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How Does A Vertical Speed Indicator Work?

Whether you're a VFR or IFR pilot, your VSI is one of the most useful instruments on your panel. But how does it work? Let's take a look.

The VSI, or Vertical Speed Indicator, is simply that. It tells you if your aircraft is climbing, descending, or in level flight.

And it does that purely off you're plane's static air source, which is actually pretty cool. There are a few main components in your VSI.

Main Components Of The VSI

Let's start with the diaphragm. The diaphragm is a fancy name for a flexible metal container that's directly connected to your static air source.

The diaphragm is also connect to a set of gears and rods that move your VSI's needle up and down, and that happens when the diaphragm expands and contracts, but we'll get to that in a second.

The next major component is the calibrated leak. The calibrated leak is a tiny hole that connects the casing of the VSI to the static source, but there's a catch. The hole in the calibrated leak is small enough that is restrict airflow, so it can't move in and out as fast as the diaphragm can.

How It Works: Differential Pressure

So here's how it all works. Let's say you start climbing. As you climb, your static pressure decreases, and as it decreases immediately in the diaphragm. But the instrument casing is a different story. Since the calibrated leak lets air out slowly, it creates a higher pressure in the casing than the diaphragm. When that happens, it creates a pressure differential, the diaphragm is squeezed down, and the gears connected to the VSI needle make it move up.

And the greater the pressure differential, the more the needle moves up.

What happens when you descend? The exact opposite.

Trend Vs. Rate

Because the VSI relies on air leaking out of (or into) the casing, it takes a second or two for everything to stabilize. That's where trend vs. rate comes into play.

When you initially start climbing or descending, your VSI needle will start moving, but it can't immediately indicate how fast you're climbing. This is what's called trend information. When you see the directing of the needle moving up, you know your climb rate is increasing, and when it moves down, you know your climb rate is decreasing. You just don't know how much...at least yet.

After a second or two, the calibrated leak has a chance to catch up and reach equilibrium, and your VSI will stabilize at a certain climb or descent rate. When that happens, you have rate information.

Putting It All Together

Your VSI tells you if you're climbing, descending, or in level flight, and it does it by giving you trend and rate information. And most impressively, it does it without any power, except for the changing pressure of the air around your plane.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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