Is the instantaneous vertical speed indicator (IVSI) a pressure differential measuring instrument?

• A. No
• B. It measures wind speed
• C. Yes
• D. It measures altitude

Answer: (C)

The instantaneous vertical speed indicator is a pressure differential instrument because it indicates how fast the ambient static pressure is changing as you move vertically. Its sensing element has two sides with pressures that are slightly different: one side sees the static pressure, and the other side is fed through a restricted path so its pressure lags a bit. The rate of change of static pressure creates a differential pressure across the diaphragm, and that differential is converted into a vertical speed reading (feet per minute). This is not measuring wind speed or altitude directly—the altitude is read from the absolute static pressure (altimeter), and wind speed isn’t what the VSI uses. The IVSI’s faster, instantaneous response just comes from how the differential pressure is generated and processed.