LTT ATX CARGOLINER DSNDED THROUGH ASSIGNED ALT DUE TO CREW FATIGUE.

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FOR THE LAST APPROX 2 YRS I HAVE FLOWN FOR AN ON-DEMAND FREIGHT AIRLINE. FROM THE MOMENT WE ARE LEGAL; WE ARE 'ON CALL' 14 HRS ON DUTY; AND 10 HRS OFF IS MORE OFTEN THE RULE THAN THE INTENDED EXCEPTION. ON SEP/THU/96 I AROSE AT XH00 AM LEGAL AND READY TO FLY. FULLY EXPECTING TO FLY SOMETIME DURING THAT DAY; I DIDN'T (COULDN'T) SLEEP UNTIL XA30 AM ON SEP/FRI/96. AT XC45 AM DISPATCH CALLED FOR A 45 MIN LAUNCH (SOP). HOWEVER; DUE TO A MAINT DELAY; WE DIDN'T DEPART YIP UNTIL XD45 AM FOR SDF. HAULED FREIGHT FROM SDF TO CAE (APCH TO MINIMUMS); CONTINUING TO SAV. OH YEAH; THE ENTIRE TRIP FLYING WITH A DEFERRED (TYPICAL) AUTOPLT. THE PROB OCCURRED DURING MY LEG HAND FLYING FROM SAV TO MIA. THINGS GOT BUSY ON THE DSCNT TO MIA. NO SLEEP; NO FOOD; AND HAND FLYING AT FL310 FOR 1 HR 20 MINS HAD TAKEN ITS TOLL. I WAS CLRED TO 8000 FT. WHEN WE PASSED THROUGH 7700 FT; THE ALT ALERTER SOUNDED. I REACTED AT 7500 FT AND CLBED TO THE ASSIGNED ALT. DURING THIS PERIOD OF A COUPLE OF SECONDS; MIA APCH ANNOUNCED 'YOU JUST BLEW THROUGH YOUR ALT.' WE WERE HANDED OFF AND LANDED WITHOUT FURTHER INCIDENT. 5 1/2 HRS LATER; BECAUSE OF THE OFTEN OCCURRING 'FREIGHT DELAY' WE DEPARTED MIA FOR ORF. WE PART 91'ED HOME FOR YIP ARRIVING AT XW20 PM LCL. THAT'S APPROX 20 HRS FROM CALL-OUT TILL BLOCK-IN. IN CONCLUSION; I BELIEVE THE MAJOR CONTRIBUTING FACTOR WAS LACK OF SLEEP; COMPOUNDED BY UNUSABLE AUTOPLT. THE CURRENT PART 135 DUTY AND REST PERIOD REGS NEED CHANGING; INCLUDING THE INFAMOUS 'FREIGHT DELAY;' WHICH CAN TURN WHAT SHOULD BE A NORMAL WORK DAY INTO 20 HRS OR MORE. ON MORE THAN ONE OCCASION; I HAVE AWAKENED FROM A LITTLE COPLT NAP; TO SEE THE CAPT STILL ENJOYING HIS. WELL; I'M STILL TIRED NOW; I HOPE I'VE MADE MY POINT.