The vertical speed indicator (VSI) in figure 3-14: "Vertical speed indicator" is also called a vertical velocity indicator (VVI) and was formerly known as a rate-of-climb indicator. It is a rate-of-pressure change instrument that gives an indication of any deviation from a constant pressure level.
Inside the instrument case is an aneroid very much like the one in an airspeed indicator. Both the inside of this aneroid and the inside of the instrument case are vented to the static system, but the case is vented through a calibrated orifice that causes the pressure inside the case to change more slowly than the pressure inside the aneroid. As the aircraft ascends, the static pressure becomes lower and the pressure inside the case compresses the aneroid, moving the pointer upward, showing a climb and indicating the number of feet per minute the aircraft is ascending.
When the aircraft levels off, the pressure no longer changes, the pressure inside the case becomes the same as that inside the aneroid, and the pointer returns to its horizontal, or zero, position. When the aircraft descends the static pressure increases and the aneroid expands, moving the pointer downward, indicating a descent.
The pointer indication in a VSI lags a few seconds behind the actual change in pressure, but it is more sensitive than an altimeter and is useful in alerting the pilot of an upward or downward trend, thereby helping maintain a constant altitude.
Some of the more complex VSIs, called instantaneous vertical speed indicators (IVSI), have two accelerometer-actuated air pumps that sense an upward or downward pitch of the aircraft and instantaneously create a pressure differential. By the time the pressure caused by the pitch acceleration dissipates, the altitude pressure change is effective.
Tag: Types of Airspeed, Indicated Airspeed, Calibrated Airspeed, Equivalent Airspeed, True Airspeed, Mach number, Maximum Allowable Airspeed, and Airspeed Color Code.
Position error: Error in the indication of the altimeter, ASI, and VSI caused by the air at the static system entrance not being absolutely still.
Kollsman window: A barometric scale window of a sensitive altimeter.
Calibrated orifice: A hole of specific diameter used to delay the pressure change in the case of a vertical speed indicator.
When flying from hot to cold, or from a high to a low, look out below!
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