The vertical speed indicator (VSI) is a static instrument that shows the rate of climb or descent of an aircraft. It relies on pressure differentials and exhibits a small response delay. VSI readings are given in units of feet per minute and have a delay of 6-9 seconds before showing accurate readings.
A vertical speed indicator (VSI) is an instrument used to indicate the speed at which the aircraft is climbing or descending. The VSI is a static instrument that is connected to the static airlines and static ports of the aircraft. These instruments are marked with a zero point which, if correctly calibrated, will indicate level flight and identical positive and negative scales above and below the zero point. A positive reading indicates and increases altitude or climb status; a negative reading shows a drop or decrease in altitude. Due to the differential pressure nature of its operation, a VSI typically exhibits a small response delay when it initially returns an accurate indication of a climb, descent, or return to level flight.
Rates of climb and descent are critical factors in the flight regimes of any aircraft; Accurate and reliable indication of these trends is one of the most basic instrumentation requirements in fixed and rotary wing types. This makes the vertical airspeed indicator one of the basic flight instruments found on most aircraft. Also known as a variometer or 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 links to a diaphragm that features 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 aircraft is in level flight, there is no pressure difference between the diaphragm and casing pressure, and the vertical speed indicator should show a zero reading. If the plane goes up or down, the static pressure increases or decreases. In each case, the diaphragm and casing will react to pressure changes at different rates due to the restriction in the static line of the casing. This known and controllable pressure differential is the mechanism that allows the diaphragm to move the needle in precise increments to indicate rates of ascent and descent. VSI readings are usually given in units of feet per minute (ft/min) and are marked in 100-foot increments with maximum readings appropriate for the aircraft type. Although ft/min is the most commonly used VSI unit, some countries use instruments marked with metric equivalents.
Although the vertical speed indicator generally reacts to changes in altitude much faster than altimeters, it does have a small delay before it begins to accurately indicate a deviation. This delay is typically between 6 and 9 seconds and is experienced before the needle shows accurate climb or descent readings and before it returns to zero or a level flight indication. Although the needle will immediately show a rising or falling trend, pilots should wait at least 10 seconds before relying on exact ft/min readings.
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