Let’s pick up with the Vertical Speed Indicator, the VSI — Page 74, Lesson 87

Let’s pick up with the Vertical Speed Indicator, the VSI. I want to walk you through how it works, starting with the metering unit, because that’s the heart of the instrument. The VSI measures the rate of change of static pressure. But here’s the problem: the static pressure change is actually measured in hectopascals per minute. The pilot, though, needs to see an indication in feet per minute, regardless of altitude. And that’s not a simple conversion, because there are more feet to a hectopascal at higher altitudes than at lower altitudes. Think about it — the atmosphere is less dense up high, so a given pressure change corresponds to a bigger altitude change. So the rate of pressure change with altitude at different altitudes needs to be compensated for. That compensation is achieved by a combination of different types of hole, called a capillary and an orifice. The restrictor — also called the choke, or the metering unit — is more complicated than a simple hole. By using a suitable combination of these two types of hole, the instrument gives a near-constant indication in feet per minute, whatever the actual altitude, and therefore whatever the actual pressure differential is needed. Now, the display. The VSI dial shows a zero in the middle, with a CLIMB scale above and a DESCENT scale below. The pointer moves up for climb, down for descent. That’s the basic face you’ll see in the cockpit. But the VSI has errors, and you need to know them cold. There are four. First, instrument error. That’s simply due to manufacturing imperfections. Every instrument has some. Second, position error, also called pressure error. If the static pressure is subject to position error, the VSI will wrongly indicate a climb or descent when speed is suddenly changed. This is most noticeable during take-off acceleration. So when you slam the throttles forward on the runway, the static pressure at the vent can be disturbed, and the VSI may show a false climb or descent. Third, manoeuvre-induced error. Any short-term fluctuations in pressure at the static vent during attitude changes will cause the instrument to indicate a false rate of climb or descent. Additionally, with most VSIs, the linkage includes a small counterbalance weight. The inertia of that weight causes delays in the indications of changes in vertical speed during manoeuvres. So when you pitch the nose up or down, the instrument can lag or overshoot because of that weight. Fourth, time lag. The pointer takes a few seconds to steady because of the time taken to build up a steady pressure difference on climb or descent. There will also be a time lag on levelling out, because of the time taken for the pressures to equalize. This error is most noticeable after a prolonged climb or descent, especially at a high rate. So after a long, fast climb, when you level off, the VSI will keep showing a climb for a few seconds while the pressures equalize. Let me show you the metering unit and the display. So the key takeaway: the VSI converts a pressure rate into a feet-per-minute rate, using a capillary and orifice combination to compensate for altitude, and it has four distinct error sources — instrument, position, manoeuvre-induced, and time lag. That’s the full picture of the VSI.