The Vertical Speed Indicator — Page 74, Lesson 90

Let’s pick up with the Vertical Speed Indicator, and I want to start with a critical safety point about blockages. If the static line or the vent becomes blocked, the needle will return to zero. That’s a key failure mode: a blocked static source doesn’t give you a false reading, it drives the needle back to zero. And here’s the important cross-check: if the supply of air to this instrument is blocked, it’s probable that the other pressure instruments—the ASI, the altimeter, and the Machmeter—will also be affected, because they all share the same static pressure source. Now, the main problem with a standard VSI is lag. To overcome that lag, we have the Instantaneous Vertical Speed Indicator, or IVSI. The IVSI incorporates an accelerometer unit, sometimes called a dashpot or a dynamic vane, which responds quickly to a change of altitude. Let me walk you through how it works during the start of a descent. The piston in the vertical acceleration pump immediately rises in the cylinder, and that causes a temporary increase of pressure in the capsule. The capsule expands, and the pointer gives an instant indication of descent. Then, as the initial acceleration is turned into a steady rate of descent, the piston slowly descends back to its original position. But by that time, the correct differential pressure between the capsule and the case has been set up, so the correct rate of descent continues to be shown. So the dashpot gives you the instantaneous response, and the normal metering system takes over for the steady state. Let me label the parts of the IVSI for you: we have the capsule, the main metering restriction, the static pressure connection, and the vertical acceleration pump, which is the dashpot. The descent condition is shown with the static pressure, Ps, acting on the system. Now, the IVSI has errors peculiar to it. Because of the sensitivity of the dashpot assembly, the instrument tends to overreact to turbulent flying conditions, so small fluctuations should be ignored. And there’s another specific error: in a steep, level turn, the piston will tend to sink towards the bottom of the cylinder, and that will give a false indication of a climb. So in a steep turn, you can see a climb indication even though you’re level—that’s a dashpot-specific error you need to be aware of. Let’s move to presentation. Two types of presentation are available: a linear scale and a logarithmic scale. The logarithmic scale is more easily read at the lower rates of climb and descent, because the scale expands at the low end. You can see this clearly in the figures: the linear scale has evenly spaced markings, while the logarithmic scale compresses the higher rates and expands the lower rates for better readability. Now, two construction details. Diaphragm overload stops may be fitted to prevent damage to the instrument should the rate of climb or descent exceed the maximum to which the instrument is calibrated. So if you exceed the calibrated range, the stops protect the diaphragm. On some instruments, a zeroing screw is fitted, which allows you to adjust the zero point. Finally, serviceability checks. On the ground, the instrument should read zero, or the error should be within permissible limits. Those limits are plus or minus 200 feet per minute at temperatures between minus 20 degrees Celsius and plus 50 degrees Celsius. Outside those temperatures, the permissible error is plus or minus 300 feet per minute. And there should be no apparent damage to the instrument. In the air, you can check the accuracy of the instrument against the altimeter and a stopwatch during a steady climb or descent, and the instrument should indicate zero climb or descent when in level flight. So to tie it together: the standard VSI lags, the IVSI solves that with a dashpot accelerometer, but that dashpot introduces its own errors in turbulence and steep turns. And always remember the blockage failure mode—needle to zero, and check the other static instruments.