B767-300F FO BELIEVES DEPARTURE WITHOUT REQUIRED MAINTENANCE DOCUMENT WAS THE RESULT OF FATIGUE.

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AN AIRWORTHINESS RELEASE IS REQUIRED PRIOR TO EACH FLT BY OUR GOM. WE DEPARTED ZZZ2 WITHOUT HAVING THE AIRWORTHINESS RELEASE SIGNED OFF BY MAINT. THIS WAS DISCOVERED ABOUT 45 MINS IN THE FLT AS WE STARTED TO FILL OUT THE MAINT LOGBOOK. 1) WE ALMOST ALWAYS FLY AS EITHER A 3 OR 4 MAN CREW. THIS WAS A SHORT TRIP; SO IT WAS ONLY A 2 MAN CREW. THERE WAS NOT THE EXTRA SET OF EYES THAT IS USUALLY CHKING THE LOGBOOK. 2) THE FLT WAS ORIGINALLY SCHEDULED FOR AN XA00 LCL DEP FROM ZZZ WITH A RETURN TO ZZZ AT XF30. THE FLT WAS DELAYED UNTIL AN XC20 DEP NOW ARRIVING BACK IN ZZZ AT XI50. WE NOW HAD AN XB00 CALLOUT. I TRIED TO GET REST BUT PROBABLY ONLY HAD 1 GOOD HR OF SLEEP PRIOR TO THE FLT. 3) THE TIME SPENT ON THE GND IN ZZZ2 WAS USED LOADING THE FMC; PERFORMANCE CALCULATIONS; AND A LONG DISCUSSION ON ENG-OUT PROCS AT THIS DEMANDING ARPT. 4) AFTER LNDG IN ZZZ THE CAPT SPOKE TO THE MECHS MEETING THE FLT AND AFTER REACHING OPS CONTACTED THE DIRECTOR OF OPS TO LET HIM KNOW ABOUT OUR SITUATION. 5) IN MY CASE; I BELIEVE THE MAIN CAUSE WAS FATIGUE. 6) IN THE CARGO INDUSTRY; WE ARE OFTEN FLYING VERY TIRED DUE TO THE VERY LIBERAL REST REQUIREMENTS AND THE CRAZY PART 91 TAIL END FERRY. TO PREVENT AN OCCURRENCE OF THIS AGAIN; I WOULD RECOMMEND THAT SOMETHING BE INCLUDED IN A CHKLIST TO ENSURE ALL PAPERWORK ITEMS ARE COMPLETE.