Understanding All Types of Airspeed and the Functionality of the Airspeed Indicator

Airspeed, the speed at which an aircraft travels through the air, is a fundamental concept in aviation that is crucial for safe and efficient flight operations. It isn’t just a single number; it’s a spectrum of measurements that each serve different purposes during a flight. Understanding airspeed is essential for pilots to make informed decisions about their aircraft’s performance, fuel consumption, and adherence to air traffic control regulations.

In this lesson, we will delve into the different types of airspeed that pilots must understand and how the airspeed indicator, a vital cockpit instrument, displays this information.

Airspeed indicator

The airspeed indicator (ASI) is a flight instrument that measures and displays the speed of an aircraft through the air. It is a critical component of an aircraft’s pitot-static system and provides the pilot with vital information necessary to maintain control of the aircraft, especially during takeoff, cruising, and landing phases. The ASI helps in determining stall speed, safe climb rate, and the appropriate speed for executing various maneuvers.

Airspeed indicator Markings

The face of the airspeed indicator is marked with various colors and symbols, each conveying important information:

  • White Arc: This is the flap operating range, indicating the safe speed range in which it’s safe to use the flaps.
  • Green Arc: This range represents the normal operating speed of the aircraft.
  • Yellow Arc: Known as the caution range, flying within this range is safe only in smooth air conditions and with caution.
  • Red Line: This is the never-exceed speed (VNE), indicating the maximum speed at which it is safe to operate the aircraft.
Airspeed Indicator (ASI)

Airspeed Indicator Errors

The airspeed indicator, although reliable, is subject to several types of errors:

Position Error

Position error in aviation refers to the discrepancy between the airspeed read by the airspeed indicator (ASI) and the true airspeed of the aircraft due to the placement and design of the pitot tube and static ports. This error is caused by the aerodynamic effects that alter the local air pressure at the point where the airspeed measurement instruments are located.

Density error 

Density error is another type of discrepancy that can affect the airspeed indicator (ASI). It is associated with changes in air density, which in turn are related to altitude and temperature variations. Since the ASI measures dynamic pressure to determine airspeed, any change in air density can lead to incorrect speed readings.

Density error occurs because the airspeed indicator is calibrated for a specific air density — usually at sea level in standard atmospheric conditions (15°C and 29.92 inch of mercury). When the actual air density deviates from these standard conditions, the ASI readings will no longer be accurate because:

  • At Higher Altitudes: The air is less dense, which means that for the same amount of air molecules hitting the pitot tube, the aircraft is actually moving faster than the ASI indicates.
  • In Warmer Temperatures: The air expands and becomes less dense, again leading to the aircraft moving faster than the ASI shows.
  • In Colder Temperatures: The air is denser, and the ASI will indicate a speed that’s higher than the true airspeed of the aircraft.
Correcting for Density Error

Pilots correct for density error by converting indicated airspeed (IAS) to true airspeed (TAS). This is typically done using a flight computer, an E6B manual flight computer, or an onboard air data computer, which takes into account the altitude and temperature. 

Lag error

Lag error is a discrepancy that occurs when there is a delay between the actual change in airspeed and the time it takes for the airspeed indicator (ASI) to reflect this change. It is most noticeable during rapid changes in airspeed, such as during abrupt power changes, steep climbs, or descents.

Causes of Lag Error
  • Mechanical Inertia: The internal components of the ASI, particularly the aneroid wafer, have a physical mass that requires time to react to changes in air pressure.
  • Fluid Dynamics: Changes in airspeed result in changes in dynamic pressure, which take time to stabilize within the pitot-static system.
  • Rapid Maneuvering: During aggressive flight maneuvers, the dynamic pressure changes so quickly that the ASI cannot keep up instantaneously.
Effects of Lag Error
  • Delayed Indications: The ASI will show an increase or decrease in airspeed a few moments after the aircraft has already accelerated or decelerated.
  • Misleading Readings: During rapid acceleration, the ASI will initially lag and show a speed less than the actual airspeed. Conversely, during rapid deceleration, the ASI will show a speed higher than the actual airspeed for a short period.
  • Stall Awareness: Lag error can be particularly dangerous during slow flight or when operating near the aircraft’s stall speed, as the ASI might not display a decrease in airspeed promptly.

Icing error

Icing error refers to the malfunction or incorrect readings of the airspeed indicator (ASI) and other pitot-static instruments due to the formation of ice on the pitot tube, static ports, or both. This is a critical issue in aviation as it can lead to a loss of reliable airspeed data, which is essential for safe flight operations.

  • If the Pitot Tube is Affected by Ice:
      • Complete Blockage: If ice completely blocks the pitot tube, the ASI will not register a change in airspeed and will essentially be « frozen » at the speed at which the blockage occurred, irrespective of the actual airspeed of the aircraft.
      • Partial Blockage:  If the pitot tube is partially blocked the ASI will show :
          • In a climb, Increasing speed as the différence in pressure between the trapped air and static pressure grows.
          • In a descent , decreasing speed as the différence in pressure between the trapped air and static pressure lessens.
pitot tube partially block error airspeed
  •  If the Static Ports are Affected by Ice:
      • Complete Blockage: If ice blocks the static ports, the ASI will still operate but it will not accurately indicate climbs or descents. The airspeed reading will become unreliable, typically overreading in a climb and underreading in a descent because the trapped static pressure does not reflect the changing ambient pressure.
      • Partial Blockage: A partial blockage can result in erratic or inaccurate airspeed readings :
          • The airspeed indicates lower than the actual airspeed when the aircraft is climbing.
          • The airspeed indicates higher than the actual airspeed when the aircraft is descending.

Water error

Water error in aviation occurs when water enters and obstructs the pitot tube or static system, which can lead to erroneous readings on the airspeed indicator (ASI).

Airspeed definitions

Each type of airspeed provides pilots with different insights into flight performance. Let’s break them down:

Indicated Airspeed (IAS)

Indicated Airspeed (IAS) is the speed read directly from the airspeed indicator (ASI) without any corrections for temperature or air density variations. It’s the most immediate form of airspeed measurement available to a pilot and is crucial for safe aircraft operation, particularly when adhering to stall speeds, maneuvering speeds, and other operational limits.

IAS is significant because it represents the dynamic pressure on the aircraft’s wings, which is directly related to lift. It’s what the pilot uses to gauge takeoff and landing speeds, to comply with speed restrictions, and to ensure the aircraft is flown within its structural speed limitations, known as V-speeds.

Calibrated Airspeed (CAS)

CAS is the indicated airspeed corrected for instrument and installation errors in the pitot-static system. Changes in the airplane’s flight attitude or configuration alter the airflow near the static pressure source, leading to inaccurate airspeed readings. The system’s pitot section can be inaccurate at high angles of attack because the impact pressure is decreased when the pitot intake is not aligned with the airflow. The airplane flight manual typically uses calibrated airspeed for performance data.

CAS is a more accurate representation of the aircraft’s airspeed than IAS, especially important when flying at lower speeds. It’s used for aircraft performance calculations, such as determining the takeoff, landing distances, and climb rates, because it closely relates to the aerodynamic performance of the aircraft.

How to Calculate CAS

To obtain CAS from IAS, pilots use a correction chart specific to their aircraft. This chart accounts for the pressure errors caused by the airflow around the aircraft’s fuselage and wings, which can distort the readings from the pitot tube and static ports. Modern aircraft with digital flight decks often calculate CAS automatically and display it to the pilot.

indicated to calibrated airspeed conversion chart

Equivalent Airspeed (EAS)

The EAS refers to the calibrated airspeed adjusted for the compressibility effects. This measurement is particularly importantl for pilots flying high-speed aircraft, but it’s less significant for those flying at speeds less than 250 knots and at altitudes under 10,000 feet.

True Airspeed

True Airspeed is the calibrated airspeed adjusted for errors in the airspeed indicator due to air density and temperature. Understanding and accurately calculating TAS is vital for flight planning. It ensures that a pilot can accurately estimate the time to reach a destination, which is crucial for flight schedules, fuel planning, and ensuring compliance with air traffic control clearances.

Factors Affecting TAS

  • Altitude: As altitude increases, air density decreases, which can cause an increase in TAS for a given Indicated Airspeed (IAS).
  • Temperature: Air temperature also impacts air density. Warmer air is less dense, thus for a given IAS, TAS would be higher in warmer conditions.
  • Pressure: Changes in atmospheric pressure can affect TAS, as pressure influences air density.

Calculating TAS

TAS can be calculated using the following methods:

  • Pilot’s Flight Computer: Also known as an E6B flight computer, this analog device can help pilots calculate TAS manually.
  • Electronic Flight Instrument Systems: Modern aircraft are equipped with systems that automatically calculate TAS and display it to the pilot.
  • TAS Formula: TAS can be calculated using the IAS corrected for air density, which is a function of altitude and temperature.

To calculate the True Airspeed (TAS) using a rule of thumb, follow this method:

Add 2% to your Indicated Airspeed (IAS) for every 1,000 feet of altitude to approximate the TAS. For instance, if you’re flying at an indicated airspeed of 150 knots at 10,000 feet, you would add 20% to the IAS, giving you an estimated TAS of 180 knots.

Ground Speed

Ground Speed (GS) is the speed of an aircraft relative to the surface of the earth. It’s the speed that determines how quickly an aircraft will reach its destination and is used for flight planning, navigation, and fuel burn calculations. that is to say, It’s the TAS adjusted for wind speed and direction.

Factors Affecting Groundspeed

  • Wind: The most significant factor that affects GS is wind. A tailwind increases GS, while a headwind decreases it.
  • Aircraft Performance: Aircraft engine output and aerodynamic efficiency also play roles in achieving the desired GS.
ground speed

Groundspeed in Use

A pilot must be aware of the GS during all phases of flight. For example, when approaching an airport, knowing the GS helps the pilot to time their descent and approach. When en route, GS informs whether the pilot is on schedule or needs to adjust altitude or power settings to correct the timing.

Understanding GS is also fundamental for handling emergency situations. For instance, if a pilot encounters unexpected headwinds and the GS drops, they may need to take action to conserve fuel or even divert to an alternate airport if the destination can no longer be reached with the available fuel.

Practical Example

Consider a scenario where a pilot is flying from City A to City B, which is 300 nautical miles away. The aircraft’s TAS is 120 knots, and there is a 20-knot headwind. The GS would be 100 knots, and the flight would take 3 hours.

  • If the wind changes to a 20-knot tailwind, the GS increases to 140 knots, reducing the flight time to approximately 2 hours and 9 minutes.

This example illustrates the significance of GS in flight operations.

Mach Number

Mach Number is a dimensionless unit used in aviation to express the speed of an aircraft relative to the speed of sound in the surrounding medium (usually air). It’s a critical parameter for high-speed flight and is particularly relevant in the transonic and supersonic flight regimes. Here’s a detailed look at Mach Number and its significance in aviation:

  • Definition of Mach Number
      • Mach 1: Defined as the speed of sound, which is approximately 660 knots (759mph or 1,222 km/h) at sea level at 59°F (15°C) but decreases with altitude due to lower temperatures.
      • Mach Number: The ratio of the aircraft’s true airspeed to the speed of sound in the air at that altitude. For instance, Mach 2 means the aircraft is flying at twice the speed of sound.
  • Measurement of Mach Number
      • Machmeter: An instrument in an aircraft that measures and displays the Mach Number. It typically uses pitot-static system inputs and outside air temperature to calculate the speed of sound and the aircraft’s Mach Number.
  • Importance of Mach Number
      • High-Altitude Flight: As an aircraft ascends, the air density decreases, and the speed of sound decreases. At high altitudes, it’s more practical to refer to Mach Number rather than airspeed because of the changing relationship between indicated airspeed and true airspeed.
      • Aerodynamic Considerations: At speeds close to and exceeding the speed of sound, shock waves form on the aircraft. This can lead to phenomena such as Mach tuck and buffeting. Pilots use Mach Number to avoid these critical flight regimes.
      • Performance Envelope: Military and some commercial aircraft have a specified Mach operating range, which ensures the aircraft performs optimally while maintaining structural integrity.
  • Mach Effects on Aircraft
      • Transonic Effects: As the aircraft approaches Mach 1, some parts of the aircraft may experience airflow at supersonic speeds, leading to instability and control issues.
      • Supersonic Flight: Beyond Mach 1, aircraft must be specifically designed to handle the increased pressure and temperature associated with supersonic flight.
  • Practical Application
      • Commercial Jets: Most fly at a cruise speed of around Mach 0.78 to Mach 0.85.
      • Supersonic Aircraft: Military fighters and the now-retired Concorde would cruise at speeds greater than Mach 1, often around Mach 2.
  • Critical Mach Number
      • Critical Mach Number (Mcr): The lowest Mach Number at which the airflow over any part of the aircraft reaches the speed of sound and produces a shockwave. Exceeding Mcr can lead to loss of lift and increased drag.

The Mach Number is a crucial parameter for flight safety and efficiency, especially in high-speed and high-altitude flight operations. It helps in defining the flight envelope of the aircraft and ensuring that the speeds flown are within the structural and aerodynamic limits of the aircraft.

The triangle of velocities

The Triangle of Velocities, often referred to as the Navigation or Wind Triangle, is a graphical method used by pilots to solve navigation problems related to airspeed and wind effect. It’s a tool that combines the aircraft’s True Airspeed (TAS), the wind’s speed and direction, and the desired course to find the Groundspeed (GS) and the True Course (TC) to be flown. This concept is fundamental in dead reckoning and flight planning.

  • Components of the Triangle of Velocities
      • True Airspeed (TAS): The speed of the aircraft through the air, which forms one side of the triangle.
      • Wind Speed and Direction: Represented by the wind vector, which will either add or subtract from the TAS vector, depending on if it’s a headwind or tailwind.
      • Desired Track or True Course (TC): The intended direction of flight over the ground, which is another side of the triangle.
      • Groundspeed (GS): The resulting speed over the ground, which is the base of the triangle.
  • Constructing the Triangle of Velocities
      • Drawing TAS Vector: Start with the known TAS of the aircraft and draw this as a vector on a navigation plotter or a flight computer. This vector points in the direction of the intended True Course (TC).
      • Adding Wind Vector: The wind vector is then plotted from the end of the TAS vector. Its direction is opposite to the wind direction (since it represents where the wind is coming from), and its length is proportional to the wind speed.
      • Resulting Groundspeed and Heading: The final side of the triangle is drawn from the starting point of the TAS vector to the tip of the wind vector. This side represents the required heading—the True Heading (TH)—that the pilot needs to fly to counteract the wind. The length of this line represents the Groundspeed (GS).

For instance, if a pilot plans to fly east (090°) with a TAS of 100 knots and there’s a north (360°) wind blowing at 20 knots, the wind triangle will help determine the TH and the GS.

  • The blue vector represents the aircraft’s TAS heading east (TC of 090°).
  • The green vector shows the 20-knot wind coming from the north.
  • The red vector is the resultant vector that represents the Groundspeed (GS) and the direction of the True Heading (TH) that the pilot must fly to counteract the wind drift.
triangle of velocities

Based on the triangle created:

  • The True Heading (TH) that the pilot needs to fly to compensate for the wind and maintain the intended easterly course is approximately 78.69°.
  • The Groundspeed (GS), or the speed over the ground considering the wind, is approximately 101.98 knots.

The actual True Heading is slightly south of east due to the need to counteract the north wind. The Groundspeed is slightly higher than the TAS because the wind is adding a component of speed to the eastward flight path. ​

Vertical Speed

Vertical speed, in aviation, refers to the rate at which an aircraft ascends or descends, usually expressed in feet per minute (fpm). This rate of climb or descent is an important parameter for pilots during various phases of flight, particularly when conforming to air traffic control instructions or navigating through different airspace layers.

Vertical Speed Indicator (VSI)

The primary instrument used to measure vertical speed is the Vertical Speed Indicator (VSI), also known as a variometer or rate-of-climb indicator. The VSI provides a real-time indication of whether the aircraft is climbing, descending, or in level flight. Here’s how it works:

  • Pressure Sensing: The VSI is connected to the static pressure system of the aircraft. It measures the rate of change of atmospheric pressure as the aircraft changes altitude.
  • Delayed Response: There’s a calibrated leak in the instrument that causes a slight delay in the instrument’s response, which helps to dampen out any short-term changes or turbulence.
  • Display: The VSI displays the rate of climb or descent with a needle moving against a scale.
Vertical Speed Indicator

Importance of monitoring vertical speed

  • Climb and Descent Phases: Pilots closely monitor the VSI during climbs and descents to maintain desired climb rates for efficiency and comfort, and to ensure compliance with altitude restrictions.
  • Approach and Landing: During approach, vertical speed is crucial for following the glideslope or for step-down fixes in a non-precision approach.
  • Terrain Avoidance: It is also vital for avoiding terrain, especially in mountainous areas or when flying in cloudy or foggy conditions.

Vertical speed in flight planning and execution

  • Climb Rate: Aircraft performance charts often specify climb rates to reach a certain altitude efficiently. These rates can vary depending on the aircraft weight, atmospheric conditions, and engine performance.
  • Descent Planning: Pilots plan descents by calculating a descent rate that allows for a gradual approach to the destination airport, factoring in the need to slow down and configure the aircraft for landing.
  • Altitude Change Restrictions: ATC may issue altitude restrictions that require a specific climb or descent rate. Pilots use the VSI to ensure they meet these restrictions.

Practical Example

Suppose a pilot is instructed by ATC to climb to 10,000 feet at a rate of 500 fpm. The pilot adjusts the aircraft’s pitch and power to achieve this rate of climb. The VSI provides immediate feedback on the climb rate, and the pilot uses this information to maintain the required ascent until the altitude is reached.

Practical applications of airspeed knowledge in flight

Understanding the different types of airspeed is more than an academic exercise; it has practical applications that are critical to the safety and efficiency of flight operations. Here’s how knowledge of airspeed is applied in the cockpit:

  1. Takeoff and Landing: Pilots rely on indicated airspeed (IAS) to ensure they are within the safe speed ranges for takeoff and landing. For instance, there is a specific speed known as Vr (rotation speed) at which pilots must lift the nose of the aircraft for takeoff. Similarly, approach and touchdown speeds are critical for a safe landing.
  2. Cruise Performance: True airspeed (TAS) is vital for flight planning and fuel burn calculations during the cruise phase of flight. Knowing TAS helps pilots to accurately estimate arrival times and manage fuel reserves.
  3. Navigation: Groundspeed (GS) is the actual speed at which an aircraft moves over the ground. It is critical for navigation, especially when flying in strong wind conditions. Pilots use GS to adjust their course and ensure they reach their waypoints as planned.
  4. Aircraft Limitations: Every aircraft has certain speed limitations, such as never exceed speed (Vne) and maximum operating speed (Vmo). These limitations are often expressed in terms of calibrated airspeed (CAS), which is IAS corrected for instrument and position errors.
  5. Altitude Changes: Equivalent airspeed (EAS) is used to understand the aircraft’s aerodynamic performance at different altitudes. As air density decreases with altitude, EAS helps pilots to maintain the correct aerodynamic speeds for maneuvers, regardless of the actual TAS.
  6. High-Speed Flight: For aircraft that operate at very high altitudes and speeds, the Mach number (the ratio of the aircraft’s speed to the speed of sound) becomes important. It is crucial for avoiding phenomena like Mach tuck or shock stall, which can occur near the speed of sound.
  7. Weather Conditions: Knowledge of the various airspeeds allows pilots to anticipate and respond to changing weather conditions. For example, understanding TAS in relation to wind speed and direction helps in making corrections for wind drift.
  8. Air Traffic Control: Air traffic controllers give speed instructions in IAS to ensure safe separation between aircraft. Pilots must be able to quickly convert these instructions into the appropriate speed type for their current flight situation.

Conclusion and key takeaways

As aviators, our command over the concept of airspeed and the proficient use of the airspeed indicator is crucial for the conduct of safe and efficient flight operations. We’ve navigated through the spectrum of airspeed measurements – Indicated Airspeed (IAS), True Airspeed (TAS), Groundspeed (GS), Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), and Mach Number – understanding that each type has its unique application and significance in a pilot’s decision-making process.

Beyond these, the notion of vertical speed must also be acknowledged. While not a type of airspeed, the vertical speed indicator (VSI) complements the airspeed indicator by providing the rate of climb or descent, which is integral to altitude management and further enhances situational awareness.

The airspeed indicator is more than just a gauge—it is an indispensable instrument that informs the pilot about the aircraft’s aerodynamic health, especially when visual cues are lacking. Its readings are essential for avoiding critical flight envelopes such as stalls or excessive speed conditions.

Here are the key takeaways from our exploration:

  1. Comprehensive Airspeed Understanding: Mastery of the different types of airspeed and their applications is essential for navigation, aircraft control, and effective flight planning.
  2. Airspeed Indicator Significance: This instrument is not just about velocity; it is a direct indicator of the aircraft’s performance, significantly influencing safety and operational efficiency.
  3. Vertical Speed Consideration: The vertical speed indicator provides critical information about the aircraft’s altitude change rate, essential for climb and descent performance and for ensuring adherence to assigned altitudes.
  4. Practical Application: The knowledge of airspeed types, including vertical speed, is integral to every facet of flying, from pre-flight planning to in-flight adjustments for wind conditions and air traffic control directives.
  5. Correcting Misconceptions: Pilots must recognize the differences between groundspeed and airspeed, and understand that groundspeed does not necessarily reflect aircraft performance but rather the combined effect of airspeed and wind.

In closing, the airspeed indicator, along with the vertical speed indicator, is not just an instrument; it’s a critical reference that reflects the dynamics of flight. A pilot’s ability to accurately read and interpret these gauges is a testament to their proficiency and their dedication to aviation safety. With continuous learning and an in-depth understanding of the instruments at hand, pilots can ensure they are always one step ahead in the cockpit.

Typical exam questions

1 :What does the Airspeed Indicator (ASI) measure?

    • A) The speed of the aircraft relative to the ground
    • B) The speed of the aircraft relative to the surrounding air
    • C) The rate of change of altitude of the aircraft
    • D) The speed of the wind around the aircraft

2 : Which type of airspeed indicates the aircraft’s speed over the ground?

    • A) Indicated Airspeed (IAS)
    • B) Calibrated Airspeed (CAS)
    • C) True Airspeed (TAS)
    • D) Groundspeed (GS)

3 : What does the True Airspeed (TAS) take into account that the Indicated Airspeed (IAS) does not?

    • A) Wind speed
    • B) Air temperature and pressure altitude
    • C) Aircraft weight
    • D) Engine power setting

4 : What instrument would you use to determine your rate of climb or descent?

    • A) Altimeter
    • B) Vertical Speed Indicator (VSI)
    • C) Airspeed Indicator (ASI)
  • D) Attitude Indicator

5 : If an aircraft is in a steady climb with a constant airspeed, what will happen to the groundspeed if a headwind starts to increase?

    • A) It will increase.
    • B) It will decrease.
    • C) It will not change.
    • D) It cannot be determined with the given information.

6 : Which airspeed is used to ensure the aircraft does not exceed its structural limits?

    • A) Indicated Airspeed (IAS)
    • B) True Airspeed (TAS)
    • C) Groundspeed (GS)
    • D) Equivalent Airspeed (EAS)

7 : During a flight, if the outside air temperature increases at altitude, what is the effect on the True Airspeed (TAS) if the Indicated Airspeed (IAS) is kept constant?

    • A) TAS will increase.
    • B) TAS will decrease.
    • C) TAS will remain the same.
    • D) TAS will fluctuate unpredictably.

8 : What is the primary purpose of the Equivalent Airspeed (EAS)?

    • A) To measure the speed over the ground
    • B) To indicate the aerodynamic performance of the aircraft
    • C) To reflect the noise level generated by the aircraft
    • D) To measure the speed of the air coming into the engine.

Answer :

1 : Correct Answer: B
2 : Correct Answer: D
3 : Correct Answer: B
4 : Correct Answer: B
5 : Correct Answer: B
6 : Correct Answer: A
7 : Correct Answer: A
8 : Correct Answer: B

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