What is the function of the "vertical speed indicator" (VSI)?

The vertical speed indicator (VSI) serves a crucial role in monitoring an aircraft's vertical movement. Its primary function is to show the rate of ascent or descent in feet per minute. This instrument provides pilots with real-time feedback on how quickly the aircraft is climbing or descending, which is vital for maintaining safe flight operations, especially during approaches and departures.

By indicating whether the aircraft is gaining or losing altitude and at what rate, the VSI helps pilots make informed decisions regarding their flight path and approach to landing. Accurate assessment of vertical speed is particularly important in mountainous or other challenging terrain, as well as in ensuring optimal performance during takeoff and landing phases.

The other options do not align with the VSI's purpose. For instance, measuring fuel consumption is done by other gauges, altitude is indicated by the altimeter, and tracking the aircraft's speed falls under the airspeed indicator's function. Thus, understanding the VSI's specific purpose helps pilots effectively manage vertical navigation during their flights.