deck. Due to the venturi effect of the air flowing around the  fuselage, the air pressure inside the flight deck is lower than  the exterior pressure.  When the alternate static source pressure is used, the  following instrument indications are observed:  Each pilot is responsible for consulting the Aircraft Flight  Manual (AFM) or the Pilot’s Operating Handbook (POH)  to determine the amount of error that is introduced into the  system when utilizing the alternate static source. In an aircraft  not equipped with an alternate static source, an alternate  method of introducing static pressure into the system should  a blockage occur is to break the glass face of the VSI. This  most likely renders the VSI inoperative. The reason for  choosing the VSI as the instrument to break is that it is the  least important static source instrument for flight.  The altimeter is an instrument that measures the height of  an aircraft above a given pressure level. Pressure levels  are discussed later in detail. Since the altimeter is the only  instrument that is capable of indicating altitude, this is one of  the most vital instruments installed in the aircraft. To use the  altimeter effectively, the pilot must understand the operation  of the instrument, as well as the errors associated with the  altimeter and how each affect the indication.  A stack of sealed aneroid wafers comprise the main  component of the altimeter. An aneroid wafer is a sealed  wafer that is evacuated to an internal pressure of 29.92  inches of mercury ("Hg). These wafers are free to expand  and contract with changes to the static pressure. A higher  static pressure presses down on the wafers and causes them  to collapse. A lower static pressure (less than 29.92 "Hg)  allows the wafers to expand. A mechanical linkage connects  the wafer movement to the needles on the indicator face,  which translates compression of the wafers into a decrease  in altitude and translates an expansion of the wafers into an  increase in altitude.  [Figure 8-2]  Notice how the static pressure is introduced into the rear of the  sealed altimeter case. The altimeter’s outer chamber is sealed,  which allows the static pressure to surround the aneroid  wafers. If the static pressure is higher than the pressure in the  aneroid wafers (29.92 "Hg), then the wafers are compressed  The presentation of altitude varies considerably between  different types of altimeters. Some have one pointer while  others have two or more. Only the multipointer type is  discussed in this handbook. The dial of a typical altimeter  is graduated with numerals arranged clockwise from zero  to nine. Movement of the aneroid element is transmitted  through gears to the three hands that indicate altitude. In  Figure 8-2 , the long, thin needle with the inverted triangle  at the end indicates tens of thousands of feet; the short, wide  needle indicates thousands of feet; and the long needle on  top indicates hundreds of feet.  The pressure altimeter is an aneroid barometer that measures  the pressure of the atmosphere at the level where the altimeter is  located and presents an altitude indication in feet. The altimeter  uses static pressure as its source of operation. Air is denser  at sea level than aloft—as altitude increases, atmospheric  pressure decreases. This difference in pressure at various levels  causes the altimeter to indicate changes in altitude.  until the pressure inside the wafers is equal to the surrounding  static pressure. Conversely, if the static pressure is less than  the pressure inside of the wafers, the wafers are able to expand  which increases the volume. The expansion and contraction  of the wafers moves the mechanical linkage which drives the  needles on the face of the altimeter.  Figure 8-2.  Altimeter.  Principle of Operation  Altimeter  1.  The altimeter indicates a slightly higher altitude than  actual.  2.  The ASI indicates an airspeed greater than the actual  airspeed.  3. The VSI shows a momentary climb and then stabilizes  if the altitude is held constant.  8-3  Aneroid wafers  Static port  100 ft. pointer  A crosshatched area appears on  some altimeters when displaying  an altitude below 10 , 000 feet MSL.  Crosshatch flag  Barometric scale adjustment knob  10,000 ft. pointer  1,000 ft. pointer  Altimeter setting window 