Vertical Speed Indicator (VSI)

The vertical speed indicator (sometimes called the VSI or rate-of-climb indicator) shows how fast an aircraft is climbing or descending. The VSI is usually calibrated in feet per minute.

Pilots use the VSI primarily during instrument flight to help them establish the correct rate of descent during approaches and to maintain steady rates of climb or descent.

How It Works

The VSI is connected to the static system. Air pressure inside the instrument case decreases as the airplane climbs and increases as the airplane descends. Inside the case, a sealed wafer, much like the one used in the altimeter, expands and contracts as the pressure changes. A needle connected to the wafer rotates as the wafer expands and contracts, indicating a rate of climb or descent. The wafer also has a small, calibrated leak to allow the pressure in the wafer to equalize with the pressure in the case when the airplane levels off. When the pressure inside the wafer equals the pressure in the case, the needle returns to zero, indicating level flight.

Reading the VSI

You shouldn't use the VSI as the primary indicator of whether you're maintaining level flight. If the airplane begins to climb or descend, the VSI initially indicates the change in the proper direction. But the indicator takes several seconds to catch up to the aircraft's actual rate of climb or descent. "Chasing" the needle on the VSI can make you feel like you're riding a roller-coaster. Rely instead on the airspeed indicator and altimeter—they give quick, accurate indications of deviations from level flight. Then cross-check the VSI to verify that the airplane is climbing or descending at the rate you want.

The VSI in the Learjet 45 and Boeing 737-400 looks almost identical to the Cessna model. But the scale has a bigger range—up to +/-6,000 fpm (1,830 m/min).