Vertical Speed Indicator (VSI)

A VSI shows the rate at which an aircraft is climbing or descending. It is very important for night flying, flying through fog or clouds, or flying when the horizon is obscured. Another use is to determine the maximum rate of climb during performance tests or in actual service.

The rate of altitude change, as shown on the indicator dial, is positive in a climb and negative in a dive or glide. The dial pointer (Figure 6-10) moves in either direction from the zero point. This action depends on whether the aircraft is going up or down. In level flight the pointer remains at zero.

The vertical speed indicator is contained in a sealed case, and it connects to the static pressure line through a calibrated leak.

Changing pressures will result in expansion or contraction of the diaphragm, which in turn will move the indicating needle through the use of internal gears and levers. The instrument automatically compensates for changes in temperature. Although the vertical speed indicator operates from the static pressure source, it is a differential pressure instrument. The difference in pressure between the instantaneous static pressure in the diaphragm and the static pressure trapped within the case creates the differential pressure.

When the pressures equalize in level flight, the needle reads zero. As static pressure in the diaphragm changes during a climb or descent, the needle immediately shows a change of vertical speed. However, until the differential pressure stabilizes at a definite ratio, indications are not reliable. Because of the restriction in airflow through the calibrated leak, the differential pressure requires a 6 to 9 second lag for the pressures to stabilize. The VSI has a zero adjustment on the front of the case. You use this adjustment with the aircraft on the ground to return the pointer to zero. While adjusting the instrument, tap it lightly to remove friction effects.