What principle does the airspeed indicator utilize to measure speed?

The airspeed indicator measures speed by utilizing dynamic pressure, which is the pressure created by the motion of air as an aircraft moves through it, commonly known as ram air. This dynamic pressure results from the difference between the static air pressure (the ambient air pressure around the aircraft) and the total air pressure that includes the dynamic pressure caused by the aircraft's speed.

As the aircraft accelerates, the increase in dynamic pressure provides a direct indication of how fast the aircraft is moving relative to the surrounding air. The airspeed indicator converts this dynamic pressure into an indicated airspeed, which is essential for flight performance calculations, allowing pilots to understand their speed and make informed decisions regarding aircraft operation.

The other choices do not directly apply to the functioning of the airspeed indicator. Static air pressure is part of the measurement but does not measure speed on its own. Temperature variations, while they can affect air density and performance, do not factor into the direct measurement of airspeed. Density altitude calculations relate to performance criteria at different altitudes but are not used in measuring speed by the airspeed indicator.