Airspeed and Groundspeed in Flight Navigation

In the world of aviation, “speed” is not a single number as it is in a car. For an aircraft to fly safely and arrive on time, pilots must manage and calculate different types of speeds, taking into account factors such as altitude, temperature, and wind. Below, we explain these fundamental concepts.

Types of Speed

To understand how an aircraft moves and how the instruments react, we must distinguish between five key definitions:

1. Indicated Airspeed (IAS): The direct, uncorrected reading from the instrument (airspeed indicator).
2. Calibrated Airspeed (CAS): IAS corrected for instrument and installation errors. It measures dynamic pressure, that is, the force with which the air strikes the aircraft. This is the critical speed for aircraft control (such as avoiding a stall).
3. Equivalent Airspeed (EAS): CAS corrected for air compressibility. At high speeds and high altitudes, the air compresses in front of the pitot tube, distorting the reading. EAS is fundamental for high-speed engineering and aerodynamic calculations.
4. True Airspeed (TAS): The aircraft’s true speed through the air mass (EAS corrected for density). As we climb, the air becomes less dense. To maintain the same dynamic pressure (CAS/EAS), the aircraft must physically fly faster. This is why, at altitude, TAS is significantly higher than CAS.
5. Ground Speed (GS): Speed over the ground. It is the result of taking TAS and adding or subtracting the effect of the wind. This is the only speed used to determine how long it will take to reach the destination.

In addition, there is the Mach Number, which is the ratio between the aircraft’s speed (TAS) and the local speed of sound (LSS). It is the primary reference for high-altitude flight.

Relationship Between Altitude, Temperature, and Speed

The environment changes as the aircraft climbs, affecting speeds in several ways:

• The 2% Rule: As a quick general rule, TAS increases by approximately 2% relative to CAS for every 1,000 feet of altitude. For example, at 10,000 feet, your true speed is about 20% higher than what the indicator shows.
• Temperature and Sound: The speed of sound (LSS) depends solely on absolute temperature (Kelvin). Sound travels more slowly in cold air. The formula is LSS = 38.95 × √Temperature(K). Since it gets colder at higher altitudes, the speed of sound decreases, causing the Mach Number to increase even if TAS remains constant.

Behavior in Climbs and Descents (ECTM Charts)

There is a strict relationship between EAS, CAS, TAS, and Mach when changing flight levels. A useful tool is the ECTM charts (EAS, CAS, TAS, Mach):

• When climbing while maintaining a constant CAS, both TAS and Mach increase.
• When climbing while maintaining a constant Mach, both TAS and CAS decrease.
• When descending at constant Mach, both TAS and CAS increase.

The Effect of Wind

The air mass in which the aircraft flies moves relative to the ground:

• Headwind: Reduces speed. GS = TAS − Wind.
• Tailwind: Increases speed. GS = TAS + Wind.
• Crosswind: Pushes the aircraft sideways.

To fly a straight line over the ground (Track), the pilot must point the nose of the aircraft (Heading) into the wind to counteract the lateral push. The angular difference between Heading and Track is called Drift.

Practical In-Flight Calculations

Pilots use tools such as the flight computer to solve mathematical problems in the air:

• Distance and Time: The basic formula is Speed = Distance / Time. If we arrive late at a checkpoint, we must increase Ground Speed to recover lost time.
• Rate of Descent: To descend comfortably toward the runway (usually on a 3° path), a mental rule is used: Multiply your Ground Speed by 5. If you are flying at 120 knots over the ground, you should descend at 600 feet per minute.
• Fuel Management: Fuel is consumed per hour of flight. If we have a strong headwind, our GS decreases, it takes longer to arrive, and therefore we need more fuel to cover the same distance.