to the element with a switch in the cockpit when ice-forming  conditions exist. The pitot tube heat switch may be wired  so that when the ignition switch is turned off when the  aircraft is shut down, a pitot tube heater inadvertently left  on does not continue to draw current and drain the battery.  Caution should be exercised when near the pitot tube, as  these heating elements make the tube too hot to be touched  without receiving a burn.  The pitot-static tube is mounted on the outside of the aircraft  at a point where the air is least likely to be turbulent. It is  pointed in a forward direction parallel to the aircraft’s line  of flight. The location may vary. Some are on the nose of  the fuselage and others may be located on a wing. A few  may even be found on the empennage. Various designs exist  but the function remains the same, to capture impact air  pressure and static air pressure and direct them to the proper  instruments.  [Figure 10-24] Most aircraft equipped with a pitot-static tube have an  alternate source of static air pressure provided for emergency  use. The pilot may select the alternate with a switch in  the cockpit should it appear the flight instruments are not  providing accurate indications. On low-flying unpressurized  aircraft, the alternate static source may simply be air from  the cabin.  [Figure 10-25]  On pressurized aircraft, cabin  air pressure may be significantly different than the outside  ambient air pressure. If used as an alternate source for static  air, instrument indications would be grossly inaccurate. In  this case, multiple static vent pickup points are employed. All  are located on the outside of the aircraft and plumbed so the  pilot can select which source directs air into the instruments.  On electronic flight displays, the choice is made for which  source is used by the computer or by the flight crew. Another type of pitot-static system provides for the location  of the pitot and static sources at separate positions on the  aircraft. The pitot tube in this arrangement is used only to  gather ram air pressure. Separate static vents are used to  collect static air pressure information. Usually, these are  located flush on the side of the fuselage.  [Figure 10-26]  There  may be two or more vents. A primary and alternate source  vent is typical, as well as separate dedicated vents for the pilot  and first officer’s instruments. Also, two primary vents may  be located on opposite sides of the fuselage and connected  Figure 10-25.  On unpressurized aircraft, an alternate source of  static air is cabin air.  Figure 10-24.  Pitot-static system heads, or pitot tubes, can be of  various designs and locations on airframes.  Figure 10-26.  Heated primary and alternate static vents located on  the sides of the fuselage.  10-14