What is the basic principle behind the airspeed indicator?

• A. The airspeed indicator measures the difference between total pressure and static pressure (dynamic pressure) to display airspeed.
• B. The airspeed indicator measures static pressure only.
• C. It measures temperature to determine speed.
• D. It uses GPS to compute speed.

Answer: (A)

The airspeed indicator works on dynamic pressure. It uses a pitot tube to capture total (ram) pressure from the air as you move, and a static port to sense the ambient static pressure. The instrument displays airspeed based on the difference between these two pressures. That pressure difference, known as dynamic pressure, rises with the square of your speed and is translated inside the instrument into the indicated airspeed. It’s calibrated for standard atmospheric density, so the reading reflects speed at sea level density, with adjustments needed for density changes at altitude. Temperature or GPS don’t determine the reading—the essential input is the pressure difference between total and static pressure.