Vertical Speed Indicator (VSI)  Principle of Operation  Instrument Check  Figure 8-5.  Vertical speed indicator (VSI).  Prior to each flight, a pilot should examine the altimeter for  proper indications in order to verify its validity. To determine  the condition of an altimeter, set the barometric scale to the  current reported altimeter setting transmitted by the local  airport traffic control tower, flight service station (FSS), or  any other reliable source, such as ATIS, AWOS, or ASOS.  The altimeter pointers should indicate the surveyed field  elevation of the airport. If the indication is off more than  75 feet from the surveyed field elevation, the instrument  should be referred to a certificated instrument repair station  for recalibration.  As an example, consider an airport with a field elevation  of 5,048 feet MSL where the standard temperature is 5 °C.  Under these conditions, pressure altitude and density altitude  are the same—5,048 feet. If the temperature changes to  30 °C, the density altitude increases to 7,855 feet. This  means an aircraft would perform on takeoff as though the  field elevation were 7,855 feet at standard temperature.  Conversely, a temperature of –25 °C would result in a density  altitude of 1,232 feet. An aircraft would perform much better  under these conditions.  A pilot must understand how the performance of the aircraft  is directly related to the density of the air. The density of  the air affects how much power a naturally aspirated engine  produces, as well as how efficient the airfoils are. If there are  fewer air molecules (lower pressure) to accelerate through  the propeller, the acceleration to rotation speed is longer  and thus produces a longer takeoff roll, which translates to  a decrease in performance.  Although the VSI operates solely from static pressure, it is a  differential pressure instrument. It contains a diaphragm with  connecting linkage and gearing to the indicator pointer inside  an airtight case. The inside of the diaphragm is connected  directly to the static line of the pitot-static system. The area  outside the diaphragm, which is inside the instrument case,  is also connected to the static line but through a restricted  orifice (calibrated leak).  Both the diaphragm and the case receive air from the static  line at existing atmospheric pressure. The diaphragm receives  unrestricted air, while the case receives the static pressure via  the metered leak. When the aircraft is on the ground or in level  flight, the pressures inside the diaphragm and the instrument  case are equal, and the pointer is at the zero indication.  When the aircraft climbs or descends, the pressure inside  the diaphragm changes immediately, but due to the metering  action of the restricted passage, the case pressure remains  higher or lower for a short time, causing the diaphragm to  contract or expand. This causes a pressure differential that  is indicated on the instrument needle as a climb or descent.  When the pressure differential stabilizes at a definite ratio,  the needle indicates the rate of altitude change.  The VSI, which is sometimes called a vertical velocity  indicator (VVI), indicates whether the aircraft is climbing,  descending, or in level flight. The rate of climb or descent is  indicated in feet per minute (fpm). If properly calibrated, the  VSI indicates zero in level flight.  [Figure 8-5]  3.  Absolute altitude—the vertical distance of an aircraft  above the terrain, or above ground level (AGL).  4.  Pressure altitude—the altitude indicated when  the altimeter setting window (barometric scale) is  adjusted to 29.92 "Hg. This is the altitude above  the standard datum plane, which is a theoretical  plane where air pressure (corrected to 15 °C) equals  29.92 "Hg. Pressure altitude is used to compute density  altitude, true altitude, true airspeed (TAS), and other  performance data.  5.  Density altitude—pressure altitude corrected  for variations from standard temperature. When  conditions are standard, pressure altitude and density  altitude are the same. If the temperature is above  standard, the density altitude is higher than pressure  altitude. If the temperature is below standard, the  density altitude is lower than pressure altitude. This  is an important altitude because it is directly related  to the aircraft’s performance.  I 0 I UP 3  4  2  2  3  VERTICAL SPEED  THOUSAND FT PER MIN  DOWN  Diaphragm  Direct static pressure  Calibrated leak  8-7 