A B757-200 IN CRUISE AT FL350 DECLARED AN EMER AND DIVERTED DUE TO SMOKE AND FIRE IN THE COCKPIT. CAUSED BY A LOOSE WINDOW ELECTRICAL TERMINAL.

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FLT XXX WAS DISPATCHED ZZZ-SFO WITH 1 MEL DEFERRAL FOR THE L RECIRCULATION FAN DUE TO AN INBOUND CREW COMPLAINT OF AN ACRID SMELL APPARENTLY EMANATING FROM THE L RECIRCULATION FAN. ACR MAINT; AFTER COORD WITH LCL CONTRACT MAINT; DETERMINED THE FLT COULD BE DISPATCHED PER THE MEL AND APPROPRIATE OPS PLACARDS. ALL OPS PLACARD LIMITATIONS WERE REVIEWED AND COMPLIED WITH AND THE FLT DEPARTED ZZZ UNEVENTFULLY. APPROX 25 NM SE OF TXK AT FL350 THE FO AND I DETECTED AN UNUSUAL ODOR IN THE COCKPIT FOLLOWED SHORTLY BY A FAINT BLUE HAZE OF WHAT APPEARED TO BE SMOKE SUSPENDED IN THE AIR. AS THE CONCENTRATION OF ODOR AND SMOKE INCREASED THE FO AND I DONNED OUR OXYGEN MASKS AND SMOKE GOGGLES AND PERFORMED THE SMOKE/FUMES/ODOR EMER QRC CHKLIST. THAT CHKLIST IN TURN LED TO THE ELECTRICAL SYS FIRE OR SMOKE IRREGULAR CHKLIST PAGE 7-8. AT THIS POINT THE SMOKE SOURCE COULD NOT BE POSITIVELY IDENTIFIED AND THE CHKLIST DIRECTED THE UTILITY BUS SWITCHES TO BE TURNED OFF. AFTER TURNING BOTH UTILITY BUS SWITCHES OFF; THE CONCENTRATION OF SMOKE AND ODOR RAPIDLY INCREASED IN INTENSITY AND A SHAFT OF FIRE ERUPTED DOWN THE INSIDE PANE OF THE FO'S FORWARD WINDSHIELD. THIS SIT NOW CONFIRMED THE SOURCE OF SMOKE; ODOR; AND FIRE WAS FROM THE WINDOW HEAT SYS; AND ALL 4 WINDOW HEAT SWITCHES WERE IMMEDIATELY TURNED OFF REMOVING ALL ELECTRICAL PWR TO THE WINDOW HEAT AND THE FLAMES SUBSIDED. AN EMER WAS DECLARED WITH ZFW DUE TO FIRE IN THE COCKPIT AND VECTORS TO THE NEAREST ARPT WERE REQUESTED DUE TO THE SEVERITY OF OUR SIT. I INSTRUCTED THE FO WHO WAS SERVING AS THE PF TO COMMENCE AN EMER DSCNT WHILE PERFORMING THE EMER DSCNT QRC CHKLIST. DURING THE EMER DSCNT THE FO'S WINDSHIELD BEGAN CRACKING AND THE ACFT WAS SLOWED AS THE STRUCTURAL INTEGRITY OF THE WINDSHIELD WAS NOW SUSPECT. THE PURSER WAS INFORMED TO PREPARE THE CABIN FOR A POSSIBLE EVAC UPON LNDG AND THAT WE HAD ABOUT 7-8 MINS UNTIL LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE UNUSUAL ODOR AND SMOKE INCREASED AND THE OXYGEN MASKS WERE DONNED AND THE CHKLIST PERFORMED. THE RPTR SAID AT THE PART OF THE CHKLIST WHERE THE UTILITY BUSSES ARE SWITCHED OFF; A SHAFT OF FIRE SHOT DOWN THE FO'S INSIDE PANE AND SET THE WINDOW SEALANT ON FIRE. THE RPTR STATED THE WINDOW HEAT SWITCHES WERE TURNED OFF AND THE FLAMES WENT OUT. THE RPTR SAID THE WINDOW INTERIOR WIRE WAS VAPORIZED AND THE EXTERNAL WIRING WAS BURNED FROM THE WINDOW CONTACTS DOWN THE WIRING. THE RPTR SAID SOOT FROM THE BURNED WIRING AND SEALANT WAS PRESENT IN THE COCKPIT FLOATING IN THE AIR. THE RPTR STATED DURING THE EMER DSCNT THE WINDOW SHATTERED AND WHEN ON THE GND; FLAKES OF GLASS PARTICLES WERE EMITTED FROM THE WINDOW INTO THE COCKPIT. THE RPTR SAID MAINT DETERMINED THE CAUSE WAS A WINDOW HEAT PWR WIRE TERMINAL SECURED TO THE WINDOW BY A SMALL SCREW WHICH CAME LOOSE; ALLOWING THE TERMINAL TO ARC AND THEN SHORT TO THE WINDOW FRAME. THE RPTR STATED THE VISIBLE FLAME HAD TRAVELED DOWN THE EDGE OF THE WINDOW AND DID NOT TRAVEL UPWARD TOWARD THE OVERHEAD PANEL. THE RPTR SAID DURING THIS EVENT NO CIRCUIT BREAKERS TRIPPED NOR WERE ANY FOUND TRIPPED WHEN CHKED ON THE GND. THE RPTR STATED THE CARRIER IS QUESTIONING THE USE OF WINDOW SEALANT THAT SUPPORTS COMBUSTION AND ALLOWING A SUITABLE SUBSTITUTE THAT WILL NOT BURN.