Narrative:

We departed flight (#) en route to ZZZ1. We were IMC at FL280 with the engine tai on when the window heating element on the L1 window (captain's windscreen) started to arc. Almost immediately; a large crack appeared that ran from the top of the windscreen by the heating element across to the left side of the window. I reached up and turned off the window heat switch and the arcing stopped. The first officer took out the QRH and we tried to determine how bad the crack was. At that point it was just 1 crack and the QRH seemed to indicate that it was ok to continue. Then as we were talking it over; we started hearing cracking noises and a series of large cracks started appearing all over the window. At that time; it was starting to get dark; we were flying in icing conditions with no window heat and we couldn't tell whether the cracks were all the way through the window or not. Also; our last leg had been from ZZZ1; so we knew that the IMC lasted all the way to ZZZ1. I decided that it would be best to assume worst case (that it was cracked all the way through) and we complied with the QRH recommendation that we get the cabin differential down to 5.0 psi. At the time it was 7.8 psi. That was my primary concern. Center told us that there was another aircraft underneath us; so I asked for an off course vector in order to start down. I noticed that the cracks were continuing to get worse; so I decided that we needed priority handling. I declared an emergency and we landed short in ZZZ2. Other than a few vectors off course about 20 mi west of ZZZ2; the approach and landing were uneventful. After about a 40 min wait on the ground; we got a new aircraft and flew to ZZZ1. Supplemental information from acn 764351: during cruise flight; around FL280; I noticed the windscreen cracking due to what appeared to be electrical arcing on the outer L1 windscreen. The captain turned off the associated window heat and directed me to reference the QRH. While referencing the QRH and talking about our options; the windscreen spider webbed and deteriorated further. Initially; we thought we could continue; however; the further deterioration of the windscreen and the close proximity of ZZZ2; we decided it was best to descend and divert to ZZZ2. We descended immediately to reduce pressurization differential and got vectors for an uneventful ILS and landing at ZZZ2.

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Original NASA ASRS Text

Title: IN IMC CONDITIONS; A B737-300 CAPTAIN'S L-1 WINDOW (FWD WINDSCREEN); HEATING ELEMENT STARTED TO ARC WITH A LARGE CRACK APPEARING FOLLOWED BY NUMEROUS OTHER LARGE CRACKS. EMERGENCY DECLARED.

Narrative: WE DEPARTED FLT (#) ENRTE TO ZZZ1. WE WERE IMC AT FL280 WITH THE ENG TAI ON WHEN THE WINDOW HEATING ELEMENT ON THE L1 WINDOW (CAPT'S WINDSCREEN) STARTED TO ARC. ALMOST IMMEDIATELY; A LARGE CRACK APPEARED THAT RAN FROM THE TOP OF THE WINDSCREEN BY THE HEATING ELEMENT ACROSS TO THE L SIDE OF THE WINDOW. I REACHED UP AND TURNED OFF THE WINDOW HEAT SWITCH AND THE ARCING STOPPED. THE FO TOOK OUT THE QRH AND WE TRIED TO DETERMINE HOW BAD THE CRACK WAS. AT THAT POINT IT WAS JUST 1 CRACK AND THE QRH SEEMED TO INDICATE THAT IT WAS OK TO CONTINUE. THEN AS WE WERE TALKING IT OVER; WE STARTED HEARING CRACKING NOISES AND A SERIES OF LARGE CRACKS STARTED APPEARING ALL OVER THE WINDOW. AT THAT TIME; IT WAS STARTING TO GET DARK; WE WERE FLYING IN ICING CONDITIONS WITH NO WINDOW HEAT AND WE COULDN'T TELL WHETHER THE CRACKS WERE ALL THE WAY THROUGH THE WINDOW OR NOT. ALSO; OUR LAST LEG HAD BEEN FROM ZZZ1; SO WE KNEW THAT THE IMC LASTED ALL THE WAY TO ZZZ1. I DECIDED THAT IT WOULD BE BEST TO ASSUME WORST CASE (THAT IT WAS CRACKED ALL THE WAY THROUGH) AND WE COMPLIED WITH THE QRH RECOMMENDATION THAT WE GET THE CABIN DIFFERENTIAL DOWN TO 5.0 PSI. AT THE TIME IT WAS 7.8 PSI. THAT WAS MY PRIMARY CONCERN. CTR TOLD US THAT THERE WAS ANOTHER ACFT UNDERNEATH US; SO I ASKED FOR AN OFF COURSE VECTOR IN ORDER TO START DOWN. I NOTICED THAT THE CRACKS WERE CONTINUING TO GET WORSE; SO I DECIDED THAT WE NEEDED PRIORITY HANDLING. I DECLARED AN EMER AND WE LANDED SHORT IN ZZZ2. OTHER THAN A FEW VECTORS OFF COURSE ABOUT 20 MI W OF ZZZ2; THE APCH AND LNDG WERE UNEVENTFUL. AFTER ABOUT A 40 MIN WAIT ON THE GND; WE GOT A NEW ACFT AND FLEW TO ZZZ1. SUPPLEMENTAL INFO FROM ACN 764351: DURING CRUISE FLT; AROUND FL280; I NOTICED THE WINDSCREEN CRACKING DUE TO WHAT APPEARED TO BE ELECTRICAL ARCING ON THE OUTER L1 WINDSCREEN. THE CAPT TURNED OFF THE ASSOCIATED WINDOW HEAT AND DIRECTED ME TO REF THE QRH. WHILE REFING THE QRH AND TALKING ABOUT OUR OPTIONS; THE WINDSCREEN SPIDER WEBBED AND DETERIORATED FURTHER. INITIALLY; WE THOUGHT WE COULD CONTINUE; HOWEVER; THE FURTHER DETERIORATION OF THE WINDSCREEN AND THE CLOSE PROX OF ZZZ2; WE DECIDED IT WAS BEST TO DSND AND DIVERT TO ZZZ2. WE DSNDED IMMEDIATELY TO REDUCE PRESSURIZATION DIFFERENTIAL AND GOT VECTORS FOR AN UNEVENTFUL ILS AND LNDG AT ZZZ2.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.