Systems - Airspeed | Training Blog

Describe the difference between indicated airspeed, calibrated airspeed, true airspeed, and ground speed.

Cover image: AI-generated editorial illustration based on the full instrument panel of GCAC Cessna 172K N84455

A pilot can see one speed on the airspeed indicator, calculate another for flight planning, and read a third from GPS. None of those numbers has to be wrong. They are answering different questions.
 
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🧠 The Answer
 

• Indicated airspeed (IAS): The speed read directly from the airspeed indicator, without correction for atmospheric density, installation error, or instrument error.
• Calibrated airspeed (CAS): IAS corrected for installation and instrument error.
• True airspeed (TAS): CAS corrected for altitude and nonstandard temperature. It is the airplane's speed relative to the air mass in which it is flying.
• Ground speed (GS): TAS adjusted for wind. It is the airplane's actual speed over the ground.

 
The short version is:

• IAS is what the instrument shows.
• CAS corrects the instrument reading.
• TAS describes movement through the air mass.
• GS describes movement over the Earth.

 
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🎛️ From IAS to CAS
 
The airspeed indicator measures the difference between pitot pressure and static pressure. That pressure difference moves the instrument's internal mechanism and produces IAS. Our earlier post on the pitot-static system explains that pressure relationship in more detail.
 
IAS is the cockpit number pilots use most often. Airplane operating speeds and limitations are commonly presented as indicated airspeeds, but always confirm the label and value in the aircraft's approved AFM or POH. Club pilots can move from this general lesson to the N84455 aircraft page and its aircraft-specific checklist and in-flight guide for practical reference material; those Club resources support, but do not replace, the approved aircraft information and placards.
 
The instrument and its installation are not perfectly accurate at every speed and configuration. Correct IAS for those errors, using the aircraft manufacturer's calibration information, and the result is CAS.

The ASI responds to the difference between pitot and static pressure. Source: FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 8, Figure 8-7, p. 8-8.

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🌡️ From CAS to TAS
 
Correct CAS for altitude and nonstandard temperature, and the result is TAS.
 
As altitude increases, air density normally decreases. The airplane must move faster through less-dense air to create the same pitot-static pressure difference. That is why TAS is normally higher than CAS in cruise at altitude.
 
TAS is especially useful for cruise, navigation, and flight planning because it describes how fast the airplane is moving through the surrounding air mass. Temperature matters here, which is why the Club's standard-temperature lesson is part of the same planning picture.
 
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🌬️ From TAS to Ground Speed
 
Wind moves the air mass over the ground. Add that wind effect to TAS, and the result is GS.
 

• A tailwind increases GS.
• A headwind decreases GS.
• A crosswind changes the airplane's track and can also affect GS along the course.

 
This is why a GPS may show a ground speed that is noticeably different from the IAS on the panel. The instruments are measuring different relationships.

The same airspeed can produce different ground speeds as the air mass moves. Source: FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 16, Figure 16-13, p. 16-9.

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✈️ Which Speed Should You Use?
 
Each speed has a practical job:

• Fly the airplane with IAS. Use the indicated values specified by the AFM or POH for takeoff, climb, stall awareness, approach, landing, and operating limitations.
• Use CAS as the corrected bridge. It accounts for instrument and installation error and is the starting point for finding TAS.
• Use TAS for cruise and flight planning. It describes the airplane's speed through the air mass and supports navigation calculations.
• Plan time and progress with GS. It determines how quickly the airplane covers distance over the ground and therefore affects estimated time en route and fuel planning.

 
One cockpit can display several speed values at once. The useful question is not simply, Which number is right? It is, Which speed answers the decision I am making?
 
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⚠️ Common Mix-Ups
 

• IAS is not ground speed. A strong headwind can reduce GS without changing the IAS needed to fly the airplane safely.
• TAS is not automatically what the airspeed indicator shows. TAS requires correction for altitude and temperature unless the aircraft's equipment computes and displays it.
• Wind does not change TAS by itself. Wind changes the motion of the air mass over the ground, which changes GS.
• GPS ground speed does not replace IAS. GS is useful for navigation and timing, but it does not provide the same stall-margin and operating-limit information as IAS.

 
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✅ The Takeaway
 
Follow the correction path:

• IAS corrected for instrument and installation error becomes CAS.
• CAS corrected for altitude and nonstandard temperature becomes TAS.
• TAS adjusted for wind becomes GS.

 
Use IAS to fly, TAS to understand speed through the air mass, and GS to understand progress over the ground.
 
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📚 FAA Sources and References
 
1.  FAA Pilot's Handbook of Aeronautical Knowledge, FAA-H-8083-25C, Chapter 8, “Airspeed Indicator,” pp. 8-8 to 8-9 and Figure 8-7; Chapter 16, “Navigation,” p. 16-9 and Figure 16-13.

2.  FAA Private Pilot for Airplane Category Airman Certification Standards, FAA-S-ACS-6C, Area I, Task D, Cross-Country Flight Planning; Area I, Task G, Operation of Systems; Area VI, Task A, Pilotage and Dead Reckoning.
 
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📅 Next Week
 
Weather - Nonstandard Pressure
 
How do you correct an altimeter for nonstandard pressure? How inaccurate is an altimeter that is using an incorrect barometric pressure setting?
 
Next week, we'll look at how a mismatch between local atmospheric pressure and the setting used by an altimeter changes what the instrument shows. Even a modest mismatch can matter when altitude accuracy and obstacle clearance are important, so we'll connect the cockpit indication to the airplane's actual position without relying on guesswork.