TM 1-1500-204-23-4 2 Slowly   apply   pressure   to   pitot line  until  the  airspeed  indicator  reads  normal  cruising speed.    Use  appropriate  power  supply  in  accordance with pilot-static system being tested.  Pinch off pressure supply at flexible tube. 3 Tap indicator to remove friction effects.    When  indicator  pointer  drops  more  than  10 miles an hour (8.7 knots) in 1 minute, a leak is indicated. Repair   faulty   connections   or   cracks   in   pitot   lines   as necessary. 4 Remove    test    apparatus    from pitot tube. 5 Remove    tape    from    the    pitot tube drain holes. (c)   Static   line   leak   test   procedures. Static line leak test connections are shown in figure 4-9. Do tests using the procedures that follow: 1 Set   altimeter   pointers   to   field elevation. 2 Tape unused static ports. CAUTION When    applying    suction    to    the static  line,  the  airspeed  indicator and  computer  equipment  may  be damaged. 3 Slowly   apply   suction   to   static line  until  airspeed  indicator  reaches  full  scale.    Close vacuum source and tap altimeter at same time until rate of   climb   stabilizes   at   zero.      After   stabilization,   the altimeter should not drop more than 100 ft.  in the next minute. 4 Slowly     remove     suction     from static lines until altimeter reads field elevation. 5 Remove    test    apparatus    from static port. 6 Remove tape from static ports. d.   Pitot Static System Heater.  Pitot static tubes, as  shown  in  figure  4-10,  are  provided  with  heaters  to prevent  icing  during  flight.    Principles  of  operation  and testing    procedures    are    described    in    the    following paragraphs. (1)   Principles   of   operation.      During   ice- forming conditions, pilot heat is selected by a switch in the  cockpit.    Heating  elements  heat  the  pitot  head  to provide pitot anti-icing. (2) Testing.    Test  pitot  static  system  heater using the following paragraphs. (a)  Required  equipment.    No  special required equipment is needed. (b) Pitot static heater test procedures. Test using the procedures that follow: WARNING Exercise   caution   when   touching pitot  tube  with  fingers  as  serious burns may result. 1 Turn aircraft battery switch ON. 2 Turn pilot heater switch ON and check  for  heat  rise  by  touching  pitot  head  lightly  with fingers. 3 When    elements    do    not    heat within  2  minutes,  and  wiring  is  correct,  pitot  head  must be replaced. e.  Airspeed Indicators.  Principles of operator and testing    procedures    are    described    in    the    following paragraphs. (1)  Principles  of  operation.      The   airspeed indicator,  as  shown  in  figure  4-11,  shows  the  speed  of the  aircraft  relative  to  the  air  mass  through  which  it  is flying.      Each   instrument   is   a   pressure-sensitive   unit actuated by the differential between impact and static air pressures.    The  impact  (pitot)  pressure  is  transmitted from   the   pitot   head   to   the   inside   of   the   pressure- sensitive diaphragm in the instrument case, while static pressure is applied to the interior of the case. Figure 4-10.  Pitot Tube 4-12