Vertical Speed Indicators (VSI)
A vertical speed indicator shows the rate at
which an aircraft is climbing or descending. It is
very important for night flying, flying through
fog, clouds, or 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 (fig. 6-11)
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
Figure 6-12.-Mechanical schematic of a VSI.
at zero.
The vertical speed indicator is contained in a
trapped within the case creates the differential
sealed case, and it connects to the static pressure
pressure.
line through a calibrated leak. Look at figure 6-12.
You can see that changing pressures expand or
When the pressures equalize in level flight, the
contract a diaphragm, which moves the indicating
needle reads zero. As static pressure in the
diaphragm changes during a climb or descent, the
automatically compensates for changes in
needle immediately shows a change of vertical
temperature. Although the vertical speed indicator
speed. However, until the differential pressure
operates from the static pressure source, it is a
stabilizes at a definite ratio, indications are not
differential pressure instrument. The difference
reliable. Because of the restriction in airflow
in pressure between the instantaneous static
through the calibrated leak, it requires a 6 to
pressure in the diaphragm and the static pressure
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.
AIR DATA COMPUTER
SYSTEM (ADC)
Aircraft operating below 0.9 Mach airspeed
use raw pitot and static pressures to develop
accurate airspeed, altitude, and vertical speed
indications. Aircraft operating in this speed range
use the pressures that the pitot-static ports
sense.
Modern supersonic aircraft operate in a higher
speed range and require more accurate pressures.
At high speed, pressures build upon the external
skin of the aircraft. These pressures cause a
distortion of the normal flow of air, causing the
Figure 6-11.-Vertical speed indicator (VSI).
6-10
