Which instrument helps maintain flight level by indicating rates of ascent or descent?

The Rate of Climb Indicator is specifically designed to show the pilot the rate at which the aircraft is ascending or descending, expressed in feet per minute. This instrument plays a crucial role in maintaining a consistent flight level, as it provides real-time feedback on whether the aircraft is climbing, descending, or maintaining altitude. By monitoring this indicator, pilots can make necessary adjustments to their aircraft's pitch and power settings to ensure level flight or to achieve a desired rate of climb or descent.

In contrast, the Turn and Slip Indicator helps the pilot understand the aircraft's turn coordination rather than altitude, the Radio Magnetic Indicator is used for navigation and heading rather than vertical movement, and the Airspeed Indicator measures the aircraft's speed but does not provide information on ascent or descent rates. Thus, the Rate of Climb Indicator is the vital instrument for directly addressing flight levels through ascent and descent rates.