How does a vertical speed indicator determine rate of climb or descent?

• A. It measures dynamic pressure
• B. It measures rate of change of static pressure with time
• C. It measures static pressure
• D. It measures altitude

Answer: (B)

The vertical speed indicator is driven by how quickly the ambient static pressure changes as you climb or descend. It uses a sealed chamber (aneroid) connected to the static pressure through a calibrated restrictor, so when you rise and the static pressure falls, the chamber expands (or contracts) at a rate proportional to the rate of change of that pressure over time. The mechanism converts that rate into a display of feet per minute. Because it’s all about dP/dt of the static pressure, the instrument shows vertical speed rather than the actual altitude. Dynamic pressure, which relates to airspeed, is what the airspeed indicator uses, and altitude is what the altimeter shows based on the static pressure itself.