A vertical speed indicator (VSI) is an instrument used to indicate the rate at which aircraft climb or descend. The VSI is a static instrument that is connected to the aircraft's static air lines and static ports. These instruments are marked with a zero point which will, if correctly calibrated, indicate level flight and identical positive and negative scales above and below the zero point. A positive reading indicates and increase in altitude or climb status; a negative reading shows a descent or decrease in altitude. Due to the pressure differential nature of its operation, a VSI generally features a small response delay when initially returning accurate indication of a climb, descent, or return to level flight.
Rates of climb and descent are critical factors in the flight regimens of any aircraft; accurate, reliable indication of these trends is one of the most basic instrumentation requirements on both fixed and rotary wing types. This makes the vertical speed indicator one of the basic flight instruments found in most aircraft. Also known as a variometer or a rate of climb and descent indicator (RCDI), the VSI is a static air pressure instrument that relies on pressure differentials for its operation. The dial needle is connected via a series of linkages to a diaphragm that features an unrestricted connection to the aircraft's static air lines. The instrument casing within which the diaphragm is housed is also connected to the static air source but has a calibrated restriction in its path.
When an airplane is in level flight, there is no pressure differential between the diaphragm and casing pressure, and the vertical speed indicator should display a zero reading. If the aircraft climbs or descends, the static pressure either increases or decreases. In each case, the diaphragm and casing will react to the changes in pressure at different rates due to the restriction in the casing static line. This known and controllable pressure differential is the mechanism which allows the diaphragm to move the needle in precise increments to indicate climb and descent rates. VSI readouts are typically given in feet per minute (ft/min) units and are marked in 100-foot increments with maximum readings appropriate for the aircraft type. Although ft/min is the most widely used VSI unit, some countries use instruments marked with metric equivalents.
Although the vertical speed indicator usually reacts to changes in altitude far quicker than altimeters, it features a short time lag before it begins to accurately indicate a deviation. This delay is usually between 6 and 9 seconds and is experienced before the needle shows accurate climb or descent readings and before it returns to zero or level flight indication. Although the needle will immediately show a climb or descent trend, pilots should wait at least 10 seconds before relying on exact ft/min readings.