DISPATCHED WITH ONE PACK INOPERATIVE; B767-200 IS UNABLE TO MAINTAIN PRESSURIZATION AFTER SUFFERING A CRACKED WINDSHIELD DURING DESCENT AT FL250.

Narrative

EVENT OCCURRED ON ARR USING VNAV DSCNT (FORGOT TO PROGRAM HIGHER AIRSPD). VNAV INITIATED DSCNT OUT OF FL350 AT TOUCHDOWN; ALL NORMAL UNTIL MACH/AIRSPD CHANGEOVER. AT 250 KTS FMC DISPLAYED DRAG DEVICE REQUIRED; AT WHICH TIME I REALIZED MY MISTAKE. SPD INTERVENED TO INCREASE SPD TO 290 KTS. AT ABOUT 260-270 KTS AND ABOUT FL250 CAPT'S WINDSHIELD CRACKED. I IMMEDIATELY REDUCED SPD TO 230 KTS AND CAPT DIRECTED FURTHER REDUCTION TO 210 KTS WHICH WAS MAINTAINED UNTIL BASE TO FINAL AND SLOWING TO CONFIGURE/LAND. AFTER THE WINDSHIELD HAD CRACKED THE CAPT OBSERVED THE CABIN ALT CLBING. SINCE WE WERE DISPATCHED WITH 1 PACK WE CONCLUDED THIS WAS A RESULT OF THE IDLE PWR DSCNT. CAPT DIRECTED USE OF SPD BRAKES IN ORDER TO INCREASE PWR IN HOPES OF REGAINING CTL OF THE CABIN ALT. THE INCREASE PWR HAD NO APPARENT EFFECT ON CABIN ALT/PRESSURIZATION. THE MAX OBSERVED CABIN ALT WAS APPROX 9500 FT AND THERE WAS NEVER A CABIN ALT WARNING LIGHT. CAPT DECLARED AN EMER WITH CTR. CAPT RAN THE QRH FOR THE CRACKED WINDSHIELD (TURNED OFF HEAT). WE WERE BOTH KEEPING A CLOSE EYE ON THE WINDSHIELD (ALTHOUGH I COULD NOT SEE IT; AFTER LNDG CAPT MENTIONED HE SAW SPARKING AT THE BOTTOM OF THE WINDOW). DUE TO THE LACK OF PRESSURIZATION WE TRIED TO MAINTAIN 600-700 FPM RATE OF DSCNT. (THERE WAS SOME DIFFERENTIAL PRESSURE UNTIL SOMETIME BELOW 10000 FT AND WE KNEW THAT EVENTUALLY THE CABIN WOULD BE DSNDING AT THE SAME RATE AS THE ACFT.) ZZZ WAS LNDG XXL AND WE DECIDED THAT WOULD WORK SINCE WE NEEDED THE DISTANCE TO MAKE OUR SLOW DSCNT (WE OVERFLEW ARPT FOR A L-HAND VISUAL PATTERN). AT ONE POINT ON A DOWNWIND LEG THE DSCNT RATE DID GET UP TO 1500 FPM IN AN ATTEMPT TO GET DOWN QUICKLY FOR LNDG. THIS RATE WAS SLOWED TO ABOUT 600 FPM AND DOWNWIND EXTENDED SLIGHTLY. AN UNEVENTFUL LNDG WAS MADE AND THE EMER TERMINATED ON CLRING THE RWY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER HAD NO ADDITIONAL INFORMATION AS TO A MAINT RESOLUTION OF THE PROBLEM. HE DID ADVISE THE CRACK IN THE WINDSHIELD WAS AN INTERNAL PANE BUT THERE WAS NO OBVIOUS INDICATION OF AIR LEAKAGE; I.E. NO LOUD HISSING AS NORMALLY ASSOCIATED WITH ANY SIGNIFICANT LOSS OF AIR PRESSURE AT ITS SOURCE.