1,000-foot counter and then add the 100-foot pointer indication. CAUTION It is possible to misinterpret the counter- pointer altimeter by 1,000 feet immediately before  or  after  the  1,000-foot  counter moves. This error is possible because the 1,000-foot   counter   changes   when   the foot-pointer   is   between   the   900-   and 1,000-foot  position. Counter-Drum-Pointer Altimeter.— The only real  difference  between  this  altimeter  and  the counter-pointer  altimeter  is  the  addition  of  a  100-foot drum  (fig.  2-9).  This  drum  follows  the  100-foot pointer,  and  it  is  this  drum  that  actuates  the 1,000-foot  counter.  In  this  way  it  prevents  the reading  error  when  the  1,000-foot  counter  switches. There are two methods of reading the indicated pressure   altitude. One   way   is   to   read   the counter-drum   window   without   referring   to   the 100-foot  pointer.  This  will  give  a  direct  readout  of both  thousands  and  hundreds  of  feet.  The  second  way is  to  read  the  counter  window  and  then  add  the 100-foot pointer indication. The pointer serves as a precise readout of values less than 100 feet. This sample altimeter has a servoed mode and a pressure  mode  of  operation.  The  mode  of  operation  is controlled  by  a  spring-loaded,  self-centering  mode switch, placarded RESET and STBY. In the servoed mode,  the  altimeter  displays  altitude,  corrected  for position error, from the synchro output of the air data computer. In the standby mode, the altimeter operates as a standard altimeter. In this mode, it uses static Figure  2-9.-Counter-drum-pointer  altimeter. pressure from the static system that is uncorrected for position  error. The  servoed  mode  is  selected  by  placing  the  mode switch to RESET for 3 seconds. The ac power must be  on.  During  standby  operation,  a  red  STBY  flag appears  on  the  dial  face.  The  altimeter  automatically switches  to  standby  operation  during  an  electrical power loss or when the altimeter or altitude computer fails.  The  standby  operation  is  selected  by  placing  the mode  switch  to  STBY.  An  ac-powered  internal vibrator  automatically  energizes  in  the  standby  mode to  lessen  friction  in  the  display  mechanism. PRESSURE ALTIMETER ERRORS.—  There are  five  categories  of  errors  relating  to  pressure altimeters. They are the mechanical error, the scale error, installation/position error, reversal error, and hysteresis  error. Mechanical Error.— Mechanical  error  is  caused by misalignments in gears and levers that transmit the aneroid  cell  expansion  and  contraction  to  the  pointers of the altimeter. This error is not constant, and it must be  checked  before  each  flight  by  the  setting procedure. Scale Error.— Scale  error  is  caused  by  irregular expansion  of  the  aneroid  cells.  It  is  recorded  on  a scale  correction  card  maintained  for  each  altimeter  in the instrument maintenance shop. Installation/Position   Error.—   Installation/ position  error  is  caused  by  the  airflow  around  the static ports. This error varies with the type of aircraft, airspeed, and altitude. The magnitude and direction of  this  error  can  be  determined  by  referring  to  the performance  data  section  in  the  aircraft  NATOPS manual. An altimeter correction card is installed in some aircraft that combines the installation/position and the scale errors. This card shows the amount of correction needed at different altitudes and airspeeds. Reversal  Error.—  Reversal error is caused by inducing false static pressure into the system. This normally  occurs  during  abrupt  or  huge  pitch  changes. This error appears on the altimeter as a momentary indication  in  the  opposite  direction. Hysteresis Error.— Hysteresis error is a lag in altitude  indication  due  to  the  elastic  properties  of  the material  within  the  altimeter.  This  occurs  after  an aircraft  has  maintained  a  constant  altitude  for  an extended period of time and then makes a large, rapid altitude  change. After  a  rapid  descent,  altimeter 2-9