Six instruments provide pilots with information about an airplane’s motion and orientation with respect to the ground. These instruments allow them to fly safely even when they cannot see the ground or horizon.
To gather air pressure information, some instruments rely on a device called a Pitot-static tube, which is mounted on the airplane’s nose or on the leading edge of a wing.
The tube faces forward and senses the pressure of the oncoming airflow. This so-called total pressure (or ram pressure) changes with the airplane’s speed through the air. Through holes along the side of the tube, another sensor measures the local atmospheric pressure, or static pressure. Static pressure goes down as you go up. The altimeter, airspeed indicator, and vertical speed indicator all rely on the Pitot-static tube.
A gyroscope is a rapidly spinning wheel mounted on a set of swivels (gimbals) that allows it to rotate. Once set in motion, the wheel tends to maintain its orientation, regardless of how the housing in which it is mounted tilts. A gyroscope within an instrument can provide a fixed plane of reference that does not change as the airplane rolls, pitches, or yaws. The heading indicator, attitude indicator, and turn coordinator rely on gyroscopes.