A B757-200 IN CRUISE FLYING INTO SUNSET ENCOUNTERED TURB; FO'S SUN VISOR SPRUNG LOOSE STRIKING THE CARGO COMPARTMENT FIRE CTL PANEL DISCHARGING THE AFT COMPARTMENT FIRE BOTTLE.

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I WAS CAPT ON A PAX FLT FROM ZZZ TO ZZZ1 ON 11/WED/03. WHILE FLYING INTO SUNSET; VISORS WERE PUT UP TO BLOCK SETTING SUN. HIT TURB; AND A VISOR FELL ONTO CTL PANEL AFT AND TO R OF THROTTLE QUADRANT. VISOR FELL WITH FORCE ACROSS THE AFT CARGO FIRE BOTTLE ARMING SWITCH; AND THEN HIT THE DISCHARGE SWITCH (GUARDED). THE FORCE OF THE IMPACT DISTORTED THE PLASTIC GUARD SUFFICIENTLY TO DISCHARGE THE AFT CARGO FIRE BOTTLE! THE ARMING SWITCH WAS THEN UN-ARMED. THE HALON RETARDANT MUST HAVE VENTED OVERBOARD; AS THERE WAS NO RESIDUE ON CARGO OR LUGGAGE; AND THE TWO ANIMALS BEING TRANSPORTED APPEARED FINE. COMPANY MAINT AND DISPATCHER WERE NOTIFIED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE WINDSHIELD SUN VISOR CLAMPS ARE MADE OF PLASTIC AND CLAMPED WITH A .250 INCH BOLT AND WING NUT AND DO NOT WORK FOR SEVERAL REASONS; WING NUT JUST SPINS OR BOTTOMS OUT ON THE PLASTIC CLAMP. THE RPTR SAID THE VISOR IS THEN POSITIONED AND SECURED IN PLACE BY WEDGING IT AGAINST THE GLARE SHIELD WITH A TWISTING FORCE PUTTING IT IN TENSION. THE RPTR STATED WHEN HITTING TURB THE SUN VISOR SPRUNG OUT WITH GREAT FORCE STRIKING THE CARGO COMPARTMENT FIRE CONTROL PANEL LOCATED AFT OF THE FO'S L KNEE. THE RPTR SAID VISOR STRUCK THE AFT CARGO COMPARTMENT ARMING SWITCH ACTIVATING THE BELL; CLOSING THE OUTFLOW VALVE AND TURNING OFF THE COMPARTMENT HEAT. THE RPTR STATED THE VISOR THEN STRUCK THE FIRE BOTTLE DISCHARGE SWITCH LOCATED AFT OF THE ARMING SWITCH AND DISCHARGED THE HALON FIRE BOTTLE INTO THE COMPARTMENT. THE RPTR SAID THE ARMING SWITCH WAS DISARMED IMMEDIATELY BY THE CREW ALLOWING THE HALON TO VENT OVERBOARD WHEN THE OUTFLOW VALVE WAS OPENED OR PARTLY OPENED. THE RPTR STATED THE SWITCHES ARE GUARDED WITH SOFT PLASTIC WING LIKE GUARDS UNLIKE THE RED PLASTIC GUARDS THAT MUST BE LIFTED TO ACCESS THE SWITCH. THE RPTR SAID THE AFT PIT WAS LOADED WITH TWO LIVE ANIMALS BOTH SURVIVING THE EVENT. THE RPTR STATED THE SUN VISOR POSITIONING CLAMP SHOULD BE MAINTAINED IN WORKING ORDER. THE RPTR SAID NO EFFECT WAS NOTED IN THE PAX CABIN WITH FIRE BOTTLE DISCHARGE.