Which instrument is vacuum powered in a Cessna 172?

The heading indicator is a vacuum-powered instrument in a Cessna 172. Its primary function is to provide the pilot with the aircraft's direction relative to a magnetic reference. Unlike some other instruments that might rely on electrical or pitot-static systems, the heading indicator operates on the principle of gyroscopic precession, which is made possible through vacuum pressure. The vacuum system helps keep the gyroscope spinning at high speeds, allowing for stable and accurate readings of the aircraft's heading.

In contrast, the airspeed indicator, turn coordinator, and vertical speed indicator operate using different principles. The airspeed indicator uses the dynamic pressure from the pitot tube, while the turn coordinator functions on electrical power and relies on both gyroscopic action and magnetic orientation. The vertical speed indicator uses static pressure changes to determine the rate of climb or descent. Thus, the unique operation of the heading indicator makes it the only vacuum-powered instrument among the listed choices.