When flying, what is the primary instrument for maintaining altitude?

The primary instrument for maintaining altitude during flight is the altimeter. This instrument measures the aircraft's altitude above sea level by comparing the atmospheric pressure outside the aircraft to a preset standard atmospheric pressure. As the aircraft ascends or descends, changes in external air pressure are indicated on the altimeter, allowing the pilot to make necessary adjustments to maintain the desired altitude.

While the vertical speed indicator can provide helpful information about the rate of ascent or descent, it does not directly indicate the aircraft's current altitude. The artificial horizon, or attitude indicator, is crucial for understanding the aircraft’s orientation relative to the horizon, but it does not provide altitude information. The magnetic compass is important for navigation and determining direction, but it is not useful for altitude control. Therefore, the altimeter is the essential instrument for altitude maintenance.