ACR CAPT FLYING WITH NEW CAPT IN TRAINING THAT IS NOT QUITE UP TO SPD CREATES A HVY COCKPIT WORKLOAD AND A HEADING AND TURN IS MISSED.

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WX DELAY INTO ORD CAUSED US TO SIT ON TXWY IN IND FOR OVER 6 HRS. PAX IRATE; NEW CAPT (I WAS CHKING OUT) WAS WEAK AND HAVING PROBS. 4 DAYS OF CONTINUAL ERRORS BY STUDENT AND LONG DELAY IN IND WITH PAX PROBS LEFT ME PHYSICALLY AND MENTALLY EXHAUSTED. ACFT MALFUNCTIONS FURTHER COMPLICATED MATTERS. ARR LEG FROM IND TO ORD WENT WELL UNTIL E OF ORD AT 7000 FT MSL AND RAPCON DIRECTED A TURN FROM 000 DEGS TO 020 DEGS. I READ BACK 020 DEGS AND LOOKED DOWN TO VERIFY ILS RWY 22R IDENT. WHEN I LOOKED BACK UP; WE WERE TURNING L; WITH A 220 DEG HDG IN THE MCP. I DIRECTED THE STUDENT TO STOP THE TURN (BUT FIRST ASKED HIM WHAT OUR ASSIGNED HEADING WAS; AS I HAD FORGOTTEN. HE SAID 020 DEGS; THEN 220 DEGS; THEN SAID HE COULD NOT REMEMBER. I COULD NOT REMEMBER AT THIS POINT DUE TO HVY COCKPIT WORKLOAD IN ORD ENVIRONMENT). WE STOPPED TURN AT 330 DEGS AND ATTEMPTED TO CONFIRM HEADING WITH ATC. THEY REPLIED 020 DEGS AND TOLD US TO DSND TO 4000 FT MSL; THEN QUICKLY REALIZED THAT WE HAD MADE A L TURN INSTEAD OF R; AND REASSIGNED US A 050 DEG HDG. AT THIS POINT; DUE TO FATIGUE; I QUICKLY BECAME TASK SATURATED AND EXPERIENCED; WHAT I CAN ONLY DESCRIBE AS; AN ALMOST COMPLETE LOSS OF SHORT TERM MEMORY. I REALIZED THIS; AND CONTINUALLY ASKED RAPCON TO REPEAT THEIR INSTRUCTIONS; WHICH WERE USUALLY GIVEN IN 2; 3; (OR 4 OCCASIONALLY) ITEMS EACH (TURN L 010 DEGS; DSND TO 4000 FT; SLOW TO 180 KIAS) AND PERHAPS (INTERCEPT RWY 22 LOC; MAINTAIN 2500 FT UNTIL ESTABLISHED 170 KTS TO THE MARKER; CONTACT TWR AT THE MARKER ON 126.9; OR OTHER FREQ). THE STUDENT FINALLY MADE ONE TOO MANY ERRORS AND I TOOK THE ACFT IN ORDER TO GET DOWN FOR THE LNDG. ONCE I TOOK THE ACFT; THE RELIEF FROM NOT HAVING TO WATCH EVERY STEP THE STUDENT MADE IMMEDIATELY RESULTED IN MY ABILITY TO BETTER RETAIN THE INSTRUCTIONS RECEIVED. I WAS NO LONGER TASK SATURATED. A THIRD PLT WOULD HAVE HELPED -- WOULD HAVE VERIFIED THE ILS; LEAVING ME TIME TO BETTER WATCH THE STUDENT ALTHOUGH I SHOULDN'T HAVE MISSED HIS HEADING ERROR; AND THE THIRD PLT MIGHT HAVE CAUGHT THE ERROR. I DON'T KNOW HOW YOU STOP FROM GETTING FATIGUED IN THIS BUSINESS -- LONG HRS; NO CONSIDERATION FOR CIRCADIAN RHYTHM; LITTLE CONSIDERATION FOR PROPER DIET (WE HADN'T HAD AN OPPORTUNITY TO EAT FOR 12 HRS). I NEEDED TO SAY IT'S OVER SOONER AND UNLOAD MYSELF -- TAKE THE ACFT. THE TRAINING WAS TERMINATED AFTER THIS LEG. WE WERE TOO TIRED TO SAFELY CONTINUE; ALTHOUGH SCHEDULED TO DO SO.