What is Airspeed

Airspeed is the speed of the aircraft relative to the surrounding air mass. It differs from ground speed, which measures how fast the aircraft moves over the Earth’s surface. Airspeed is an important measurement for pilots because it directly affects aircraft performance, lift, and drag.

Different Types of Airspeed

A pilot must understand the different types of airspeed to ensure safe flight. Each type serves a specific purpose depending on factors like altitude, wind conditions, and the phase of flight.

Let’s dive into five different types of airspeed: Indicated Airspeed (IAS), Calibrated Airspeed (CAS), Equivalent Airspeed (EAS), True Airspeed (TAS), and Ground Speed (GS).

1. Indicated Airspeed (IAS)

Indicated airspeed is what you see on the cockpit’s airspeed indicator. It measures dynamic air pressure, but doesn’t account for changes in air density. IAS is crucial for aircraft control and is most accurate at lower altitudes where air density remains constant.

Use Case: Pilots rely on IAS for basic flight operations such as takeoff, landing, and to ensure they are within speed limits set by the aircraft’s design.

2. Calibrated Airspeed (CAS)

CAS is IAS corrected for instrument and position errors. The pitot-static system on an aircraft can sometimes give slightly inaccurate readings due to airflow disruptions around the sensors.

Use Case: CAS becomes relevant in performance calculations, ensuring more accurate navigation and aircraft handling by factoring in instrumentation quirks.

3. Equivalent Airspeed (EAS)

EAS corrects CAS for compressibility effects at higher speeds and altitudes. This is particularly relevant when the aircraft approaches the speed of sound or flies at higher altitudes, where the air becomes thinner and compressibility becomes a factor.

Use Case: Primarily used in high-speed flight, typically in military or commercial jet operations above 10,000 feet.

4. True Airspeed (TAS)

TAS reflects the aircraft’s actual speed through the air, accounting for changes in air density at altitude. TAS increases with altitude because the aircraft encounters fewer air molecules as it ascends.

Use Case: Pilots use TAS for navigation, particularly at higher altitudes, to ensure they are on track and flying efficiently, particularly on long-haul flights.

5. Ground Speed (GS)

Ground speed is the actual speed over the ground and is influenced by wind. It’s the sum of the true airspeed plus or minus the wind speed. A tailwind increases ground speed, while a headwind decreases it.

Use Case: GS is critical for navigation, especially in flight planning. It determines how long it will take to reach a destination and fuel consumption rates.

What is Mach Number (M)

Mach number is a fundamental concept in aviation that directly relates to TAS. It provides a valuable metric for understanding aircraft performance, limitations, and the effects of compressibility at high speeds.

The Mach Number (M) is the ratio of an object’s speed to the speed of sound in the surrounding medium. It’s used to describe the velocity of an aircraft relative to the speed of sound, where:

  • Mach 1 represents the speed of sound (approximately 767 mph or 1,235 km/h at sea level in standard conditions).
  • Mach < 1 means subsonic flight.
  • Mach 1 – Mach 5 denotes supersonic flight.
  • Mach > 5 indicates hypersonic flight.

Mach numbers vary with altitude, as the speed of sound changes with temperature and air density.

When discussing true airspeed, especially in relation to high-speed flight, Mach number becomes a relevant and important parameter. It provides a standardized way to compare aircraft speeds across different altitudes and conditions. This is because Mach number is a dimensionless quantity that represents the ratio of the speed of an object to the speed of sound in the surrounding medium.

Airspeed Indicator (ASI)

An Airspeed Indicator (ASI) is a flight instrument that measures how fast an aircraft moves through the air relative to the surrounding air. The airspeed indicator primarily shows indicated airspeed (IAS).

  • How it works: The ASI compares static air pressure to ram pressure created by the aircraft’s forward motion. The difference is indicated on the ASI dial.
  • What it measures: Typically measured in knots, but can also be shown in mph, km/h, or m/s.
  • What it’s used for: Guides pilots during climbs, descents, landings, and aerobatic maneuvers.
  • How it’s displayed: Analog gauges use a moving pointer; modern aircraft use a digital speed tape.

Airspeed Indicator (ASI) Errors

The Airspeed Indicator (ASI) can be subject to several errors, which may affect its accuracy:

  • Position Error: Caused by airflow disturbances near the pitot tube or static port due to aircraft design, leading to inaccurate pressure readings.
  • Instrument Error: Mechanical imperfections in the ASI itself can lead to minor inaccuracies.
  • Density Error: As air density decreases with altitude, the ASI may underreport actual airspeed since it doesn’t account for changes in air pressure.
  • Compressibility Error: At higher speeds, air becomes compressible, which can result in errors, particularly in high-speed flight or near Mach 1.

Pilots need to be aware of these errors and make corrections based on altitude and other factors to ensure accurate speed readings. Pilots can correct airspeed indicator errors using the following methods:

  • Position Error Correction (PEC): Pilots consult performance charts in the aircraft’s manual, which account for specific position errors at various speeds and configurations.
  • Density Correction: By converting Indicated Airspeed (IAS) to True Airspeed (TAS) using altitude and temperature data, pilots can adjust for density errors.
  • Calibrated Airspeed (CAS): Pilots use calibration charts or formulas to correct IAS for instrument and position errors.

Thumb Rule to Estimate TAS

There is a thumb rule to estimate True Airspeed (TAS) based on Indicated Airspeed (IAS) and altitude. This rule of thumb states that for every 1,000 feet of altitude above sea level, TAS increases by approximately 2% of IAS.

Example: If your IAS is 100 knots at 5,000 feet, your estimated TAS would be:
100 knots + (5 * 2%) = 100 + 10 = 110 knots.

Note: This is a rough estimate and may not be perfectly accurate in all conditions, especially at higher altitudes or in areas with significant temperature variations. For more precise calculations, pilots typically use flight planning software or specialized calculators that take into account factors such as temperature, pressure, and humidity. Additionally, it’s important to remember that TAS is affected by wind speed and direction. A headwind will decrease groundspeed, while a tailwind will increase it.

Airspeed Conversion Calculator • True Airspeed Calculator • Indicated Airspeed Calculator

Why Understanding These Speeds Matters

Each airspeed type serves different phases of flight. For example, IAS is crucial for ensuring safe takeoffs and landings, TAS is essential for efficient cruise at altitude, and GS is key for navigation and estimating arrival times.

Practical Example: Imagine you’re cruising at 30,000 feet with an indicated airspeed of 250 knots. Due to the thinner air, your true airspeed might be 400 knots. With a 50-knot tailwind, your ground speed would then be 450 knots.

AviationHunt Team

Our team at AviationHunt is a group of aviation experts and enthusiasts. We aim to provide aviation insights, technical notes, best aircraft maintenance practices, and aviation safety tips.