4 Types of Airspeed Explained: IAS, CAS, TAS, and EAS
Learn the four main types of airspeed: indicated, calibrated, true, and equivalent airspeed, plus how ground speed and Mach number fit in.
Airspeed sounds simple until you realize pilots use several different kinds of it. The number on the airspeed indicator is not always the airplane's actual speed through the air, and neither one is always the same as speed across the ground.
For student pilots, the goal is not to turn airspeed into a math class. The goal is to know which number matters for aircraft control, performance, navigation, and planning. If your bigger challenge is holding a target speed and altitude together, review airspeed and altitude control after this overview.
Indicated Airspeed (IAS)
Indicated airspeed is the speed shown on the airspeed indicator. It is the number pilots use most often while flying the airplane.
The airspeed indicator compares ram air pressure from the pitot system with static pressure from the static system. That pressure difference is displayed as airspeed.
IAS is important because many operating speeds in the airplane are based on it. Rotation speed, approach speed, stall speed markings, flap operating ranges, and many limitation speeds are normally flown by reference to indicated airspeed.
For basic flying, IAS is the number you protect. If your instructor says to climb at 79 knots, fly the indicated number unless your aircraft procedure says otherwise.
Calibrated Airspeed (CAS)
Calibrated airspeed is indicated airspeed corrected for instrument and position error. Those errors happen because airflow around the airplane and imperfections in the system can affect what the instrument senses.
At some speeds and configurations, especially slower speeds or certain flap settings, IAS and CAS may differ by several knots. At typical cruise speeds in many training airplanes, they may be very close.
You normally find calibrated airspeed information in the aircraft's Pilot's Operating Handbook or approved flight manual. It may appear as a chart that converts IAS to CAS for different configurations. The instrument itself is covered separately in this airspeed indicator guide.
CAS matters because performance charts and certification data often rely on corrected values. For everyday flying, you still fly the airplane using the speeds provided in the aircraft manual.
True Airspeed (TAS)
True airspeed is the airplane's actual speed through the surrounding air mass. It is calibrated airspeed corrected for altitude and temperature.
As altitude increases, air density decreases. To create the same pressure difference at a higher altitude, the airplane must move faster through the air. That is why true airspeed is usually higher than indicated airspeed in cruise at altitude.
Pilots use TAS for flight planning because it helps estimate time en route and fuel burn before wind is applied. A flight computer, electronic flight bag, or avionics system can calculate TAS using pressure altitude, temperature, and calibrated or indicated airspeed.
A common rough estimate is that true airspeed increases about 2 percent for each 1,000 feet of altitude above sea level, but this is only a rule of thumb. Use approved charts, the aircraft manual, or reliable calculations when accuracy matters.
Equivalent Airspeed (EAS)
Equivalent airspeed is calibrated airspeed corrected for compressibility. It becomes more important at higher speeds and higher altitudes, where air compressibility affects the relationship between pressure and speed.
Most student pilots in typical training aircraft will not use EAS during normal flight planning. It is more common in aircraft performance, structural design, flight testing, and high-speed operations.
Still, it is useful to know where EAS fits: IAS is what the instrument shows, CAS corrects instrument and position error, TAS corrects for density effects, and EAS accounts for compressibility effects.
Ground Speed Is Different
Ground speed is the airplane's speed across the ground. It is true airspeed adjusted for wind.
If you have a tailwind, ground speed is higher than true airspeed. If you have a headwind, ground speed is lower. This is why a flight can take longer in one direction than the return trip even when the airplane flies the same airspeed.
Ground speed is important for navigation, estimated time of arrival, and fuel planning. It is not the number you use to judge stall margin or approach speed.
What About Mach Number?
Mach number compares the aircraft's true airspeed to the local speed of sound. It matters most for high-speed aircraft and jets operating at altitude.
For primary flight training, Mach number is usually just a concept. Later, if you study turbine aircraft or high-altitude operations, it becomes more practical.
Which Airspeed Should a Student Pilot Use?
Use indicated airspeed to fly the airplane. Use the aircraft manual for required speeds and performance planning. Use true airspeed and wind correction for cross-country planning. Use ground speed to monitor progress once wind is included.
The mistake is asking, "Which airspeed is the real one?" They are all real. They answer different questions.
IAS tells you what the airplane feels aerodynamically through the pitot-static system. CAS cleans up measurement error. TAS tells you how fast you are moving through the air mass. Ground speed tells you how fast you are moving over the earth.
Once you understand the job of each airspeed, the terms become much less intimidating.
Official References
Need help applying this to your training?
Use this guide as a starting point, then bring the confusing parts to a focused ground lesson. Diego works with Louisville-area and remote students on FAA knowledge, oral-prep, and practical training decisions.