The Vertical Speed Indicator or VSI is essentially an altimeter with a hole drilled in it. Changing pressures expand or contract a diaphragm connected to the indicating needle through gears and levers, but because the static pressure chamber has a calibrated leak, the needle always goes back to zero. Pilots should remember that the VSI is not accurate until aircraft is stabilized; sudden or abrupt changes in aircraft attitude will cause erroneous instrument readings as airflow fluctuates over the static port—these changes are not reflected immediately by the VSI due to the calibrated leak. The pitch attitude should produce a rate of change on the VSI about twice the size of the altitude deviation (If the aircraft is 100 feet off the desired altitude, a 200 fpm rate of correction would be used). When pressure is applied to the controls and the VSI shows an excess of 200 fpm from that desired, over-controlling is indicated (If attempting to regain lost altitude at the rate of 500 fpm, a reading of more than 700 fpm would indicate over-controlling).

If the static port becomes blocked the VSI will remain on zero.