The vertical speed indicator (VSI) is one of the six pack instruments found in nearly every cockpit. The VSI displays an aircraft rate of climb or descent in hundreds of feet per second, gathering such measurements via a pitot-static system. The VSI may also be known as a variometer, vertical velocity indicator, and rate-of-climb indicator. When you look at each individual part, the VSI and its function are relatively easy to understand. There are just six main parts that comprise a VSI. In this blog, we will look at each part and how the device works.
The first part of the VSI is the static pressure port. This is a flush mounted opening placed on the outside of the aircraft in an area of relatively undisturbed airflow. It collects the ambient air from outside the aircraft and channels it into the next part: the static line. The static line is a hollow tube connecting the static pressure port to the case and diaphragm of the VSI. The third part of the VSI is the case. This is the housing that contains the other components of the VSI. it is connected to the static line through a connection with a calibrated leak.
The fourth part, the calibrated leak, is a special type of connection between the case and static line. The calibrated leak prevents the pressure inside the case from changing too rapidly when air flows into the case. The penultimate part of the VSI is the diaphragm. This is a flexible metal container located within the case where it is directly attached to the static line. Rods and gears also connect the diaphragm to the needle on the face of the VSI cockpit display gauge. The final part of the VSI is the face. It is the front part of the VSI which is visible in the cockpit and denotes the vertical speed reading to the pilot. The needle on the face is connected to the diaphragm through a series of gears and rods.
Now that you are familiar with the components of a VSI, let’s look at how the instrument works. To operate, VSIs use the pitot-static system. External air is collected via the static pressure ports and flows through the static line before entering the case and diaphragm. The air then flows into the case via the port with the calibrated leak, while the diaphragm receives air directly from the static line.
As an aircraft climbs, the pressure outside the aircraft lowers. This lower-pressure air flows through the static line, decreasing the pressure in the diaphragm. The case will initially be at higher pressure than the diaphragm, because the calibrated leak slows the rate at which pressure changes inside the case. The pressure differential causes the diaphragm to collapse in on itself, turning the rods and gears and thereby moving the needle on the face of the VSI. When the plane is descending, the opposite of this occurs. The external static pressure increases, traveling to the diaphragm and case. During this, the diaphragm will be at a higher pressure than the case, causing it to expand and move the needle in the opposite direction.
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