Vertical Speed Indicator (VSI), also known as the Rate of Climb and Descent Indicator (RCDI), is an instrument that 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.
- When the aircraft climbs, the static pressure outside the aircraft decreases. This lower pressure air flows through the static line and the pressure in the diaphragm decreases. The diaphragm collapses down on itself due to the pressure differential. This movement turns the rods and gears, moving the needle on the face of the VSI 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 the 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. The movement of the capsule is translated into the movement of a needle by a mechanical system.