Vertical Speed Indicator

The Vertical Speed Indicator (VSI), also called the Rate of Climb and Descent Indicator (RCDI), is a flight instrument that displays the rate of change of altitude in feet per minute (ft/min). It operates by measuring the rate of pressure change in the static pressure system.

How the VSI Works

The VSI is connected to the static pressure system. A metered leak allows the case pressure to approach the static pressure slowly; the diaphragm inside responds to the instantaneous static pressure. The difference between the lagging case pressure and the current static pressure drives the indicator needle, showing climb or descent rate.

VSI Lag

The VSI has an inherent lag — it takes 6–9 seconds to stabilize after a change in altitude. During instrument flying, this lag means the VSI must be cross-checked with other instruments. Instantaneous VSI (IVSI) systems reduce this lag using an accelerometer that detects the initial rate of change.

Instrument Scan

During level flight, the VSI should read zero. In a climbing turn, the VSI shows positive rate while the altimeter shows increasing altitude. The VSI is used to establish and maintain the desired rate of climb or descent — typically 500 ft/min for gentle manoeuvres, 1,000 ft/min for normal climb/descent.

Related Terms

See also: Altimeter, Attitude Indicator, IFR.