What instrument is powered by a vacuum system?

The correct answer is the attitude indicator, which is powered by a vacuum system because it relies on the differential pressure created by a vacuum to operate its gyroscopic components. The attitude indicator displays the aircraft's orientation relative to the horizon by utilizing a gyroscope that maintains its plane of rotation regardless of changes in the aircraft's position, thus indicating whether the aircraft is climbing, descending, or in a level flight.

In contrast, other instruments like the Horizontal Situation Indicator (HSI), Turn Coordinator, and Airspeed Indicator operate using different principles. The HSI primarily relies on electrical input and navigational data rather than a vacuum system. The Turn Coordinator uses a small electric gyroscope to show the rate of turn and coordination of the aircraft. Meanwhile, the Airspeed Indicator measures dynamic pressure and does not require a vacuum for its function. Understanding the operational principles of these instruments is critical for effectively interpreting their readings during flight.