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The Vertical Speed Indicator (VSI), one of the simplest yet most important instruments at a pilot’s disposal, is a tool used to indicate the rate of climb or descent of an aircraft. The VSI uses the common pitot-static system to determine vertical speed and displays the result via a needle and circular scale instrument. In this blog, we will take a look at VSIs, what components they are made of, and how they work.

The two main components of the VSI are the diaphragm and the calibrated leak. The diaphragm is the flexible metal container that connects to the VSI’s source of static air. In addition to this, the diaphragm is connected to a set of rods and gears that move the VSI’s needle up and down. The calibrated leak is a miniscule opening that connects the VSI casing to the static source. The calibrated leak’s opening is designed small enough to restrict airflow to prevent air from moving in and out of the diaphragm faster than it can handle.

As the aircraft climbs, static pressure in the air begins to decrease. As such, the pressure in the diaphragm decreases. Despite this, the pressure in the VSI casing actually increases because the calibrated leak slowly releases air. Varying levels of pressure within the casing and diaphragm causes a pressure differential, squeezing the diaphragm and moving the gearshift connected to the VSI needle. The exact opposite of this phenomenon occurs when the aircraft descends.

The VSI relies on air moving in and out of the casting to make readings, so it is not always an immediate indication of vertical speed. It typically takes a second or two for the readings to stabilize and provide operators with the accurate information. When you first begin ascending or descending, the needle will move, but can’t yet accurately determine the vertical speed. This is called trend information. Once the calibrated leak catches up and stabilizes, the VSI will begin provide accurate readings known as rate information.