Vertical Speed Indicator (VSI)

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Vertical Speed Indicator

Indicating "Climbs"and "Descents" - The Vertical Speed Inidicator

The Vertical Speed Indicator, abbreviated as the VSI, presents the pilot with information on how fast the aircraft climbs or descents. This is achieved by measuring how fast the ambient pressure changes and translating this as a rate of change in feet per minute. Besides displaying rate of change information, the VSI is also useful to observe specific trends. If, for example, the pilot initiates a turn, the VSI is the first indicator that will tell a small pitch correction is needed in order to maintain altitude. This is because the VSI instantaneously indicates a change in vertical speed, while the attitude indicator gives a far less precise indication of pitch.

There are basically two types of Vertical Speed Indicators, being the VSI and the IVSI, or Instantaneous Vertical Speed Indicator. The VSI is the most common type, using static pressure changes solely to present vertical speed. The IVSI however, combines accelerometric readings with barometric sources, which enables the instrument to display a change in pressure instantly, hence its name.

Vertical Speed Indicator (VSI)

The VSI consists of several components that are all housed in a sealed case which is connected to the static pressure source. These components comprise of a metering unite, a diaphragm, and an indicating element. A diaphragm inside the instrument expands or contracts according to the pressure change caused while the aircraft is gaining or losing altitude. Movement of the diaphragm is then translated through a mechanical linkage into a needle movement. The needle then indicates the rate of change on a dial presentation which is such that zero is at the 9 o'clock position (see VSI image above).

The diaphragm itself is directly connected to static pressure, and indirectly connected to the area around it through a calibrated leak. Its purpose is to create a lag in static pressure across the system and in this way establish the required pressure differences. If the aircraft climbs, the static pressure inside the diaphragm instantly decreases, while the metering unit restricts the airflow in and out of the sealed case. As a result a differential pressure is created, causing the diaphragm to collapse and so make the pointer indicate a climb.

Instantaneous Vertical Speed Indicator (IVSI)

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.

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.