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Vertical Speed Indicator
From SKYbrary Wiki
A Vertical Speed Indicator (VSI), also known as a Rate of Climb and Descent Indicator (RCDI) is an instrument which indicates the rate of climb or descent of an aircraft.
The VSI uses the aircraft pitot-static system to determine the vertical speed and depicts the result on a conventional needle and circular scale instrument, or on a ribbon at the side of an Electronic Flight Instrument System EADI.
Two typical VSI indications are depicted below. The first is a conventional indication while on the second instrument, the vertical speed is indicated on the scale at the extreme right of the instrument.
In a simple VSI, a barometric capsule is contained in a sealed case. The capsule is fed with static pressure from the pitot-static system, while the case is also connected to that system through a calibrated nozzle. The nozzle restricts the passage of air so that there is a time delay between a change in static pressure and that pressure being experienced within the case. Thus, if the aircraft climbs (or descends), the pressure within the capsule will decrease (increase) while that within the case will decrease (increase) at a lower rate due to the presence of the nozzle. Movement of the capsule is translated into movement of a needle by a mechanical system.