"What is Calibrated Airspeed (CAS)?"

"Calibrated Airspeed (CAS) is the indicated airspeed (IAS) corrected for errors caused by instrument imperfections and the position of the pitot tube and static port. CAS offers a more accurate measure of the aircraft’s speed through the air, and is critical for calculating performance, stall speeds, and regulatory compliance."

"Why is CAS important in aviation?"

"CAS is used to ensure that performance calculations, stall speeds, and safety margins are accurate. Regulations and aircraft manuals specify critical speeds—like stall speed and V-speeds—in CAS because it corrects for errors in the airspeed measurement system, ensuring consistent and safe operations."

"How is CAS different from Indicated Airspeed (IAS)?"

"Indicated Airspeed (IAS) is the value directly displayed by the airspeed indicator, without corrections. CAS is IAS corrected for instrument and position (installation) errors. At cruise speeds and clean configuration, the difference is small, but at low speeds or with flaps/gear extended, the difference can be significant."

"How do pilots determine CAS in flight?"

"Pilots use an airspeed calibration chart found in the aircraft’s Pilot’s Operating Handbook (POH) or Aircraft Flight Manual (AFM). By referencing the chart, they convert IAS to CAS for different configurations and operating conditions. Modern avionics may automate this process, but understanding the manual method is essential."

"What causes instrument and position errors in airspeed measurement?"

"Instrument error comes from mechanical imperfections or calibration drift in the airspeed indicator. Position error results from airflow disturbances at the pitot tube or static port location, especially during high angles of attack, with flaps/gear extended, or nonstandard flight attitudes. These errors skew the IAS reading, so correcting them yields CAS."

"How does CAS relate to TAS, EAS, and Groundspeed?"

"CAS is the starting point for further airspeed calculations. True Airspeed (TAS) is CAS corrected for altitude and temperature. Equivalent Airspeed (EAS) is CAS corrected for compressibility effects at high speed or altitude. Groundspeed (GS) is TAS adjusted for wind. Each correction refines the airspeed for specific uses."

"Are calibration charts universal for all aircraft?"

"No, calibration charts are specific to each aircraft make, model, and sometimes individual airframes. Always use the chart provided in the specific aircraft’s POH/AFM to ensure accuracy."

Calibrated Airspeed (CAS)

Calibrated Airspeed (CAS) is indicated airspeed corrected for instrument and position errors, ensuring accuracy for aircraft performance and safety.

Aviation Flight Instruments Aircraft Performance

Calibrated Airspeed (CAS) – Aviation Glossary

Calibrated Airspeed (CAS): Definition and Fundamental Concepts

Calibrated Airspeed (CAS) is the indicated airspeed (IAS) of an aircraft, corrected for errors caused by inaccuracies in the pressure measurement system and the airspeed indicator itself. These errors are categorized as instrument error (mechanical imperfections in the airspeed indicator) and position or installation error (due to the mounting location of the pitot tube and static port). The International Civil Aviation Organization (ICAO) defines CAS as the airspeed shown after correcting for these errors, making it a vital reference for aircraft operation and certification.

The pitot-static system—comprising a pitot tube facing the airflow and static ports sensing ambient pressure—measures airspeed by comparing total and static pressure. However, disturbances in airflow near the sensing ports, and mechanical imperfections in the indicator, introduce errors. CAS corrects for these, resulting in a value that more accurately represents the aircraft’s speed relative to the surrounding air.

CAS is foundational for regulatory and safety-critical parameters: stall speeds, V-speeds (such as V1, VR, VREF), and all key performance charts are specified in CAS. At cruise in clean configuration, CAS and IAS may differ little, but during takeoff, landing, or with high drag configurations, the difference can exceed 10 knots—making CAS essential for safety and compliance.

The Pitot-Static System and Its Impact on CAS

The pitot-static system is central to airspeed measurement. The pitot tube captures total pressure (static plus dynamic), while static ports record ambient pressure. The airspeed indicator displays IAS based on the pressure difference.

However, aircraft structure, configuration changes (like flaps and gear), and flight attitudes can disturb the airflow around these sensors, leading to position errors. Instrument error arises from mechanical imperfections in the airspeed indicator. Both errors can cause IAS to diverge from the actual airspeed.

CAS is obtained by referencing calibration tables or charts (from the POH/AFM), which correct IAS for these errors. These resources are developed through rigorous flight testing, ensuring pilots have accurate airspeed data for every configuration and flight phase.

Why Calibrated Airspeed (CAS) is Essential in Aviation

CAS is indispensable for both pilots and engineers:

Safety: Performance charts, stall speeds, and V-speeds are specified in CAS. This ensures pilots maintain correct safety margins, especially in critical phases like takeoff and landing.
Accuracy: CAS corrects for position and instrument errors, providing a reliable reference when IAS is most likely to be inaccurate—at low speeds or with high-drag configurations.
Regulatory Compliance: Authorities like ICAO, FAA, and EASA require performance data and operational limitations to be expressed as CAS to standardize safety margins and procedures across all aircraft.

Using CAS instead of IAS ensures that approach speeds, stall margins, and performance calculations are always based on the most accurate available data.

Determining Calibrated Airspeed: Flight Testing and Calibration Charts

CAS is determined during aircraft certification by comparing the aircraft’s IAS to a reference air data system (often a “boom” sensor positioned in undisturbed airflow). Flight tests are conducted across various configurations and attitudes, and the results are compiled into calibration tables or charts in the POH/AFM.

How to use a calibration chart:

Identify the current configuration (flaps, landing gear, etc.).
Read the IAS.
Use the chart to find the corresponding CAS.

Example table:

IAS (knots)	Flaps Up (CAS)	Flaps 20° (CAS)	Flaps 40° (CAS)
40	41	43	48
60	61	62	65

Modern avionics may automate this, but manual use is a vital skill.

Instrument and Installation Errors: Detailed Technical Analysis

Instrument Error:
Caused by imperfections in the airspeed indicator (e.g., mechanical friction, calibration drift). Usually small (1–2 knots), but can be larger in older or poorly maintained instruments.

Position (Installation) Error:
Results from disturbed airflow at the pitot tube or static port, especially with high angles of attack, flaps/gear extended, or nonstandard attitudes. Errors are configuration-dependent and can be significant.

Real-World Examples: IAS and CAS Discrepancies

Stall Speed Example: A Cessna 150’s POH may list a full-flap stall speed of 48 knots CAS, while the IAS at stall could be as low as 39 knots. If a pilot relied only on IAS, they might approach too slowly and risk a stall.
Cruise Flight: At 100 knots IAS in cruise, CAS may be 100–101 knots—a negligible difference.
High Drag Configurations: With flaps/gear extended, IAS–CAS discrepancies can reach 10 knots or more, depending on the aircraft.

Relationships: CAS, IAS, TAS, EAS, and GS

Airspeed Type	What It Means	Use
IAS	Direct reading from airspeed indicator	Basic flight reference, regulatory limits
CAS	IAS corrected for instrument/position error	Performance charts, stall speeds, V-speeds
EAS	CAS corrected for compressibility	High-speed/structural calculations
TAS	CAS corrected for altitude/temperature	Navigation, flight planning
GS	TAS adjusted for wind	Ground navigation, progress

Flowchart: IAS ➔ CAS (correct for errors) ➔ EAS (compressibility) ➔ TAS (density) ➔ GS (wind)

CAS in Aircraft Performance and Certification

CAS is the reference for stall speeds, takeoff/landing distances, and V-speeds in the POH/AFM. Certification tests are conducted and performance limits published using CAS, ensuring safety margins are consistent across aircraft types and conditions.

Operational speed limits (like VFE, VLO, VA) are specified in CAS to protect structural integrity and ensure safe operation throughout the flight envelope.

Using CAS in Day-to-Day Flight Operations

Pilots use the calibration chart to convert IAS to CAS for:

Performance calculations (takeoff/landing distances)
Setting approach speeds
Ensuring regulatory compliance

Example:
If approach speed is 65 knots CAS (flaps extended), and the chart shows this equals 60 knots IAS, the pilot flies the approach at 60 knots IAS for the correct margin.

Visual Aids and Example Calibration Charts

Calibration charts and graphical aids help pilots make quick, accurate corrections. Always use the chart for your specific aircraft and configuration.

Advanced Technical Considerations: ICAO and Certification Standards

ICAO and national authorities require CAS for operational and certification data (see ICAO Annex 6, Doc 4444). Performance, stall, and limitation speeds are published in CAS to ensure consistency and safety industry-wide. Manufacturers conduct flight tests and publish data as CAS, forming the basis for all operational limitations in the POH/AFM.

Troubleshooting and Maintenance: When CAS Reveals System Problems

Significant or unexpected differences between IAS and CAS may indicate:

Obstructed pitot tubes/static ports (ice, debris)
Leaks or kinks in pitot-static lines
Faulty or uncalibrated airspeed indicators

Comparing IAS to expected CAS can help diagnose instrumentation issues—vital for safe and accurate operation.

Summary

Calibrated Airspeed (CAS) is a cornerstone of safe flight operations, providing the reliable, standardized speed reference required for accurate aircraft performance, compliance, and safety. Understanding CAS, its determination, and its operational use is an essential skill for every pilot and aviation professional.

For more on airspeed accuracy, instrumentation, or to improve your flight operations, reach out or schedule a demo with our experts.

Frequently Asked Questions

What is Calibrated Airspeed (CAS)?: Calibrated Airspeed (CAS) is the indicated airspeed (IAS) corrected for errors caused by instrument imperfections and the position of the pitot tube and static port. CAS offers a more accurate measure of the aircraft’s speed through the air, and is critical for calculating performance, stall speeds, and regulatory compliance.
Why is CAS important in aviation?: CAS is used to ensure that performance calculations, stall speeds, and safety margins are accurate. Regulations and aircraft manuals specify critical speeds—like stall speed and V-speeds—in CAS because it corrects for errors in the airspeed measurement system, ensuring consistent and safe operations.
How is CAS different from Indicated Airspeed (IAS)?: Indicated Airspeed (IAS) is the value directly displayed by the airspeed indicator, without corrections. CAS is IAS corrected for instrument and position (installation) errors. At cruise speeds and clean configuration, the difference is small, but at low speeds or with flaps/gear extended, the difference can be significant.
How do pilots determine CAS in flight?: Pilots use an airspeed calibration chart found in the aircraft’s Pilot’s Operating Handbook (POH) or Aircraft Flight Manual (AFM). By referencing the chart, they convert IAS to CAS for different configurations and operating conditions. Modern avionics may automate this process, but understanding the manual method is essential.
What causes instrument and position errors in airspeed measurement?: Instrument error comes from mechanical imperfections or calibration drift in the airspeed indicator. Position error results from airflow disturbances at the pitot tube or static port location, especially during high angles of attack, with flaps/gear extended, or nonstandard flight attitudes. These errors skew the IAS reading, so correcting them yields CAS.
How does CAS relate to TAS, EAS, and Groundspeed?: CAS is the starting point for further airspeed calculations. True Airspeed (TAS) is CAS corrected for altitude and temperature. Equivalent Airspeed (EAS) is CAS corrected for compressibility effects at high speed or altitude. Groundspeed (GS) is TAS adjusted for wind. Each correction refines the airspeed for specific uses.
Are calibration charts universal for all aircraft?: No, calibration charts are specific to each aircraft make, model, and sometimes individual airframes. Always use the chart provided in the specific aircraft’s POH/AFM to ensure accuracy.

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