What is the primary bank instrument used to maintain heading in level flight?

The primary bank instrument used to maintain heading in level flight is the heading indicator. This instrument displays the aircraft's current heading in degrees, allowing pilots to maintain a specific course. When flying level, it is essential for the pilot to keep the aircraft on the desired heading, and the heading indicator provides a clear reference for this purpose.

The heading indicator is particularly important because it helps pilots make the necessary adjustments to ensure that the aircraft remains on track, countering any drifting that may occur due to wind or other factors. This instrument is often used in conjunction with other navigational aids, but its role in providing a direct indication of heading is vital for effective flight management.

Other instruments, such as the turn coordinator or the attitude indicator, offer valuable information regarding the aircraft's orientation and rate of turn, but they do not specifically indicate the aircraft's heading. The model altimeter measures altitude rather than heading, further distinguishing its function from that of the heading indicator. As such, the heading indicator's specific design and functionality make it the correct answer for maintaining heading in level flight.