The Vertical Speed Indicator: Reading the Rate of Change

Propeller Breeze

In this episode of Aviation Essentials, we explore one of the most informative yet often misunderstood instruments in the cockpit—the Vertical Speed Indicator (VSI), also known as the Vertical Velocity Indicator (VVI). This episode unpacks how this differential pressure instrument helps pilots interpret an aircraft’s rate of climb or descent in feet per minute, and why understanding its lag and limitations is essential to accurate altitude control.

We’ll break down:

• The Core Function: How the VSI translates changes in static pressure into readable vertical speed.

• Inside the Instrument: The role of the diaphragm, airtight case, and calibrated leak that make the VSI work.

• Trend vs. Rate Information: How the VSI first shows the direction of change, then stabilizes to show the rate of change.

• Instrument Lag: Why every standard VSI has a 6–9 second delay—and what that means in turbulence or during aggressive maneuvers.

• The IVSI Advantage: How the Instantaneous Vertical Speed Indicator uses accelerometers to provide immediate, accurate readings.

• Preflight Checks: The vital steps to ensure the VSI is functioning correctly before and during flight.

Whether you’re a student pilot learning your first scan or an experienced aviator refining your instrument awareness, this episode will give you a deeper appreciation for the VSI’s role in maintaining smooth, controlled vertical flight.