So, the Vertical Speed Indicator displays your rate of climb or descent — Page 74, Lesson 85

I want to walk you through the Vertical Speed Indicator — the VSI — and I’ll start with what it actually does for you in the cockpit. You can get a rough idea of your rate of climb or descent just by watching the altimeter pointer rotate. But that’s crude. There are times when you need something far more accurate — for example, achieving a certain height loss within a specified time on airways, or setting up a smooth, steady rate of descent on a glide path during an instrument approach. That’s exactly the job the VSI is built for. So, the Vertical Speed Indicator displays your rate of climb or descent. The instrument senses the rate of change of static pressure by comparing the present static pressure with the static pressure measured 4 to 6 seconds earlier. Hold onto that 4–6 second figure — it’s central to how the instrument works and to one of its big limitations. The VSI is also known by another name: the Rate of Climb and Descent Indicator, or RCDI. So if you hear RCDI, it’s the same instrument. Now the principle. When the aircraft departs from level flight, the static pressure changes. The VSI measures the pressure difference between each side of a restricted choke — also called the metering unit. In level flight, the pressures on each side of the choke are the same. But during a climb or descent, the air fed to the choke immediately responds to the change in atmospheric pressure, while the choke transmits that change at a lower rate. That difference in response is what the instrument reads. Let me make that concrete with the construction. Inside an airtight case there’s a capsule, and that capsule is fed with static pressure directly. The case itself is also fed with static pressure, but through that restricted choke. So when the static pressure changes, the pressure surrounding the capsule changes at a slower rate than the pressure inside the capsule. That’s the whole trick. Here’s the example from the text: if the aircraft is climbing, the pressure in the capsule will be less than the pressure in the case. The consequent compression of the capsule is converted by a suitable linkage to a pointer indication of rate of climb. So the capsule physically compresses, a linkage moves, and the pointer shows you the climb rate. That figure shows you the metering unit — the choke and capillary — and the two conditions: climbing and descending, with the static pressure increasing on one side. Now, before we go further, I want to flag the key limitation that comes straight out of this design. Because the case pressure lags behind the capsule pressure by that 4 to 6 seconds, the VSI does not respond instantly. When you make a sudden pitch change, the instrument takes a moment to settle on the true rate. That lag is the source of the instrument’s errors, and it’s exactly why a more advanced version exists — the Instantaneous Vertical Speed Indicator, which we’ll get to shortly. So let me summarise where we are: the VSI compares present static pressure with static pressure from 4–6 seconds ago, using a capsule inside a case, with the case fed through a restricted choke. In level flight the pressures balance; in a climb the capsule pressure drops below the case pressure, compressing the capsule and driving the pointer. That’s the core mechanism. Now let’s move on to the display and then the errors, because the display is how you actually read this thing, and the errors are what you have to compensate for in real flying.