CAPT OF AN LTT ACR ACFT OVERSHOT FINAL APCH INTERCEPT ALT DUE TO THE FO NOT PROVIDING A 500 FT REMINDER.

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WE WERE ON VECTORS FOR THE ILS 29 AT PWM; CLRED TO DSND TO 1800 FT ON A HDG TO INTERCEPT THE LOC. I; AS PNF; HAD LEFT THE ATC FREQ AFTER ACKNOWLEDGING THE HDG AND ALT; TO SPEAK TO COMPANY OPS. I HAD SET 1800 FT IN THE ALERTER BEFORE LEAVING THE FREQ. JUST AS I CAME BACK ON THE FREQ; I NOTICED THE CAPT DSNDING THROUGH 1800 FT AT ABOUT 800- 1000 FPM. I QUICKLY X-CHKED FOR FULL SCALE LOC AND GS INDICATIONS; LOOKED AT THE ALT ALERTER; AND CALLED OUT 'LEVEL 1800.' THE CAPT IMMEDIATELY BEGAN A CORRECTION. SIMULTANEOUSLY; THE ALT ALERTER CHIMED; AND ATC CALLED WITH A LOW ALT ALERT. THE CAPT HAD DSNDED TO APPROX 1400 FT. WHEN WE GOT ON THE GND; THE CAPT BLAMED HIS EXCURSION ON MY FAILURE TO MAKE A '500 FT TO GO' CALLOUT. I HAD; HOWEVER; BEEN OFF THE FREQ; AND IN THIS CASE; COMPANY POLICY REQUIRED HIM TO MAKE THE CALLOUT. I BELIEVE THE CAUSE OF THIS OCCURRENCE TO HAVE BEEN FATIGUE ON THE PART OF THE CAPT CAUSED BY COMPUTER SCHEDULING TAKING FULL ADVANTAGE OF THE DUTY TIME LIMITS OF FAR 135 RESULTING IN EXTREMELY LONG DUTY DAYS; SCHEDULED REDUCED REST LAYOVERS; AND UP TO 10-12 FLTS PER DAY WITH NO AUTOPLT OR FLT DIRECTOR.