What type of indicator shows the relationship of ascent or descent in an aircraft?

The vertical velocity indicator is the device that specifically shows the relationship of ascent or descent in an aircraft. It measures the rate of climb or descent in feet per minute or meters per second, allowing pilots to easily gauge whether the aircraft is rising or falling and at what rate. This is crucial during takeoff, landing, and in-flight adjustments, as maintaining the desired climb or descent rate is essential for safe flight operations.

Other indicators mentioned in the options serve different purposes. A speed indicator measures how fast the aircraft is moving through the air, which is unrelated to whether the aircraft is climbing or descending. The pressure altimeter provides information on altitude based on changes in air pressure, giving a static view of height rather than the dynamic rate of change. Lastly, a fuel flow indicator monitors the fuel consumption rate of the engines, which is important for efficiency but does not inform the pilot about climb or descent rates.