Measuring airspeed in an aircraft is very important in determining its safety and capabilities. To understand KIAS in aviation, you require more knowledge than the simple reading of an airspeed indicator.

When measuring the speed of an aircraft, a lot of variables come into play to figure out the actual speed. It is not as simple as reading a speedometer in a car. Pilots employ airspeed during crucial flight maneuvers like takeoff, landing, and ascent. Additionally, it helps an airplane maintain its structural integrity while in flight. But first, we need to know what airspeed is and how to measure it.

We will take a deep dive into understanding how to measure the speed of an aircraft and the variables that come into play.

What does KIAS mean in aviation?

KIAS is defined as “knots of indicated airspeed”. In aviation, knots are used as a unit of speed and are similar to kilometers per hour (KPH) and miles per hour (MPH). The Air Speed Indicator displays indicated speed. It is used in performing most tasks in the cockpit since it is quite easy to reference although it may be subject to a lot of inaccuracies.

For a pilot to plan their flight properly they need to correct the indicated speed for many factors. This way they can determine the ground speed between the two points they will be traveling across.

KIAS holds a lot of significance when it comes to determining the safe flying limits of airplanes. It directly indicates the airflow; that is the dynamic air pressure, around the wings and the fuselage of an airplane.

The airflow around an aircraft directly affects its ability to lift off, stall, or maintain structural integrity, hence the pilots continuously check KIAS throughout all phases of flight.

Airspeed Indicator
Airspeed Indicator

V-speeds are usually identified in terms of KIAS and are used to determine the all-important limits in an aircraft.

V1, VR, VS, VNE, and VMO are all represented in terms of indicated airspeed in this instance since V-speeds are directly related to the airflow.

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Here’s a quick summary of what these V-speeds indicate:

  • V1 – indicates the speed at which takeoff cannot be canceled.
  • VR– It is the rate of rotation or the rate of takeoff for an airplane.
  • VS – The stall speed, or the lowest speed at which an airplane can lose control.
  • VMO– The speed at which an airplane may operate at its maximum capacity before sounding an alarm.
  • VNE– An aircraft should never surpass this speed since doing so could cause structural failure.

KIAS vs KCAS: Types of Airspeed in Aviation

Aviation uses four different categories of airspeed. They include:

  • Indicated Airspeed (IAS)
  • True Airspeed (TAS)
  • Calibrated Airspeed (CAS)
  • Groundspeed (GS)

Below we will take a deeper look at each one of them.

  1. Indicated Airspeed (IAS): This is the speed that is usually read off the airspeed indicator in the cockpit. It is used to reference speed changes. To express the speed limits of the sky, we use airspeed values.
  2. True Airspeed (TAS): True Airspeed refers to the plane’s speed in relation to the air around it. At higher altitudes, true airspeed is often higher than indicated airspeed. In actuality, for every thousand feet above sea level, true airspeed is typically 2% greater than indicated airspeed. This results from the fact that pressure decreases with altitude.
  3. Groundspeed (GS): Groundspeed refers to the movement of your airplane relative to the ground. You can attain groundspeed by correcting true airspeeds for wind. For example, an aircraft with a true airspeed of 150 knots and a tailwind of 25 knots will have a ground speed of 175 knots.
  4. Calibrated Airspeed (CAS): Indicated airspeed corrected for positional and instrument errors is what is known as calibrated airspeed. When an aircraft is flying at certain airspeeds with certain flap settings, the total instrument and installation errors may be several knots.

At lower airspeeds with nose-high pitch altitudes, the installation and instrument errors are observed to be at their highest.

Under the International Standard Atmosphere, calibrated airspeed will be identical to true airspeed when flying at sea level (ISA). The ISA conditions include 15 degrees Celsius, 29.92 inches of mercury and 0% humidity. Calibrated airspeed will equal ground speed if there is no wind.

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Avionics AirSpeed Panel
Avionics Airspeed Panel


KCAS refers to Knots Calibrated Airspeed and just like its name suggests, the airspeed is calibrated in knots. In locations like the UK, calibrated airspeed is known as direct airspeed.

KTAS, on the other hand, refers to an aircraft’s airspeed in relation to the air it is traveling through.


Pilots must understand the distinction between calibrated and indicated airspeed depending on the type of aircraft they are operating.

In most circumstances, the difference between these two airspeeds is usually negligible and in situations where the flaps are extended and the plane demonstrates a nose-up attitude, it tends to increase to several knots.


Consider an airplane traveling at about 200 knots at sea level whereby the atmospheric pressure is 101 kPa. An increase in altitude to 15000ft means that the indicated speed will drop to around 160knots even while the true airspeed remains constant at 200 knots. Less air is striking the pitot tube as a result of the altitude decreasing to 57kPA.

As an aircraft increases its altitude, the air surrounding it becomes thinner resulting in a decrease in the dynamic pressure that is being measured by the pitot tube.

At typical atmospheric conditions, which are at sea level with temperatures of 15 degrees Celsius, KTAS is equivalent to KIAS or KCAS. However, at higher altitudes, the true airspeed and the indicated airspeed diverge.

What is KNOT in aviation?

A knot stands for nautical miles per hour. It is used for navigation purposes whether by air or the sea. The unit is the standard measurement for all charts that use longitudes and latitudes worldwide.

A nautical mile is approximately 1.85 kilometers or 1.15 statute miles long and represents the distance between one minute of latitude.

Pitot Aircraft
Pitot Aircraft

How Is Airspeed Measured?

Airspeed, as previously stated, is the velocity of an aircraft when in flight. To calculate airspeed, we convert the pressure of air that is being sucked into the pitot tube to linear velocity.

The airspeed increases as the pitot tube’s internal pressure increases. The pilot can view and analyze the airspeed by reading a dedicated airspeed indicator gauge or a linear scale on a Primary Flight Display.

In two-pilot planes, both the pilots are provided with similar ASI in the cockpit. Each ASI is provided by a different pitot-static system. In the case that one system fails, both can be fed by the same system. Commercial aircraft typically have a third independent ASI, which is typical of the pointer/scale kind, regardless of the type of indication.

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Pitot-static systems on board the aircraft are linked to airspeed indicators. To gather their data, they rely on pressure readings taken outside the fuselage. They are simple instruments that can be read easily.

The pitot tube, which is often positioned on an aircraft’s wing, is the most crucial component of an airspeed indicator. It is placed pointing forwards into the airflow.

More air enters the pitot tube when an aircraft flies faster, and the air pressure conveyed to the airspeed indicator presses a flexible diaphragm that is vented to the static port on the opposite side.

A static port is an opening on the side of an aircraft. It is usually open to the atmospheric pressure outside the aircraft. As more air goes into the pitot tube, the airspeed indicator’s diaphragm gets pressed and the needle on the indicator moves up.

Since it is connected to the static air on the other side, the indicator displays the pressure differential between the moving and static air.

The dial on the airspeed indicator is somewhat tuned to display accurate speeds when flying straight and level. Even though older models were created with miles per hour, the airspeed indicator’s dial displays airspeed in knots.

ASIs may be equipped with adjustable “bugs” that can be used to set crucial takeoff and landing speeds.


For successful flights to happen, several factors come into play and when it comes to the safety of an aircraft, airspeed plays a huge role. Just like how the speed of a car affects the safety of a driver and its passengers so does airspeed in aviation.

We have debunked airspeed and seen how pilots analyze and monitor it during flight and how critical phases such as take-off, landing and climb depend on it. Additionally, we have seen the various kinds of airspeeds and how they impact flying.

Pilots need to learn and recall the different V-speeds that we saw earlier to know when it is safe to launch the different phases of flight. The airspeed indicator makes all this easier for pilots and understanding how it works and how the readings on it come into play is key to safer flights.

James Blake

By James Blake

Does it fly? Then I am interested!

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