The vertical speed indicator (VSI), also called a vertical velocity indicator (VVI) or rate-of-climb indicator measures how fast the static (ambient) pressure increases or decreases as the airplane climbs or descends. It then converts and displays this pressure change as a rate of climb or descent in feet per minute. Unlike the altimeter, the VSI is not affected by air temperature since it measures only changes in air pressure.

Figure 1 - Vertical Speed Indicator Front

The VSI displays rate information and trend information. Although the instrument is designed to display rate of climb or descent, it can take roughly six to nine seconds of lag for the needle to stabilize on an accurate vertical speed indication after you change power or pitch. Even though it takes a few moments to indicate the exact vertical speed, the VSI is valuable because it instantaneously indicates changes in vertical speed, or trend information. When making a steep turn, the VSI usually is the first instrument to tell you a small correction in pitch is needed, since the attitude indicator does not give a precise enough indication of pitch. As you study attitude instrument flying and maybe practise it in the airplane, you will appreciate the VSI's early warning of deviations from the desired pitch. This instrument provides even more essential pitch information in the event of a failure of the gyroscopic indicator.

Since the VSI is not designed to instantaneously indicate the rate of climb or descent, it will not give a clear indication during turbulence or when applying abrupt control inputs. You may be able to average the erratic readings during turbulence or determine whether you are climbing or descending. Because the VSI uses static air pressure, this instrument will not function if the static port is clogged (read more on this subject right here). In some advanced aircraft, you may find an instantaneous vertical speed indicator (IVSI). This device incorporates acceleration pumps to compensate for the limitations of the calibrated leak, eliminating the lag found in the typical VSI.

Figure 2 - Vertical Speed Indicator

During changes in static air pressure, the diaphragm inside the vertical speed indicator expands or contracts (figure 2). Movement of the diaphragm is translated into a needle movement through a mechanical linkage. Static pressure is directly connected to the inside of the diaphragm, and indirectly connected to the area outside the diaphragm via a restricted orifice (calibrated leak) that prevents the pressure outside the diaphragm from changing instantaneously.

To get into a little bit more detail of the IVSI, a detailed description of this instrument is described here. The IVSI (figure 3) consist of the same basic elements as conventional VSIs, but in addition they emply an accelerometer unit which is designed to create a more rapid differential pressure effect, specifically at the initiation of a climb or descent.

Figure 3 - Instantaneous Vertical Speed Indicator

The accelerometer comprises of a small cylinder, or dashpot, containing a piston held in balance by a springs and its own mass. The cylinder is connected in capillary tube leading to the capsule, and is thus open directly to the static pressure source. When a change in vertical speed is initiated, the piston is immediatly displayed under the influence of a vertical acceleration force, and this creates an immediate pressure change inside the capsule. For example, at initiation of a descent, the piston moves up and thereby decreases the volume of chamber 'A' to produce an immediate increase of pressure inside the capsule. The capsule displacement in turn produces instantaneous deflection of the indicator pointer over the descent portion of the scale. At initiation of an ascent, the converse of the foregoing responses would apply. The accelerometer response decays in each case after a few seconds, but by this time the change in actual static pressure becomes effective, so that a pressure differential is produced by the metering unit in the conventional manner. The purpose of the restrictor in the bypass line is to prevent any loss of pressure change effects created by displacements at the acceleration pump.