Vertical speed indicatorThe Vertical Speed Indicator (VSI) is an instrument in an aircraft used to inform the pilot of his rate of descent or climb.
Like the altimeter, the VSI is a pressure instrument, and is connected to the aircraft's static port. Whereas the altimeter measures the absolute pressure, the VSI is designed to leak away pressure at a slow rate. The rate of the leak is calibrated to drive a display indicating feet per minute. Because of the way it works, the VSI is a very slow-responding instrument, taking several seconds to "catch up" with the reality of what is actually happening. It is common for beginner pilots to "chase the needle", trying to establish a steady rate of descent or climb, or simply staying at zero. This usually results in the aircraft wandering up and down in altitude - a form of pilot-induced oscillation (PIO). More experienced pilots know to use the instrument purely as a measure after establishing performance in some other manner.