. V2 is always higher than VR and provides a defined safety margin above the single-engine stall speed.
Vmc is the minimum speed at which directional control can be maintained following failure of the critical engine with the remaining engine at maximum continuous power. Below Vmc with one engine inoperative, the aircraft cannot be controlled with rudder alone. Vmc is marked as a red radial line on the ASI of multi-engine aircraft and must never be approached at low altitude.
The table below lists the most common aircraft V-speeds with their official names and definitions.
V-Speed Basis
V-speeds are based on Indicated Airspeed (IAS) because IAS directly represents the aerodynamic loads acting on the aircraft.
Indicated Airspeed (IAS) reflects dynamic pressure measured by the pitot-static system. Lift, drag, and structural stress depend on dynamic pressure, not true airspeed or groundspeed. This makes Indicated Airspeed (IAS) the correct reference for defining safe operating limits.
V-speeds such as Vne, Vno, Va, Vfe, Vs, and Vso use Indicated Airspeed (IAS) to ensure consistent aircraft behaviour across all altitudes and temperatures. The same Indicated Airspeed (IAS) produces the same aerodynamic force regardless of air density.
Using Indicated Airspeed (IAS) removes the need to correct for altitude, temperature, or density during flight operations. This allows pilots to maintain structural limits and stall margins using direct instrument readings.
Indicated Airspeed (IAS) also ensures immediate cockpit usability. The airspeed indicator displays Indicated Airspeed (IAS) in real time, which allows pilots to respond instantly during critical phases of flight such as takeoff, approach, and turbulence.
Because Indicated Airspeed (IAS) is tied to aerodynamic loading, it provides a universal reference for certification standards, flight testing, and Pilot Operating Handbook (POH) performance data.
Stall Speed
Stall speed is published in Indicated Airspeed (IAS) because IAS directly represents the aerodynamic conditions that determine when a wing reaches its critical angle of attack.
Stall occurs when lift decreases due to airflow separation over the wing. This condition depends on dynamic pressure rather than true airspeed or groundspeed. Indicated Airspeed (IAS) measures this dynamic pressure using the pitot-static system, which makes it the correct reference for stall behaviour.
The same Indicated Airspeed (IAS) produces the same stall condition regardless of altitude, temperature, or air density. As altitude increases, True Airspeed (TAS) increases for the same Indicated Airspeed (IAS), but the stall still occurs at the same IAS value because aerodynamic loading remains unchanged.
Aircraft manufacturers use Indicated Airspeed (IAS) for stall speed (Vs and Vso) to ensure pilots can identify stall conditions directly from the airspeed indicator. This allows immediate recognition of low-speed aerodynamic risk without requiring corrections for atmospheric conditions.
Indicated Airspeed (IAS) provides a consistent safety reference because it links directly to lift generation. This ensures that stall margins remain predictable across all phases of flight and all operating environments.
Structural Limits
Aircraft structural limits are defined in Indicated Airspeed (IAS) because IAS directly represents the aerodynamic loads acting on the airframe.
Exceeding structural limits can cause permanent deformation of the airframe, failure of control surfaces, flutter, or catastrophic structural separation. Each structural speed limit exists to prevent a specific failure mode. Vne protects against flutter and structural divergence — above Vne, oscillations in the airframe can amplify rather than dampen and may result in loss of the aircraft. Vno protects against gust loading — above Vno, atmospheric turbulence can generate forces that exceed the aircraft's design load factor even without pilot input.
Va (maneuvering speed) provides structural protection through a different mechanism. At or below Va, a full or abrupt deflection of a single flight control will cause the wing to stall before aerodynamic loads can reach the structural design limit. This means the aircraft absorbs the input through a stall rather than through structural damage. Above Va, the same control input generates lift forces that exceed the load limit before the wing stalls, which risks permanent structural damage or failure.
The yellow arc on the Airspeed Indicator (ASI) spans from Vno to Vne and defines the caution range. Flight in the yellow arc is permitted only in smooth air and with caution, because any gust or turbulence in this speed range can generate loads beyond the certified design limit. The yellow arc does not mean the aircraft will fail immediately — it means the available structural margin against gust loads has been reduced to zero or less.
Indicated Airspeed (IAS) provides a direct operational reference for avoiding overstress during turbulence, high-speed descent, and abrupt control inputs. Pilots use IAS to ensure the aircraft remains within certified structural limits at all times.
System Errors
Indicated Airspeed (IAS) is affected by pitot-static system errors because the system measures pressure differences that can be distorted by airflow conditions and instrument limitations.
The airspeed indicator calculates Indicated Airspeed (IAS) from the difference between pitot (total) pressure and static pressure. Errors occur when these pressure readings do not accurately represent true atmospheric conditions around the aircraft.
Position error affects Indicated Airspeed (IAS) when disturbed airflow around the airframe changes the pressure reaching the pitot tube or static port. This error varies with aircraft configuration, angle of attack, flap position, and airspeed, and is corrected using Pilot Operating Handbook (POH) calibration data.
Instrument error affects Indicated Airspeed (IAS) when the airspeed indicator itself has mechanical or calibration inaccuracies. This error is generally small in modern aircraft but can still affect precision at specific speed ranges.
Blockage of the pitot tube or static port produces significant Indicated Airspeed (IAS) errors. A blocked pitot tube can freeze Indicated Airspeed (IAS) at a constant value, while a blocked static port can cause Indicated Airspeed (IAS) to behave incorrectly during climbs and descents.
Despite these limitations, Indicated Airspeed (IAS) remains the primary reference for aircraft control because it directly reflects dynamic pressure, which determines aerodynamic performance and structural loading.
Altitude Effect
Altitude does not directly affect Indicated Airspeed (IAS) because IAS is based on dynamic pressure measured by the pitot-static system.
The airspeed indicator calculates Indicated Airspeed (IAS) from the difference between total pressure and static pressure. This pressure difference depends on aerodynamic forces on the aircraft, not on altitude or air density directly.
At higher altitude, air density decreases, which changes True Airspeed (TAS) for the same Indicated Airspeed (IAS). The aircraft must travel faster through thinner air to generate the same dynamic pressure required for a given Indicated Airspeed (IAS).
Indicated Airspeed (IAS) remains constant across altitude for the same aerodynamic condition. Stall speed, maneuvering speed (Va), and structural limits remain unchanged in Indicated Airspeed (IAS) regardless of height or temperature.
Altitude affects True Airspeed (TAS) and groundspeed, but Indicated Airspeed (IAS) remains a direct measure of aerodynamic loading. This makes Indicated Airspeed (IAS) a stable reference for aircraft control in all flight phases.
Temperature Effect
Temperature does not directly affect Indicated Airspeed (IAS) because IAS is determined by dynamic pressure measured by the pitot-static system.
The airspeed indicator compares total pressure from the pitot tube with static pressure from the static port. This pressure difference determines Indicated Airspeed (IAS) and is independent of outside air temperature.
Higher or lower temperatures change air density, which affects True Airspeed (TAS) rather than Indicated Airspeed (IAS). In warmer air, the aircraft must travel faster through the air mass to produce the same Indicated Airspeed (IAS). In colder air, the same Indicated Airspeed (IAS) corresponds to a lower True Airspeed (TAS).
The aerodynamic forces associated with a given Indicated Airspeed (IAS) remain the same regardless of temperature. For this reason, aircraft stall speeds, maneuvering speed (Va), and structural speed limits are published in Indicated Airspeed (IAS) instead of True Airspeed (TAS).
Temperature affects aircraft performance and navigation calculations through True Airspeed (TAS), climb performance, and density altitude, but it does not directly change the Indicated Airspeed (IAS) displayed on the airspeed indicator.
POH Charts
Indicated Airspeed (IAS) is used in Pilot Operating Handbook (POH) performance charts to define operating speeds that depend on aerodynamic forces rather than atmospheric conditions.
The Pilot Operating Handbook (POH) publishes stall speeds, takeoff speeds, approach speeds, climb speeds, maneuvering speed (Va), and structural speed limits in Indicated Airspeed (IAS). These speeds remain valid regardless of altitude or temperature because Indicated Airspeed (IAS) directly represents dynamic pressure acting on the aircraft.
Pilots use Indicated Airspeed (IAS) when referencing Pilot Operating Handbook (POH) charts for normal aircraft operation. Flying the published Indicated Airspeed (IAS) ensures the aircraft achieves the intended aerodynamic performance, whether maintaining climb performance, preventing stall, or operating within structural limits.
Some Pilot Operating Handbook (POH) performance charts also include Calibrated Airspeed (CAS) because CAS removes instrument and position errors from Indicated Airspeed (IAS). Navigation and cruise performance charts may additionally use True Airspeed (TAS), particularly for range, endurance, and fuel planning.
Using the correct airspeed reference is essential when interpreting Pilot Operating Handbook (POH) data. Indicated Airspeed (IAS) is used for aircraft handling and operating limitations, while Calibrated Airspeed (CAS) and True Airspeed (TAS) are used when greater accuracy is required for performance calculations and flight planning.
IFR & VFR
Indicated Airspeed (IAS) is used in both Instrument Flight Rules (IFR) and Visual Flight Rules (VFR) operations to control the aircraft safely and maintain published operating speeds.
In Instrument Flight Rules (IFR) operations, pilots use Indicated Airspeed (IAS) to fly instrument departures, climbs, cruise segments, holding patterns, approaches, and missed approaches at the correct operating speeds. Maintaining the published Indicated Airspeed (IAS) ensures predictable aircraft performance and adequate stall margins throughout each procedure.
In Visual Flight Rules (VFR) operations, pilots use Indicated Airspeed (IAS) during takeoff, climb, cruise, descent, approach, and landing to maintain safe aircraft handling. Indicated Airspeed (IAS) provides a consistent reference for flying traffic patterns, maintaining approach speeds, and preventing aerodynamic stall.
Indicated Airspeed (IAS) is used instead of True Airspeed (TAS) because aerodynamic performance depends on dynamic pressure rather than the aircraft's actual speed through the air. This allows the same Indicated Airspeed (IAS) to produce the same aerodynamic behaviour regardless of altitude or temperature.
Although Indicated Airspeed (IAS) is the primary airspeed reference for flying the aircraft, Instrument Flight Rules (IFR) and Visual Flight Rules (VFR) navigation use True Airspeed (TAS) and groundspeed to calculate headings, Estimated Time En Route (ETE), and fuel planning. This separation allows pilots to control the aircraft with Indicated Airspeed (IAS) while planning navigation with True Airspeed (TAS) and groundspeed.
Common Mistakes
Common Indicated Airspeed (IAS) mistakes occur when pilots misunderstand what IAS represents or use it incorrectly for aircraft operation and flight planning.
A frequent mistake is confusing Indicated Airspeed (IAS) with True Airspeed (TAS). IAS represents dynamic pressure acting on the aircraft, while True Airspeed (TAS) represents the aircraft's actual speed through the surrounding air mass. Using Indicated Airspeed (IAS) instead of True Airspeed (TAS) for navigation produces incorrect groundspeed, Estimated Time En Route (ETE), and fuel calculations.
Another common mistake is assuming Indicated Airspeed (IAS) changes automatically with altitude or temperature. The same Indicated Airspeed (IAS) produces the same aerodynamic loading regardless of atmospheric conditions. Altitude and temperature primarily affect True Airspeed (TAS), not the Indicated Airspeed (IAS) displayed on the airspeed indicator.
Pilots also make errors by ignoring instrument and position errors. Indicated Airspeed (IAS) is the direct instrument reading, but some aircraft require corrections to obtain Calibrated Airspeed (CAS), particularly during low-speed flight or when using different flap configurations.
Another mistake is exceeding aircraft operating limits by focusing on True Airspeed (TAS) or groundspeed instead of Indicated Airspeed (IAS). Structural limits such as Vne, Vno, Va, and Vfe are all published in Indicated Airspeed (IAS) because they are based on aerodynamic loads.
Pitot-static system problems can also lead to incorrect Indicated Airspeed (IAS) indications. A blocked pitot tube, blocked static port, or instrument malfunction can produce inaccurate airspeed readings. Cross-checking Indicated Airspeed (IAS) with aircraft attitude, power setting, altitude trend, and other flight instruments helps pilots recognise abnormal indications promptly.
Common Questions
The airspeed indicator colour arc system defines the aircraft's approved operating speed ranges using standardized colour markings.
The white arc extends from Vso to Vfe and identifies the flap operating range. The green arc extends from Vs1 to Vno and identifies the normal operating range. The yellow arc extends from Vno to Vne and identifies the caution range, where flight is permitted only in smooth air. The red radial line marks Vne (never-exceed speed).
Although the numerical speeds differ between aircraft, the colour coding is standardized by FAR Part 23 and EASA CS-23 and is published in the Pilot Operating Handbook (POH).
Va (maneuvering speed) is the maximum Indicated Airspeed (IAS) at which full or abrupt control inputs will not overstress the aircraft structure. At or below Va, the wing stalls before aerodynamic loads exceed the aircraft's structural design limit.
Va decreases as aircraft weight decreases because a lighter aircraft stalls at a lower IAS. The relationship is:
Va (actual weight) = Va (maximum weight) × √(Actual weight ÷ Maximum weight)
Vne (never-exceed speed) is the maximum permitted Indicated Airspeed (IAS) under any flight condition. It is shown by the red radial line on the airspeed indicator.
Flying above Vne can cause structural failure or aerodynamic flutter, which is a self-amplifying vibration that can rapidly destroy the aircraft. Manufacturers establish Vne below the demonstrated dive speed (Vd) to provide a structural safety margin.
Unlike Vno, Vne must never be exceeded.
The white arc identifies the approved flap operating range on the airspeed indicator. It extends from Vso (stall speed in landing configuration) to Vfe (maximum flap extended speed).
Pilots may extend or operate flaps only while the aircraft remains within this speed range. Extending flaps above Vfe can damage the flap system or wing structure.
KIAS stands for Knots Indicated Airspeed. It expresses Indicated Airspeed (IAS) in knots, where one knot equals one nautical mile per hour.
KIAS is the standard unit used for V-speeds, airspace speed limits, and aircraft operating speeds published in the Pilot Operating Handbook (POH). It distinguishes IAS from KCAS (Knots Calibrated Airspeed), KTAS (Knots True Airspeed), and groundspeed.
The maximum indicated airspeed below 10,000 ft mean sea level (MSL) in the United States is 250 KIAS under 14 CFR 91.117.
Additional limits apply in some airspace. Within 4 nautical miles of the primary airport and at or below 2,500 ft above ground level (AGL) in Class C, Class D, and certain Class B airspace, the limit is 200 KIAS.
These limits use IAS because aircraft separation and aerodynamic performance depend on dynamic pressure rather than True Airspeed (TAS).
Indicated Airspeed (IAS) measures the aircraft's speed relative to the surrounding air mass, while groundspeed measures the aircraft's speed over the Earth's surface.
IAS is determined by dynamic pressure measured by the pitot-static system and is used for aircraft control. Groundspeed depends on True Airspeed (TAS) and wind, making it the correct speed for navigation, Estimated Time En Route (ETE), and fuel planning.
Angle of attack (AoA) is the angle between the wing's chord line and the relative airflow. A stall occurs when the angle of attack exceeds the wing's critical angle, regardless of True Airspeed (TAS) or groundspeed.
At a given aircraft weight and configuration, the critical angle of attack is reached at approximately the same Indicated Airspeed (IAS) because IAS directly represents the dynamic pressure that produces lift. This is why stall speeds are published in IAS.
Pitot heat is an electrical heating system that prevents ice from blocking the pitot tube. A blocked pitot tube prevents the airspeed indicator from receiving correct ram air pressure and can cause incorrect Indicated Airspeed (IAS) readings.
Pilots should switch on pitot heat before entering visible moisture in conditions where icing is possible, including clouds, rain, snow, or temperatures near or below freezing. Many aircraft procedures also require pitot heat during Instrument Meteorological Conditions (IMC).
Yes. Two aircraft can fly at the same Indicated Airspeed (IAS) while having different True Airspeeds (TAS) if they are at different altitudes. At higher altitude, lower air density requires the aircraft to move faster through the air to generate the same dynamic pressure.
For example, 120 KIAS corresponds to approximately 120 KTAS at sea level but about 139 KTAS at 10,000 ft under International Standard Atmosphere (ISA) conditions. Both aircraft experience the same aerodynamic loads and stall margins, but the higher-altitude aircraft travels through the air more quickly.