The vertical speed indicator is one of the 3 instruments in the pitot-static system of an aircraft. The vertical speed indicator or VSI tells a pilot the rate at which they are climbing or descending, usually in feet per minute. The VSI does this by comparing the air instantaneous static air pressure with a lagged or delayed static air pressure. Because static air pressure can be easily correlated to altitude, the VSI can then come up with an altitude gain or loss rate.

So how does the VSI work? The VSI uses a sealed diaphragm called an aneroid wafer. This diaphragm expands and contracts when the air inside is at a different pressure than the air outside of it. The diaphragm is filled with unmetered static air while the air surrounding the diaphragm is filled with metered static air. The static air outside the diaphragm is metered by a calibrated nozzle set to allow in and out at a certain rate. Because of this, the air pressure outside of the wafer changes at a slower rate than the air pressure inside the wafer.

For example, if an aircraft is climbing the static air pressure is decreasing. Since the air inside the wafer is simply unmetered static air... it feels this decrease in pressure instantaneously. However, the air pressure outside of the diaphragm is trying to decrease by releasing air molecules out of the metered nozzle but the nozzle is slowing this process. Therefore, the air pressure outside of the diaphragm will be higher than the air pressure inside the diaphragm until the aircraft levels off and gives the metered air outside the diaphragm time to catch up to the decrease in pressure.

This pressure differential during a climb or descent between the inside of the diaphragm and the outside of the diaphragm causes the diaphragm or wafer to expand or contract (expands when the pressure inside is larger and contracts when it is smaller). When the diaphragm expands or contracts, it spins a system of gears that turns the needle on the face of the instrument giving an indication of vertical speed.

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🛫 Timestamps:
00:00 Intro
00:19 How does a Vertical Speed Indicator or VSI works?

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