What is CAS in Aviation? (Calibrated Airspeed)

Calibrated airspeed (CAS), also known as indicated airspeed (IAS), is an essential term used in aviation to measure the speed of an aircraft. It is one of the key parameters that pilots rely on to ensure safe and efficient flight operations. CAS refers to the airspeed reading displayed on the aircraft’s airspeed indicator, which is adjusted for instrument and installation errors. In simpler terms, CAS is the indicated airspeed corrected for the instrument and position errors. Understanding calibrated airspeed is crucial for pilots as it helps them accurately determine their aircraft’s performance and comply with regulatory requirements.

The Importance of Calibrated Airspeed

Calibrated airspeed plays a vital role in aviation for several reasons:

  • Performance Calculation: CAS is used in the calculation of various flight performance parameters such as true airspeed (TAS), mach number, and the aircraft’s climb and descent profiles. These calculations are essential for flight planning and operating within specific aircraft performance limitations.
  • Aircraft Systems: Many aircraft systems, such as the flight control system and autopilot, rely on accurate airspeed information to operate correctly. CAS feeds into these systems, providing the necessary data for precise control and automation.
  • Regulatory Compliance: Aviation regulatory authorities, such as the Federal Aviation Administration (FAA) and the European Union Aviation Safety Agency (EASA), have specific requirements regarding aircraft performance and safety. Compliance with these regulations often requires accurate CAS readings for documentation and reporting purposes.

How is Calibrated Airspeed Determined?

Calibrated airspeed is determined by adjusting the indicated airspeed (IAS) reading on the aircraft’s airspeed indicator. The calibration process aims to compensate for instrument and installation errors, ensuring accurate readings.

To determine calibrated airspeed, the following factors are considered:

  • Instrument Error: Each airspeed indicator has its own inherent errors, which may vary from instrument to instrument. These errors can be caused by manufacturing tolerances or other factors. Instrument error correction involves comparing the displayed IAS to a known calibrated reference and applying corrections accordingly.
  • Installation Error: The aircraft’s airspeed indicator is typically positioned in a specific location relative to the airflow. However, factors such as aircraft design and modifications can cause installation errors, affecting the accuracy of the airspeed reading. Installation error correction involves adjusting the IAS reading based on the specific installation characteristics of the aircraft.
  • Instrument Error Correction Chart: Manufacturers provide instrument error correction charts or tables specific to each aircraft type. These charts help pilots determine the corrections required based on the indicated airspeed and altitude.

By applying the appropriate instrument and installation error corrections, pilots can obtain the calibrated airspeed, which reflects the true speed of the aircraft through the air.

Importance of Calibrated Airspeed in Different Flight Phases

Calibrated airspeed is particularly important during different flight phases due to the varying flight conditions and operational requirements. Let’s explore its significance in key flight phases:

Takeoff and Landing

During takeoff and landing, knowing the calibrated airspeed is essential for a safe and efficient operation. The CAS readings help pilots ensure they are within the safe limits of their aircraft’s performance, such as the minimum control speed (Vmc), rotate speed (VR), and approach speed (Vref).

Vmc is the minimum calibrated airspeed at which an aircraft is still controllable with an engine failure. VR is the calibrated airspeed when the pilot applies back pressure on the control column to lift the nose wheel off the runway during takeoff. Vref is the calibrated airspeed at the threshold of the runway during landing approaches. These speeds are critical for the safe execution of takeoff and landing maneuvers.

Climb and Descent

Different climb and descent profiles require specific calibrated airspeed values to ensure optimal performance and safe operations. During climb, the CAS assists pilots in maintaining the aircraft within the desired climb speed range, which affects fuel efficiency and climb time. Similarly, during descent, the CAS helps pilots control the aircraft’s speed within the required limits for a stabilized approach and landing.

For example, during a climb, the aircraft’s best rate of climb airspeed (VY) is determined using calibrated airspeed. VY provides the optimal climb speed to gain altitude most efficiently. Similarly, the approach speed (Vapp) during descent is determined using calibrated airspeed to ensure the aircraft descends at a safe and controlled rate.


During cruise, maintaining a specific calibrated airspeed is crucial for several reasons, including fuel efficiency, aircraft performance, and compliance with air traffic control requirements.

One essential concept related to calibrated airspeed during the cruise phase is the aircraft’s true airspeed (TAS). TAS is the calibrated airspeed corrected for non-standard atmospheric conditions such as altitude and temperature. Accurate TAS readings enable pilots to determine the aircraft’s groundspeed and actual performance.

For example, knowing TAS is crucial when calculating fuel consumption, estimated time of arrival, and determining the aircraft’s range. Pilots with accurate calibrated airspeed and TAS readings can optimize their flight planning and make informed decisions during long-range flights.

In Conclusion

Calibrated airspeed (CAS) is a critical parameter in aviation that provides pilots with accurate speed readings necessary for safe and efficient flight operations. Understanding and using calibrated airspeed correctly allows pilots to determine their aircraft’s performance, comply with regulatory requirements, and make informed decisions during various flight phases.

By adjusting the indicated airspeed (IAS) for instrument and installation errors, pilots can obtain the calibrated airspeed readings needed for precise calculation of flight parameters, control system operations, and compliance with regulatory requirements. Through an accurate understanding of calibrated airspeed and its importance, pilots can enhance the safety and efficiency of their flights.

For more information about the different airspeed indicators and their indications, you can visit Skybrary – Indicated Airspeed.

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