Which flight instrument operates using a vacuum system?

The attitude indicator operates using a vacuum system as it relies on gyroscopic principles to provide the pilot with information about the aircraft's orientation relative to the horizon. A vacuum system spins a gyroscope, maintaining its orientation and allowing the attitude indicator to reflect the aircraft's pitch and roll. This instrument is essential for visualizing the aircraft's attitude, especially in conditions where the horizon is not visible.

In contrast, the altimeter, airspeed indicator, and turn coordinator rely on different principles. The altimeter measures altitude based on changes in atmospheric pressure, the airspeed indicator uses a pitot-static system to measure dynamic pressure, and the turn coordinator utilizes a small electrical gyroscope to indicate the rate of turn. These instruments serve different purposes and operate based on their respective physical principles rather than a vacuum system.