Whether it be F1, NASCAR, World Rally, or MotoGP, our fascination with speed is what makes them so popular. This allure is also evident from other sporting events such as the Olympics in which we have several competitions like the 100m dash, marathons, swimming, etc.
Now, when you combine speed with flying, a very important parameter of flight is born, the airspeed.
The airspeed of an aircraft, whether military or commercial, is quite significant not just in terms of the capabilities of that aircraft, but also from a safety point of view.
If an aircraft is flying slow, the pilot must ensure that sufficient airspeed is maintained to avoid a stall. On the other hand, while going full throttle, pilots ensure that the aircraft does not exceed the maximum airspeed limits (VMO).
When we talk about the speed of cars or ground vehicles in general, we understand that it can be calculated by converting the rotational speed of its wheels. But what is airspeed and how do we measure it?
Table of Contents
What is Airspeed: How Is It Measured?
Airspeed is the speed of aircraft as it moves through air during the flight. It is calculated by converting the pressure of air being sucked into the pitot probe (See image above) to linear velocity.
In simpler terms, the greater the pressure of the air inside the pitot tube, the higher the airspeed.
The airspeed is always indicated to the pilot through either a dedicated airspeed indicator gauge or as a linear scale on Primary Flight Display (PFD).
Different types of airspeeds
But may have heard that there are different types of airspeeds; well, that is true.
There are a couple of them, and in this article, we are going to look closer at:
- Knots Indicated Airspeed,
- Knots Calibrated Airspeed
- Knots True Airspeed.
Did you notice something common between them? You guessed it! It is ‘Knots’.
Well, that is because the airspeed is always measured in knots.
Now, let us go over what these different types of airspeeds are and why are they important.
KIAS vs KCAS vs KTAS
Let’s start by comparing the three in a table, to give you a quick overview of the differences.
| Airspeed Type | What Is it? |
| KIAS (Knots Indicated Airspeed) | The speed of the airplane through the air. |
| KCAS (Knots Calibrated Airspeed) | Like KIAS, but corrected for instrumentational or positional errors |
| KTAS (Knots True Airspeed) | Airspeed corrected for changing atmospheric pressure at different altitudes |
Here is a nice video showcasing the differences between the three.
With that out of the way, let’s delve deeper into each!
1. What is Knots Indicated Airspeed (KIAS)?
The Knots Indicated Airspeed (KIAS) is the airspeed directly measured from the pitot probe and is indicated to the pilot on his airspeed display.
KIAS is a direct indication of airflow (dynamic air pressure) around the fuselage and wings of an airplane and this value, therefore, holds significance in terms of safe flying limits of airplanes.
This type of airspeed is directly indicated to pilots as all-important limits or V-speeds are identified in terms of KIAS.
The V-speeds such as V1, VR, VS, VMO, VNE are all represented in terms of indicated airspeed as they are directly related to the airflow around the structure of an aircraft.
Let us have a quick go through what important these indicated airspeeds mean:
- V1, – The speed during take-off after which take-off cannot be aborted
- VR – The rotation speed or the speed at which an airplane can lift-off
- VS, – The stall speed or minimum speed below which an aircraft does not remain controllable
- VMO – The maximum operating speed limit of an aircraft; beyond which the aircraft triggers an alarm
- VNE – This is the never-exceed-speed of an aircraft; beyond which, an aircraft may structurally disintegrate
As the take-off, stall or structural integrity of an airplane directly depends upon the airflow around the aircraft, KIAS is constantly monitored by the pilots during all phases of flight.
2. What is Knots Calibrated Airspeed (KCAS)?
The Knots Calibrated Airspeed is the airspeed measured by the pitot probe and corrected for the instrumentational or positional error of the pitot probe. In other words, it is slightly more complicated!
Well, let us make it easier to understand then!
The Pitot Tube or Pitot Probe of an aircraft is mounted on aircraft in the forward section of the fuselage in most cases and on the wings in some cases.
Although, the aircraft designers make sure that the pitot probe is positioned to ensure minimal errors due to changing flap configuration or so on, the errors still exist.
Similarly, like any other sensing device, a certain instrumentation error is also present in the pitot probe and indicating devices.
The calibrated airspeed is calculated after incorporating all such positional and instrumentational errors into the value of indicated airspeed.
Is Calibrated Airspeed Displayed to Pilots?
Although KCAS is not displayed to the pilot, it is still important for pilots to know and understand the difference is calibrated and indicated airspeed for the type of aircraft they are flying.
Mostly the difference in calibrated and indicated airspeed is negligible but in some cases, with nose-up attitude or when flaps are extended, it may increase to several knots.
Now we have seen that Calibrated Airspeed or the Indicated Airspeed are directly related to the dynamic pressure being measure by the pitot tube, but the True Airspeed is slightly different. Why don’t we have a look at what it represents!
3. What is Knots True Airspeed (KTAS)?
KTAS or Knots True Airspeed is the airspeed of the airplane relative to the (stationary) air, it is flying through.
But how is it any different to Indicated Airspeed or calibrated airspeed?
Well, as the altitude of an aircraft increases, the air around it becomes thinner. Increasing altitude, therefore, results in a decrease of dynamic pressure being measured by the pitot tube.
Think of an airplane traveling at 200knots at sea level where atmospheric pressure is 101kPA. As the altitude of the airplane increases to 15000ft, the indicated airspeed of the airplane will drop (roughly to 160knots) even if it is still traveling at a constant true airspeed of 200knots.
This is because the atmospheric pressure at this altitude has dropped to 57kPA, and less air is hitting the pitot tube.
When Is True Airspeed Equal to KIAS/KCAS?
At standard atmospheric conditions (sea level and 15 Celsius temperature) the KTAS is equivalent to KIAS or KCAS. But as the altitude of an aircraft increases, the indicated airspeed and the true airspeed of the airplane diverge.
So, at higher altitudes, even if an aircraft is flying at 200knots relative to the static air around it, the indicated airspeed may be shown as 160knots.
How is true airspeed measured?
Unlike the Calibrated Airspeed, true airspeed is mostly displayed to the pilots in modern aircraft on their Flight Displays. True airspeed is also displayed to the passengers on their cabin entertainment systems in modern commercial airliners.
But how is it measured; you may ask?
For true airspeed, we need to not only consider the dynamic air pressure, but also the static air pressure around the aircraft.
Often, we see static air pressure sensing ports on the pitot tube itself or the fuselage of the aircraft.
These sensors provide a measurement of the static air pressure outside, as the aircraft is flying.
By incorporating the dynamic pressure sensed by the pitot tube and static air pressure sensed by static ports, the aircraft calculates its true airspeed.
Summary and Conclusion
The airspeed of any aircraft holds significance both in terms of flight safety and structural integrity of any aircraft. Pilots constantly monitor the airspeed during all critical phases of flight such as take-off, climb, and landing to ensure that they do not exceed the maximum and minimum limits of airspeed.
Airspeed is commonly measured in terms of ‘knots’ which is a standard aeronautical term for airspeed. There are three distinct types of airspeed and each one holds its value to the pilots flying the aircraft.
- Indicated airspeed is the type of airspeed that is directly proportional to the airflow over any aircraft and is indicated to the pilots on their airspeed indicator. The airspeed limits are defined in terms of indicated airspeed by the manufacturer for different phases of flight and different airplane configurations.
Pilots flying the aircraft learn and remember these airspeed limitations to recall during flight. For example, during take-off, pilots have to remember the minimum airspeed required for take-off so that they don’t try to lift the aircraft prematurely. - Calibrated airspeed is similar to indicated airspeed but corrected for positional and instrumentational errors within the airspeed sensing system.
- True airspeed represents the actual speed of the aircraft compared to the stationary air around it.
Each one of these airspeeds has its utilization for a pilot. Pilots ensure safe flight by maintaining a constant awareness of airspeed and knowing the limits for the aircraft that is being flown. Safe flying!
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