by Jonathan M. Stern
Vertical Speed Indicator
The vertical speed indicator, like the altimeter, is connected only to the static vent. The vertical speed indicator shows whether the airplane is flying at a constant altitude, climbing, or descending, and, if climbing or descending, at what rate. The face of the instrument is graduated in hundreds of feet per minute, with the top half indicating climbs and the bottom half indicating descents (see Figure 2.27).
Figure 2.27. The vertical speed indicator uses a calibrated leak across a diaphragm to measure rate of pressure change.
In a typical general aviation airplane such as the Cessna 182RG, the VSI has a lag time of six to nine seconds. In other words, the VSI displays the vertical speed of the aircraft some six to nine seconds earlier. The lag is caused by the mode of operation of a VSI, which uses a calibrated leak to create a pressure differential across a diaphragm. When the pressures equalize, the VSI displays a 0 rate of climb.
More expensive aircraft, such as the Lear 35A, may incorporate an IVSI, or Instantaneous Vertical Speed Indicator. Through the use of acceleration pumps, the IVSI gives an instantaneous readout of vertical speed.