Figure 10-36.  The effect of air temperature on aircraft performance  is expressed as density altitude.  Static system leaks can affect the static air input to the  altimeter or ADC resulting in inaccurate altimeter indications.  It is for this reason that static system maintenance includes  leak checks every 24 months, regardless of whether any  discrepancy has been noticed. See the instrument maintenance  section toward the end of this chapter for further information  on this mandatory check. It should also be understood  that analog mechanical altimeters are mechanical devices  that often reside in a hostile environment. The significant  vibration and temperature range swings encountered by  An analog vertical speed indicator (VSI) may also be referred  to as a vertical velocity indicator (VVI), or rate-of-climb  indicator. It is a direct reading, differential pressure gauge  that compares static pressure from the aircraft’s static system  directed into a diaphragm with static pressure surrounding  the diaphragm in the instrument case. Air is free to flow  unrestricted in and out of the diaphragm but is made to flow  in and out of the case through a calibrated orifice. A pointer  attached to the diaphragm indicates zero vertical speed when  the pressure inside and outside the diaphragm are the same.  The dial is usually graduated in 100s of feet per minute.  It should be noted that many modern altimeters are  constructed to integrate into flight control systems, autopilots,  and altitude monitoring systems, such as those used by ATC.  The basic pressure-sensing operation of these altimeters is the  same, but a means for transmitting the information is added.  The mechanical nature of the analog altimeter’s diaphragm  pressure measuring apparatus has limitations. The diaphragm  itself is only so elastic when responding to static air pressure  changes. Hysteresis is the term for when the material from  which the diaphragm is made takes a set during long periods  of level flight. If followed by an abrupt altitude change,  the indication lags or responds slowly while expanding or  contracting during a rapid altitude change. While temporary,  this limitation does cause an inaccurate altitude indication. the instruments and the pitot static system (i.e., the tubing  connections and fittings) can sometime create damage or a  leak, leading to instrument malfunction. Proper care upon  installation is the best preventive action. Periodic inspection  and testing can also assure integrity.  Figure 10-37.  The location of the static vent is selected to keep  altimeter position error to a minimum.  Vertical Speed Indicator Pitot tube probes Static ports Density Altitude Chart Outside air temperature Approximate density altitude (thousand feet) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 S.L. 14,000 13,000 12,000 11,000 10,000 9,000 Pressure altitude (feet) 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 –1,000 Sea level –18 –12° –7° –1° 4° 10° 16° 21° 27° 32° 38° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90°  100° Standard temperature F C 28.0 1,824 28.1 1,727 28.2 1,630 28.3 1,533 28.4 1,436 28.5 1,340 28.6 1,244 28.7 1,148 28.8 1,053 28.9 957 29.0 863 29.1 768 29.2 673 29.3 579 29.4 485 29.5 392 29.6 298 29.7 205 29.8 112 29.9 20 29.92 0 30.0  −73 30.1  −165 30.2  −257 30.3  −348 30.4  −440 30.5  −531 30.6  −622 30.7  −712 30.8  −803 Altimeter setting  ("Hg) Pressure altitude conversion factor 10-22