Indicated Airspeed Calculator

Indicated airspeed (IAS) is the airspeed reading directly from the airspeed indicator (ASI) in the cockpit. The ASI measures the difference between pitot pressure (the ram air pressure in the pitot tube) and static pressure (the ambient atmospheric pressure from the static ports). This difference — called dynamic pressure — is displayed as a speed. Pilots use IAS because all aircraft performance limits are defined in IAS. The aerodynamic forces on the aircraft structure — lift, drag, and control surface loads — depend on dynamic pressure, not on true airspeed. At a given IAS, the aircraft always experiences the same aerodynamic load regardless of altitude or air density. This means Vne (never exceed speed), Va (maneuvering speed), and all structural limits are always the same number on the ASI regardless of altitude. IAS is the primary speed reference in the cockpit for exactly this reason.

Indicated Airspeed (IAS) is the raw reading from the airspeed indicator, which may include a small instrument error due to imperfections in the instrument calibration. Calibrated Airspeed (CAS) is IAS corrected for position error — the error caused by the location of the pitot tube and static ports in non-ideal airflow around the aircraft. When the aircraft is at a high angle of attack (slow flight, flaps extended), the airflow around the fuselage is disturbed, and the static ports may not measure ambient pressure accurately. This creates a position error that makes IAS differ from CAS, sometimes by as much as 5–10 kt at approach speeds. The correction table (IAS to CAS) is published in every POH in the Limitations or Performance section. At typical cruise speeds, the difference between IAS and CAS is usually less than 3 kt.

V-speeds are defined in IAS because the critical aerodynamic and structural phenomena they mark occur at specific dynamic pressures — and dynamic pressure is what IAS measures. The stall, for example, occurs when the wing reaches its critical angle of attack. The lift coefficient at that angle depends on dynamic pressure. At a given weight, the aircraft always stalls at the same IAS regardless of altitude because the same dynamic pressure (IAS) is needed to generate sufficient lift. This means the pilot applies the same technique regardless of altitude — look for the same numbers on the ASI. If V-speeds were given in TAS, they would be different at every altitude, making normal operations impossibly complex. The only exception is Va (maneuvering speed), which decreases with weight because at lighter weights, the aircraft can reach the stall before structural limits are exceeded at a lower speed.

The airspeed indicator in most GA aircraft is colour-coded for quick reference: White arc: flap operating range from Vso (bottom) to Vfe (top). Flaps may be extended within this range. Green arc: normal operating range from Vs1 (bottom) to Vno (top). The aircraft may be operated in smooth air throughout this range without structural concern. Yellow arc: caution range from Vno to Vne. The aircraft may be operated in this range only in smooth air — flight in turbulence above Vno risks structural damage. Red line: Vne (never exceed speed). Structural failure from flutter or excessive loading is possible above this speed. Red radial: Vso (lower limit of white arc) also marks the stall speed in landing configuration. Some aircraft have a second red line indicating minimum takeoff/landing speed or other limits.

Position error is the difference between the actual static pressure and the pressure measured by the static port due to the location of the port in disturbed airflow around the aircraft. The static ports are located at points on the fuselage where the manufacturer attempted to measure true ambient pressure, but the airflow around the fuselage is never exactly at ambient pressure. At high angles of attack, the fuselage disrupts the flow more severely, creating larger position errors. The correction is determined during aircraft certification testing by flying alongside a calibrated airspeed source (a pace aircraft or a towed parachute static source). The resulting correction table is published in the POH. For most cruise operations, position error is small (1–3 kt). For slow flight, approach, and landing configurations, it can be larger and matters for approach speed accuracy.

The indicated stall speed does not change with altitude — the aircraft always stalls at the same IAS regardless of altitude. This is because stall is an aerodynamic event determined by angle of attack and dynamic pressure, and the ASI measures dynamic pressure. However, the true airspeed at stall does increase with altitude. At 20,000 ft, the true stall speed might be 30% higher than at sea level, even though the indicated stall speed is the same. This means: the aircraft covers the ground faster when it stalls at altitude, and the landing gear and brakes must dissipate more kinetic energy if a stall occurs near the ground at a high-elevation airport. Additionally, stall speed changes with weight: heavier aircraft stall at higher IAS. At 90% of maximum weight, stall speed is approximately 95% of the published maximum weight stall speed (Vs ∝ √W).

Va is the maximum speed at which full or abrupt single control deflection will not overstress the airframe. At Va or below, the aircraft will stall before aerodynamic forces reach the structural limit — the stall is a protective mechanism. Above Va, full control deflection can generate forces exceeding the structural design load, risking airframe damage or failure. Va changes with weight because at lighter weights, the aircraft stalls at a lower IAS. If the aircraft stalls at 50 kt at light weight but the structural limit corresponds to 65 kt, then Va at that weight is 50 kt (not the published 65 kt at max weight). Va_actual = Va_max_weight × √(actual weight / max weight). This is critically important in turbulence: operating at the published Va when below max gross weight does not provide the same protection — pilots should reduce to the weight-corrected Va.

Vne (never exceed speed) is the maximum speed at which the aircraft is certified to be operated. It is defined by the manufacturer based on aeroelastic analysis and flight testing. Above Vne, control surface flutter can occur — a self-reinforcing oscillation that can destroy the aircraft within seconds. Flutter occurs when aerodynamic forces drive structural vibration faster than damping can prevent amplification. Once started, flutter typically cannot be stopped — the speed must be reduced immediately and drastically. Vne is not a conservative limit; it is where structural failure begins to become possible. Exceeding Vne even briefly can initiate structural damage (delamination, rivet fatigue) that weakens the structure without visible evidence. Vne is published as IAS and typically indicated by a red radial on the ASI.

The white arc on the ASI spans from Vso (bottom) to Vfe (top). Within the white arc, flap extension is permitted. Vso is the stall speed in the landing configuration (gear down, full flap) — the lower limit of the white arc and also a red radial. Vfe is the maximum flap extended speed — the upper limit of the white arc. Operating flaps above Vfe risks structural damage to the flap structure from excessive aerodynamic loads. The flap limit is based on the structural strength of the flap hinges and actuators, not on the wing structure. Extending flaps at speeds above Vfe can cause flap separation, which creates an extreme asymmetric lift condition. Always reduce speed below Vfe before extending flaps, and do not accelerate above Vfe with flaps extended.

IAS is not used directly for flight planning. For navigation, pilots calculate TAS from IAS (using altitude and temperature), then apply wind correction to get groundspeed. Groundspeed = TAS ± wind effect. IAS tells the pilot how the aircraft is flying; groundspeed tells the pilot how fast they are covering the ground. An aircraft flying at 120 kt IAS at 8,000 ft in ISA conditions has a TAS of approximately 140 kt. With a 30 kt headwind, groundspeed is 110 kt. At 30 kt tailwind, groundspeed is 170 kt. The IAS remains 120 kt throughout — the pilot cannot detect the wind from the ASI alone. ETE and fuel burn per nautical mile always use groundspeed, not IAS or TAS.