A vertical speed indicator (VSI), also known as a vertical velocity indicator, is a vital component within the pitot-static system. Proper functionality is crucial for determining whether an aircraft is level, climbing or descending during flight. To ensure accuracy and safety of both the VSI and the pitot-static system, instrument calibration should be maintained and scheduled regularly.
Similarly to the ASI and the altimeter, the VSI is a type of pressure gauge. The VSI is sealed within a case and is connected to the static line through a calibrated leak. Within the case is a diaphragm, which is key to the measurement of pressure drops to indicate climbing/descending. When pressures are equalized during level flight, the needle of the VSI is calibrated to read zero. During pre-flight, it is extremely important to verify the VSI needle is reading zero. If the needle indicates anything other than zero feet per minute on the ground, then the calibration of the VSI is off. The instrument can still be used, but it is highly discouraged. If a VSI is not reading properly on ground, chances of serious hazard in flight are likely, especially in weather.
Many will argue that malfunction of the VSI is no big deal and that flight can still take place. Although it is true there are other ways to calculate the climbing of an aircraft, proper functionality of the pitot-static systems relies on the correct functionality of the VSI. This is why calibration of the VSI should be performed on a frequent basis – to ensure overall accuracy of the VSI itself and also of the pitot-static system.