Now, the metering unit is not just a simple hole — Page 74, Lesson 87

Let’s start with the Vertical Speed Indicator, the VSI. This is the instrument that tells you, in feet per minute, whether you are climbing, descending, or holding level. The heart of it is the metering unit — also called the restrictor or the choke. I want you to hold onto all three names, because you’ll see them used interchangeably. Now, the metering unit is not just a simple hole. Here’s the problem it solves. The static pressure change the instrument senses is actually measured in hectopascals per minute. But the pilot needs to see an indication in feet per minute, and that has to work at any altitude. The catch is that there are more feet to a hectopascal at higher altitudes than at lower altitudes. In other words, the rate of pressure change with altitude is different at different altitudes. So the instrument has to compensate for that, or your reading would drift with altitude. The way it compensates is by using a combination of two different types of hole — a capillary and an orifice. By choosing the right combination of these, the instrument gives you a near-constant indication in feet per minute, regardless of the actual altitude, and therefore regardless of the actual pressure differential needed. So the metering unit is a tuned restriction, not just a hole. Now let’s talk about the errors of the VSI, because every instrument has them and you need to know each one by name. First, instrument error. This is simply due to manufacturing imperfections. No instrument is perfect. Second, position error — also called pressure error. If the static pressure is subject to position error, the VSI will wrongly indicate a climb or a descent when speed is suddenly changed. This is most noticeable during take-off acceleration. Think about that: you’re accelerating down the runway, and the static pressure at the vent is disturbed by the airflow, so the instrument can 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 the mechanical linkage itself introduces lag. 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, to tie it together: the VSI senses static pressure, feeds it through a metering unit made of a capillary and an orifice, and displays your rate of climb or descent in feet per minute. And you have four distinct error sources to remember — instrument, position, manoeuvre-induced, and time lag.