Rate-of-climb indicator

(aerospace engineering) A device used to indicate changes in the vertical position of an aircraft by comparing the actual outside air pressure to a reference volume that lags the outside pressure because a calibrated restrictor imposes a lag-time constant to the reference pressure volume. Also known as rate-of-descent indicator; vertical speed indicator.

An aircraft instrument that provides an indication of the vertical change of the aircraft position within the air mass. It is more commonly known as the vertical-velocity or vertical-speed indicator. Contained within a sealed case, it is connected to the aircraft static pressure source, the so-called at-rest air pressure outside the aircraft, through a calibrated leak. Although the instrument operates from a static pressure source, it is a differential pressure indicator. The differential pressure is established between the static pressure in the diaphragm or pressure capsule and the trapped static pressure within the case. When the aircraft changes vertical position, the static pressure in the diaphragm changes immediately but, because of the metering action of the calibrated leak, the case pressure will remain at its prior value and cause the needle to show a change in vertical speed. The needle is usually calibrated in feet per minute but may be calibrated in any appropriate unit of length over time.

In many modern aircraft with flight computers, the rate of climb or descent is electronically calculated by differentiating the altitude from the pitot-static source. See also Aircraft instrumentation; Pitot tube.