Understanding the Airspeed Indicator: How Pilots Measure Airplane Speed.

Flying an airplane is a complex and demanding task that requires skill, knowledge, and a range of instruments to ensure safety and precision. Among these instruments, the airspeed indicator is one of the most crucial, as it provides pilots with accurate information about the airplane's speed and performance in the air. In this article, we'll explore the basics of the airspeed indicator, its uses, and its role in modern aviation.

What is an Airspeed Indicator?

An airspeed indicator, also known as an ASI, is a flight instrument that displays the airplane's speed relative to the surrounding air. It measures the dynamic pressure of the air that flows around the airplane and converts it into a speed indication, typically in knots or miles per hour (mph). The ASI consists of a sensitive diaphragm that detects the pressure difference between the static pressure (the atmospheric pressure outside the airplane) and the dynamic pressure (the air pressure generated by the airplane's motion). By comparing these pressures, the ASI determines the airplane's airspeed and displays it on a dial or digital screen.

How Does the Airspeed Indicator Work?

The basic principle behind the ASI is Bernoulli's equation, which states that the pressure of a fluid (such as air) decreases as its speed increases. In the case of an airplane, the ASI measures the difference between the static pressure, which is obtained from a port on the airplane's skin, and the dynamic pressure, which is obtained from a pitot tube that faces forward and collects the air flowing around the airplane. The difference between these two pressures is proportional to the airplane's speed and is used to move a mechanical or electronic indicator that shows the speed.

This is a very basic airspeed indicator which is used to measure an aircraft’s speed as related to air surrounding it. As that air could be moving in any direction it does not necessarily relate to the speed the aircraft is moving over the ground.

The ASI is designed to compensate for changes in air density, temperature, and altitude, which can affect the accuracy of the speed reading. For example, at high altitudes, where the air is less dense, the ASI may overestimate the airplane's speed unless it is calibrated to account for the lower pressure. Similarly, in hot weather, the air expands and becomes less dense, which can cause the ASI to indicate a lower speed than the actual one. To address these issues, the ASI may incorporate additional sensors, such as an altimeter and a thermometer, to adjust the speed reading accordingly.

Types of Airspeed Indicators

There are several types of ASIs, each with its own features and limitations. The most common types are:

The basic ASI: This type displays the indicated airspeed, which is the speed relative to the surrounding air, regardless of wind or other factors. The indicated airspeed is used as a reference for most flight operations, such as takeoff, climb, cruise, and descent.

The true airspeed (TAS) indicator: This type shows the airplane's speed relative to the air mass, taking into account the effects of wind and air density. The TAS is calculated based on the indicated airspeed and the altitude, temperature, and pressure data, which are fed into a computer or a slide rule. The TAS is important for navigation and performance calculations, such as fuel consumption, range, and time en route.

The airspeed indicator works by comparing two types of air. First the ram air which is fed by a forward facing pitot head and into a diaphragm much like a barometer. The air forced down the pitot tube increases and decreases depending on the aircraft’s forward motion through the air. Second a static air line measures the air pressure at the altitude the aircraft is flying to ensure like for like is being measured.

The calibrated airspeed (CAS) indicator: This type shows the indicated airspeed that has been corrected for instrument and position errors. The CAS is calculated based on the indicated airspeed and the airspeed calibration data, which are provided by the manufacturer or the operator. The CAS is used for aircraft certification and performance testing, as it represents the true airspeed in a standard atmosphere.

The ground speed indicator: This type displays the airplane's speed relative to the ground, taking into account the effects of wind and the airplane's heading. The ground speed is calculated based on the true airspeed and the wind speed and direction, which are obtained from the airplane's navigation system. Ground speed is useful for navigation and flight planning, such as determining the arrival time and fuel consumption.

The Airspeed Indicator in Action

The airspeed indicator is a critical instrument for pilots, as it provides them with essential information about the airplane's speed and performance. Here are some examples of how the ASI is used in various phases of flight:

Takeoff: During takeoff, the ASI is used to ensure that the airplane reaches the required speed for liftoff, known as the takeoff speed or rotation speed. This speed varies depending on the airplane's weight, runway length, and environmental conditions. The ASI may also show the speed at which the airplane should retract its flaps and gear for better aerodynamic performance.

Climb: During climb, the ASI is used to maintain the proper airspeed and climb rate, which depend on the airplane's weight, engine power, and altitude. The climb speed is typically lower than the cruise speed, as the airplane needs to gain altitude rather than speed. The ASI may also show the maximum climb speed that the airplane can achieve, known as the service ceiling.

Cruise: During cruise, the ASI is used to maintain the desired airspeed and fuel efficiency, which depend on the airplane's weight, altitude, and wind conditions. The cruise speed is typically higher than the climb speed and may vary depending on the airplane's performance goals, such as speed, range, or endurance. The ASI may also show the recommended cruise speed for optimal performance.

Descent: During descent, the ASI is used to maintain the proper airspeed and descent rate, which depend on the airplane's weight, altitude, and environmental conditions. The descent speed is typically higher than the cruise speed and may require the use of speed brakes or flaps to reduce the speed and drag. The ASI may also show the maximum descent speed that the airplane can achieve, known as the dive speed.

Landing: During landing, the ASI is used to ensure that the airplane reaches the proper approach speed, which is the speed at which the airplane should be configured for landing, such as with flaps and gear extended. The approach speed varies depending on the airplane's weight, runway length, and wind conditions. The ASI may also show the stall speed, which is the minimum speed at which the airplane can maintain lift and control.

Here on this American Airlines Boeing 777-300 we can see the forward-facing pitot head. The static air intake is the round disk to the upper right of the pitot head. In times gone by pitot heads have been located on the top of the tail or on one of the wing tips.

Conclusion

The airspeed indicator is a vital instrument for pilots, as it provides them with accurate and timely information about the airplane's speed and performance. By using the ASI, pilots can ensure that they fly the airplane safely, efficiently, and in compliance with the regulations and procedures. Modern airliners have advanced ASIs that incorporate various sensors, computer algorithms, and displays to enhance the pilot's situational awareness and decision-making. However, the ASI is still subject to errors and malfunctions, which require the pilots to cross-check the speed indications with other instruments and use their judgment and training to respond appropriately. Therefore, the airspeed indicator remains a critical component of the aviation system, and pilots must understand its principles, uses, and limitations.