Groundspeed and Airspeed

Airspeed Explained

Airspeed is the speed of an aircraft relative to the air mass in which it is flying. The most important types for navigation are:

• Indicated Airspeed (IAS): Shown on the cockpit instrument, affected by instrument and position errors.
• Calibrated Airspeed (CAS): IAS corrected for instrument and position errors.
• True Airspeed (TAS): CAS corrected for atmospheric pressure and temperature; represents actual speed through the air.

TAS increases with altitude for a given IAS due to decreasing air density. Pilots use a flight computer or rule of thumb (add 2% TAS per 1,000 ft increase in altitude) to convert CAS to TAS.

Groundspeed Explained

Groundspeed is the aircraft's speed relative to the Earth's surface. It is the actual rate of progress over the ground and is crucial for estimating arrival times and fuel consumption. Groundspeed is affected by wind:

• Tailwind: Increases groundspeed (GS = TAS + tailwind component)
• Headwind: Decreases groundspeed (GS = TAS - headwind component)

Calculating Groundspeed and Airspeed

• Groundspeed Calculation: Use the formula GS = TAS ± wind component (add for tailwind, subtract for headwind).
• From Fixes: If you know the distance between two points and the time taken, GS = Distance / Time.
• Mechanical Computer: Use the navigation computer (E6B or CRP-5) to solve for GS, heading, and wind correction angle (WCA).

Relationship and Practical Use

• The difference between groundspeed and airspeed becomes more significant with stronger winds or lower TAS.
• For navigation, always use GS to estimate time en route and TAS for planning air distance.
• When wind is not aligned with the track, use the triangle of velocities to resolve GS and heading.

Exam Application

• Be able to calculate average groundspeed from two fixes.
• Apply the correct formulas and understand the impact of wind.
• Know how to convert CAS to TAS and vice versa using both rule of thumb and flight computer.