Understanding the Vertical Airspeed Indicator (VSI)

The Vertical Speed Indicator (VSI) – Video Transcript

Today, we are going to talk a little bit about the vertical speed indicator. We’re going to dive into a little bit about how it works and how to read the instrument as well. To get us started, what I want to do is look here to the back of the instrument, away from the face, and look at what’s going on in the guts inside.

What we’ll find inside of the sealed case is a diaphragm kind of similar to that of the airspeed indicator, or something to that effect. What we’re going to see then is this, we’re going to use the static pressure source and that static pressure source is going to go directly into this diaphragm. Then off of that source, there’s going to be what is known as a calibrated leak. Really what that is, and what I try to show with this image is the static pressure that’s going into the diaphragm is a normal entry here. This calibrated leak is a really small line so it’s sort of restricting the amount of air that can pass in or out of the case around the diaphragm.

What that means then is this, what we basically then have is, we’ve set up a situation where inside the diaphragm always has the most current information. It always has the most accurate static pressure. We can say that the diaphragm has the current pressure. That means then because we restrict the amount of air that can move in or out of the case, the case pressure represents our old or previous pressure. So, in essence, all the vertical speed indicator is really doing is comparing what our current static pressure is to what our old or our previous static pressure was.

So, if it knows, if let’s say, the pressure inside of the diaphragm is relatively higher than the pressure outside of the diaphragm, what does that mean? It means our current pressure is greater than our old pressure, which means that we’ve descended. So, in this case, if the pressure were greater in the diaphragm, it would expand. If it expands, we know that this arm would move down. Vice Versa, if the pressure inside the diaphragm was less than our old pressure, that would imply that we are climbing. It would also cause the diaphragm to contract. As that diaphragm contracts, that will raise the needle up to indicate a rate of climb.

Once we’re stable at a particular rate of climb or rate of descent, what happens then, is these pressures are sort of matching together. So, what you’ll find is when we initially start a descent for example, the pressure inside the diaphragm would change immediately and that would cause the hand to move pretty aggressively up front. Then slowly, the calibrated leak allows for this pressure to stabilize at wherever rate we’re descending. The result is then, it’ll show us an appropriate indicated vertical speed. The same would be true obviously in a climb. Well hopefully this helped to explain a little bit as to how the vertical speed indicator works and maybe answer some of your questions.