We’re starting a brand-new instrument today: the Vertical Speed Indicator… — Page 74, Lesson 85

We’re starting a brand-new instrument today: the Vertical Speed Indicator, or VSI. This is the instrument that tells you, at a glance, whether you’re climbing, descending, or holding level, and how fast you’re doing it. Let me set the scene first. Before the VSI existed, a pilot could get a rough idea of rate of climb or descent just by watching the altimeter pointer rotate. But that’s crude. There are times when you need a far more accurate indication — 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 was built for. So, the definition. The Vertical Speed Indicator displays rate of climb or descent. It 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, because it’s the heart of how this instrument works. And note the alternative name: the VSI can also be known as the Rate of Climb and Descent Indicator, or RCDI. Now the principle. When an aircraft departs from level flight, the static pressure changes. The VSI measures the pressure difference between each side of a restricted choke, which is also called the metering unit. In level flight, the pressures on each side of the choke are the same — no difference, so the needle sits at zero. But during a climb or descent, air fed to the choke immediately responds to the change of atmospheric pressure, while the choke transmits that change at a lower rate. That difference in response is what the instrument measures. Let me walk you through the construction, because that’s where it all becomes clear. Inside the instrument there’s an airtight case, and within that case sits a capsule. The capsule is fed with static pressure directly. The case is also fed with static pressure, but through that restricted choke. So if the static pressure changes, the pressure surrounding the capsule changes at a slower rate than the pressure within the capsule. Think about what that means in a climb: the pressure in the capsule will be less than the pressure in the case. The capsule consequently compresses, and that compression is converted by a suitable linkage to a pointer indication of rate of climb. In a descent, the whole thing reverses — the capsule expands, and the pointer moves to the descent side. That figure shows you exactly this arrangement — the metering unit or choke, the capillary, and how the capsule sits inside the case. The key idea to lock in: the capsule sees the immediate static pressure, the case sees the delayed static pressure, and the difference between them is your rate of climb or descent. Now, one thing I want to flag before we go further. This chapter has a full structure ahead of us — the VSI metering unit, the display, the errors of the VSI, the Instantaneous Vertical Speed Indicator, presentation, and serviceability checks. We’ve just covered the introduction, the principle, and the construction. The next piece is the metering unit itself, which is where we’ll go when you’re ready.