Air Speed

Airspeed is the relative speed of an aircraft with respect to the air mass through which it moves. This parameter is fundamental for flight safety, performance analysis, structural limitations, and navigation. The concept of airspeed encompasses various subtypes that account for correlations between the value displayed on the instrument and the actual speed relative to the air mass.

Pitot-Static System and Fundamental Measurement Principles

The airspeed indicator (ASI) operates via the pitot-static system. The pitot tube, mounted at the aircraft’s forward point, measures total pressure (static plus dynamic). Static ports provide ambient pressure. The value displayed on the instrument is based on the principle of converting the difference between total and static pressure into dynamic pressure. In modern aircraft, this measurement may be presented in a band (speed tape) format within electronic flight systems instead of traditional mechanical gauges. Independent pitot-static systems may be provided for each pilot; commercial aircraft also feature a third independent airspeed indicator as a backup.

The Bernoulli principle serves as the fundamental physical basis of the pitot-static system: subtracting static pressure from total pressure yields dynamic pressure, which is directly related to airspeed.

Types of Airspeed

Indicated Airspeed (IAS)

IAS is the airspeed directly read from the instrument. This value is uncorrected for instrument errors, position (installation) errors, or atmospheric conditions. Although IAS closely approximates true airspeed at sea level under standard atmospheric conditions, it tends to be lower than true airspeed as altitude increases and air density decreases.

Calibrated Airspeed (CAS)

CAS is obtained by correcting IAS for instrument and position errors. It serves as a critical control reference for evaluating the aircraft’s response to dynamic pressure. CAS represents the first correction step between IAS and true airspeed.

Equivalent Airspeed (EAS)

EAS is the speed derived from CAS after accounting for compressibility effects. At low speeds, the difference between CAS and EAS is negligible; at high speeds and high altitudes, this difference becomes more pronounced. EAS can be used directly to assess aerodynamic forces acting on the airflow.

True Airspeed (TAS)

TAS is the speed obtained by correcting EAS for variations in altitude and temperature. TAS represents the actual relative speed between the aircraft and the air mass. At high speeds or altitudes, TAS can differ significantly from IAS or CAS. Approximately, TAS can be calculated by dividing CAS (or EAS) by a factor dependent on air density.

Relationships Between Airspeed Types

The sequence of conversions between airspeed types is as follows: IAS → (instrument and position corrections) → CAS → (compressibility correction) → EAS → (temperature and pressure altitude correction) → TAS.

Through this process, the raw value from the instrument undergoes successive corrections to yield the most accurate representation of actual airspeed (TAS) under prevailing flight conditions.

Airspeed Indicators and Markings

Analog airspeed indicators feature color-coded speed bands that enable pilots to visually monitor safe and limit speeds. Critical speeds—such as stall speed and structural limit speeds—are typically defined in terms of IAS, allowing pilots to reference them directly from the instrument.

In aircraft flight manuals, V-speeds (e.g., VNE, VFE, VS, VNO) are published in IAS and correspond to specific markings on the instrument (e.g., red line, yellow arc, green arc).

Flight Tests and Calibration Methods

Various flight test methods are employed to verify the accuracy of airspeed systems and calibrate instruments:

• Tower fly-by: Radar-measured ground speed is compared with pitot-static system readings.
• Trailing cone method: A tethered cone is deployed from the aircraft to provide reference measurements of static and total pressure.
• Pacer aircraft comparison: The test aircraft flies in parallel with a reference aircraft to compare speeds.
• Radar tracking: Data from radar systems are used for comparison.
• Dynamic maneuvers: The system’s dynamic response is analyzed during controlled changes in speed and direction.
• Additional corrections are applied under special conditions such as high angle of attack, high speed, and sensor body effects.

Unreliable Airspeed

Partial or complete blockage of the pitot tube or static ports due to icing, contamination, damage, or system failures can cause instrument discrepancies. In such cases, IAS may become inconsistent. Indicators include erratic readings, abnormal increases or decreases in speed, or persistent stagnation. Altitude and vertical speed indicators (VSI) may also be affected. In these situations, emergency procedures specified in the flight manual must be followed.

Ground Speed and Wind Effects

Ground speed (GS) is the vector sum of true airspeed (TAS) and wind velocity. When flying with the wind, GS exceeds TAS; when flying against the wind, GS is less than TAS. This difference plays a critical role in navigation calculations, route planning, and fuel management.

Airspeed is a decisive parameter across all phases of flight, from flight dynamics and aerodynamic force analysis to navigation planning. The IAS value obtained via the pitot-static system is progressively corrected to CAS, EAS, and finally TAS to reflect true airspeed. Color-coded bands and V-speed definitions on the instrument enable pilots to directly monitor safety limits. System accuracy is verified through flight tests; in the event of malfunctions or icing, unreliable airspeed procedures are implemented. V-speeds serve as fundamental references for maintaining safe performance limits in both training and professional flight operations.