TM 1-1510-223-10 2-76           Change 3 CABIN LIGHTS, in the overhead circuit breaker panel. (4)    Cabin door latching mechanism light. A light is provided to check the cabin door latching mechanism. It is controlled by a red pushbutton switch located adjacent to the round observation window, which is just above the second step. 2-77.   EMERGENCY LIGHTING SYSTEM. An independent battery-operated emergency lighting system is installed. The system is actuated automatically by shock, such as a forced landing. It provides adequate lighting inside and outside the fuselage to permit the oc- cupants to read instruction placards and locate exits. An inertia switch, when subjected to a 2 G (minimum) shock will illuminate the interior lights in the cockpit, forward and aft cabin areas, exterior lights aft of the emergency exit, and aft of the cabin door. The battery power source is au- tomatically recharged by the aircraft electrical system. a.   Emergency Lighting System Operation. An emer- gency  lights  override  switch,  located  on  the  overhead control panel (fig. 2-13), is provided to turn the system off if  it  is  accidentally  actuated.  The  switch  is  placarded EMERGENCY OFF/RESET - AUTO - TEST. Should the system accidently actuate, the emergency lights will illu- minate. Placing the switch in the momentary OFF/RE- SET position will extinguish the lights. To test the system, place the switch in the momentary TEST position. The lights should illuminate. Moving the switch to the OFF/ RESET position will turn the system off and reset it. The switch is normally in the AUTO position. Section  XI. FLIGHT INSTRUMENTS 2-78.   PITOT SYSTEM. The pitot system (fig. 2-33) provides ram air pressure for the airspeed indicators and air data computer. The pi- tot system consists of two pitot masts (one located on each side of the lower portion of the nose), and associat- ed  plumbing.  The  pitot  masts  are  protected  from  ice formation by internal electric heating elements. 2-79.   STATIC AIR SYSTEM. a.   Description. The static system (fig 2-33) provides static air pressure for the pilot’s and copilot’s airspeed indicators, copilot’s altimeter, air data computer, and pi- lot’s and copilot’s vertical speed indicators. The static air pressure ports are located on the right and left sides of the aft fuselage exterior skin. b.   Alternate Static Air Source. An alternate static air line, which terminates just aft of the rear pressure bulk- head, provides a source of static air for the pilot’s instru- ments in the event of source failure from the pilot’s static air line. A control on the pilot’s subpanel placarded PI- LOTS STATIC AIR SOURCE, may be actuated to select either the NORMAL or ALTERNATE air source by a two position selector valve. The valve is secured in the NOR- MAL position by a spring clip. 2-80.   TURN-AND-SLIP INDICATOR. A turn-and-slip indicator is installed on the pilot’s side of the instrument panel (fig. 2-16). This indicator is gyro- scopically operated and pneumatically powered. 2-81.   AIRSPEED INDICATORS. Two identical airspeed indicators are installed sepa- rately on the pilot’s and copilot’s sides of the instrument panel (fig. 2-16). These indicators require no electrical power for operation. The indicator dials are calibrated in knots from 40 to 300. A striped pointer automatically dis- plays the maximum allowable airspeed at a given aircraft altitude. 2-82.   COPILOT’S BAROMETRIC ALTIMETER. a.   Description. The copilot’s barometric altimeter (fig. 2-34), provides an indication of the aircraft’s pressure altitude above sea level. b.   Controls, Indicators, and Functions. (1)    Deleted. (2)    Altitude indicator needle. Used in conjunction with altitude scale to display aircraft altitude in hundreds of feet. (3)    Barometric  pressure  counter-drum  indicator (millibars). Indicates barometric pressure in millibars that has been set by the barometric pressure setting knob. (4)    Barometric  pressure  counter-drum  indicator (Inches  of  mercury).  Indicates  barometric  pressure  in inches of mercury that has been set by the barometric pressure setting knob. (5)    Barometric  pressure  setting  knob.  Used  to manually set barometric pressure displayed in the IN HG and MB windows. (6)    Counter-drum  altitude  display.  Indicates  air- craft   altitude   in   tens   of   thousands,   thousands,   and hundreds of feet above sea level.