vertical speed indicator
The vertical speed indicator (VSI)
or vertical velocity indicator indicates whether the aircraft is climbing,
descending, or in level flight. The rate of climb or descent is indicated in
feet per minute. If properly calibrated, this indicator will register zero in
The VSI is in a sealed case connected to the static line through a calibrated
leak (restricted diffuser). Inside the case, a diaphragm attached to the pointer
by a system of linkages is vented to the static line without restrictions.
As the aircraft climbs, the diaphragm contracts and the pressure drops
faster than the case pressure can escape through the restrictor, resulting in
climb indications; the reverse is true during descent. If level flight is
resumed, pressure equalizes in the case and diaphragm within six to nine seconds
and the pointer returns to zero rate of climb. The vertical speed indicator has
100-ft calibrations with numbers every 500 ft (see Face
of Vertical Speed Indicator figure below).
Although the vertical-speed indicator operates from the
static pressure source, it is a differential pressure instrument. The
differential pressure is established between the instantaneous static pressure
in the diaphragm and the trapped static pressure within the case.
When the pressures are equalized in level flight, the needle
reads zero. As static pressure in the diaphragm changes during entry to a climb
or descent, the needle immediately shows a change of vertical direction.
However, until the differential pressure stabilizes at a definite ratio,
reliable rate indications cannot be read. Because of the restriction in air flow
through the calibrated leak, a 6- to 9-second lag is required to equalize or
stabilize the pressures.
Limitations in the use of the vertical-speed indicator are
due to the calibrated leak. Sudden or abrupt changes in aircraft attitude cause
erroneous instrument readings as the air flow fluctuates over the static ports.
Both rough control technique and turbulent air result in unreliable needle
indications. When used properly, the instrument provides reliable information to
establish and maintain level flight and rate climbs or descents.
The instantaneous vertical-speed indicator incorporates
acceleration pumps to eliminate the limitations associated with the calibrated
leak. For example, during climb entry, vertical acceleration causes the pumps to
supply extra air into the diaphragm to stabilize the pressure differential
without the usual lag time. During the level flight and steady rate climbs and
descents, the instrument operates on the same principles as the earlier
Adjustment. The needle of the vertical velocity indicator
should indicate zero when the aircraft is on the ground or maintaining a
constant pressure level in-flight. Most instruments can be adjusted to a zero
reading by turning a screw on the lower left corner of the instrument case. If
this adjustment cannot be made, you must allow for the error when interpreting
the indications in flight.