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PilotSchool Knowledge Base

Airspeed Definitions and Their Importance

This section explores the definitions and significance of different types of airspeeds — Indicated Airspeed (IAS), Calibrated Airspeed (CAS), and True Airspeed (TAS) — as they apply to aspiring pilots preparing for the CASA PPL pilot license theory exam. These concepts fall under the topic of flight instruments within the broader category of power plants and systems in General Aeronautical Knowledge.

Indicated Airspeed (IAS)

Definition

Indicated Airspeed (IAS) is the airspeed read directly from the aircraft’s airspeed indicator. It is derived from the dynamic pressure sensed by the pitot tube and displayed on the instrument panel.

Characteristics

  • Uncorrected Measurements: IAS is not corrected for factors such as instrument error, installation error, or atmospheric density variations.
  • Sea Level Comparison: At sea level under standard atmospheric conditions, IAS approximates the true airspeed due to denser air.
  • Altitude Impact: As an aircraft climbs to higher altitudes, IAS tends to decrease in comparison to True Airspeed (TAS), due to reduced air density.
  • Practical Application: IAS is crucial for maintaining safe airspeeds, especially during key phases of flight such as takeoff, landing, and maneuvers to avoid stalling.

Source: Airspeed Definitions - AviationFile

Calibrated Airspeed (CAS)

Definition

Calibrated Airspeed (CAS) is the Indicated Airspeed corrected for instrument and position errors. It represents a more accurate speed reading used for aircraft control.

Characteristics

  • Error Correction: CAS accounts for discrepancies introduced both by the airspeed indicator itself and the positioning of the pitot-static system on the aircraft.
  • Accuracy in Control: Provides a reference for more accurate speed monitoring, especially critical during low-speed operations and when using different flap settings.
  • Environmental Influence: At sea level, under standard atmospheric pressure and temperature, CAS is similar to both Equivalent Airspeed (EAS) and True Airspeed (TAS).

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True Airspeed (TAS)

Definition

True Airspeed (TAS) is the speed of the aircraft relative to the air mass through which it moves. It reflects the actual speed that affects flight duration and fuel consumption.

Characteristics

  • Altitude and Temperature Adjustments: TAS increases with altitude for a given CAS, generally rising about 2% per 1,000 feet increase in altitude due to decreasing air density.
  • Performance and Fuel Efficiency: Understanding TAS is vital for planning accurate flight performance metrics, including fuel consumption and time en route.
  • Calculation: TAS can be calculated manually using a flight computer like the E6B, or automatically via onboard systems, considering variables such as pressure altitude, ambient temperature, and IAS.

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Understanding these different airspeeds underpins safe and efficient flight operations, ensuring pilots can accurately assess aircraft performance and make informed decisions throughout their flights.