What is CAS on Boeing 737? (Computed Airspeed)

The Computed Airspeed (CAS) is an essential measurement used in aviation, including on the Boeing 737 aircraft. It is a reliable indicator of an aircraft’s airspeed and is calculated based on the difference between the total pressure and the static pressure. Understanding CAS is crucial for pilots, as it helps them navigate the aircraft safely and efficiently.

In this article, we will dive deeper into what Computed Airspeed is and how it is utilized on the Boeing 737. We will explore its significance, calculations, and the factors affecting CAS. So, let’s get started!

How is Computed Airspeed Calculated?

Computed Airspeed is derived from the difference between the total pressure (Pt) and the static pressure (Ps) encountered by an aircraft in flight. The total pressure represents the pressure exerted by the air flowing around the aircraft, while the static pressure represents the atmospheric pressure at a certain altitude.

To calculate CAS, the dynamic pressure (q) is determined by subtracting the static pressure from the total pressure. Formulaically, it can be expressed as:

CAS = sqrt((2*q)/rho)

Here, rho represents the air density and q is given by:

q = Pt – Ps

The square root ensures that the calculated airspeed represents the equivalent airspeed (EAS) accurately, which is the airspeed that would produce the same dynamic pressure at sea level. CAS takes into account the variations in air density at different altitudes to provide a standardized measurement of airspeed.

It is important to note that CAS is not the same as Indicated Airspeed (IAS), which is the reading from the aircraft’s airspeed indicator. IAS requires further correction to determine CAS due to instrument errors and installation characteristics specific to each aircraft model.

The Importance of Computed Airspeed

Computed Airspeed is a critical parameter for pilots, as it helps them make accurate judgments and decisions during flight operations. Here are some key reasons why CAS holds significant importance:

1. Performance Estimation: CAS provides pilots with a measurement of the aircraft’s airflow speed, allowing them to estimate its performance. By monitoring CAS, pilots can determine the aircraft’s capabilities, including takeoff, climb, cruise, and landing speeds. This information is crucial for maintaining optimal performance and safety throughout the flight.

2. Navigation and Route Planning: CAS also aids pilots in navigation and route planning. By considering CAS along with other factors like wind speed and direction, pilots can calculate groundspeed and time en-route accurately. This information allows for efficient flight planning, fuel management, and adherence to flight schedules.

3. Air Traffic Control Instructions: CAS is used by air traffic controllers to provide precise instructions to pilots, particularly during approach and landing phases. By referencing CAS, controllers ensure appropriate separation between aircraft and prevent conflicts. CAS also helps in determining the appropriate speed limits for different phases of flight based on the aircraft’s weight and conditions.

Factors Affecting Computed Airspeed

Computed Airspeed is influenced by several factors, which pilots must consider for accurate airspeed readings and safe flight operations. Here are some key factors affecting CAS:

1. Altitude: As altitude increases, the air density decreases, resulting in a higher CAS reading for the same dynamic pressure. This is because the lower air density at higher altitudes requires higher true airspeed to maintain the same dynamic pressure. Pilots must account for this variation in CAS for accurate speed measurements during climb and cruise.

2. Temperature: CAS is also affected by temperature variations. Warmer air has lower density, requiring a higher CAS to produce the same dynamic pressure as colder air. Conversely, colder air increases air density, leading to a lower CAS for the same dynamic pressure. Pilots adjust their speed accordingly based on the temperature to maintain the desired CAS.

3. Instrument Errors: The accuracy of CAS readings can be influenced by instrument errors, including calibration errors, installation discrepancies, and mechanical limitations. It is crucial for pilots to be aware of these potential errors and apply necessary corrections to obtain reliable CAS readings.

4. Aircraft Configuration: The configuration of the aircraft, such as flap settings, landing gear position, and external stores (if applicable), can also impact CAS. Changes in the aircraft’s configuration alter the flow of air around it, which affects the total and static pressure readings. Pilots must consider these aircraft-specific factors when interpreting CAS values for accurate airspeed control.

By understanding these factors and applying appropriate corrections to the CAS readings, pilots can ensure precise airspeed control and maintain flight safety.


Computed Airspeed (CAS) plays a crucial role in aviation, providing pilots with an accurate measurement of an aircraft’s airspeed. By considering the differences between total and static pressure, CAS is calculated and used by pilots for performance estimation, navigation, and adherence to air traffic control instructions. It is influenced by factors such as altitude, temperature, instrument errors, and aircraft configuration. Pilots must understand and account for these factors to ensure precise airspeed control and safe flight operations on the Boeing 737 aircraft.

If you want to delve even deeper into the topic of airspeed measurement, you can explore this resource provided by the Federal Aviation Administration (FAA). It offers comprehensive information on airspeed indicators, errors, and other relevant concepts.

For More: What is SAARU on Boeing 737? (Standby (Secondary) Attitude Air Data Reference Unit)