A vertical speed indicator (VSI)
, is a type of flight instrument that measures the rate of altitude change of an aircraft. It provides the pilot with information about whether the aircraft is climbing, descending, or in level flight.
The VSI works by measuring the rate of change of pressure in the aircraft's static system. The static system is a part of the aircraft's pitot-static system, which measures air pressure at the aircraft's reference point. This reference point is typically located at the fuselage and is used as the basis for measuring altitude, airspeed, and other flight parameters.
The static pressure from the static system is fed into the VSI, which uses a differential pressure gauge to measure the difference between the static pressure and the ambient pressure. This difference is proportional to the rate of altitude change of the aircraft.
The VSI uses a needle to display the rate of altitude change on a dial. The dial typically has markings for climb and descent rates in feet per minute (fpm). A typical VSI can indicate climb or descent rates of up to 4,000 fpm.
In summary, the VSI works by measuring the difference in pressure between the static system and the ambient air, which is proportional to the rate of altitude change of the aircraft. This information is then displayed on the instrument's dial, providing the pilot with a clear indication of the aircraft's vertical motion.
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