What instrument is primarily used to maintain level flight?

The attitude indicator is the primary instrument used to maintain level flight because it directly displays the aircraft's orientation relative to the horizon. By indicating whether the wings are level and if the aircraft is in a climb, descent, or level attitude, the pilot can make the necessary adjustments to maintain straight and level flight. This instrument shows the aircraft's pitch and roll, allowing the pilot to ensure that the plane is not climbing or descending without desired control.

In contrast, while the vertical speed indicator provides information on whether the aircraft is climbing or descending, it does not provide direct information on the aircraft's bank or levelness in pitch. The heading indicator reflects the direction the aircraft is facing but does not provide information on altitude or level flight. The airspeed indicator displays the current speed of the aircraft, which is important for performance but does not indicate level flight status. Thus, the attitude indicator is the most critical instrument for maintaining level flight.