DC3 FLT CREW HAS ALT HDG DEV WHILE ENRTE.

Narrative

NEARLY 5 HRS INTO OUR FLT; I WAS REQUESTED (BY HAND SIGNAL) TO COME TO THE FLT DECK. THE PLT WAS GETTING UP OUT OF THE L SEAT; AND IN THE R SEAT WAS A MECH. I STEPPED TO THE L; TO ALLOW THE PLT TO PASS REARWARD. AS HE PASSED ME; HE SAID 'WE'RE SOMEWHERE IN FLORIDA ON A VECTOR.' I THEN SAT IN THE L SEAT; TURNED THE AIRPLANE TO THE HDG BUGGED. I ASKED THE MECH (WHOSE HANDS WERE STILL ON THE WHEEL) 'DO WE GO TO 11000 FT OR 10000 FT?' WE WERE AT 10600 FT. I FLEW TOWARD 11000 FT WHEN HE REPLIED 'NO PROB.' I FASTENED MY SEATBELT. I THEN CALLED 'CTR; CONFIRM ALT AND HDG.' THERE WAS A LONG PAUSE; AND A VOICE REPLIED 'WHO WAS THAT?' REPLIED 'ACFT X; PLEASE CONFIRM HDG AND ALT ASSIGNED.' A SHORT PAUSE; THEN 'ACFT X; YOUR LAST ASSIGNED ALT WAS 10000 FT.' AS I RECALL; WITHIN SEVERAL SECONDS; CTLR INFORMED US WE SHOULD BE AT 11000 FT OR 9000 FT FOR DIRECTION-OF-FLT. I ASKED FOR AND GOT 9000 FT. I HAD THE MECH LEAVE THE FLT DECK; AND GOT THE RELIEF PLT IN THE FRONT. HE FLEW WHILE I DETERMINED OUR POS USING GPS; VOR AND DME WITH A PAPER CHART. I ESTIMATE WE WERE OFF ALT LESS THAN 3 MINS. HUMAN PERFORMANCE CONSIDERATIONS. I; AS PIC; SHOULD HAVE SET A ROTATION SCHEDULE FOR THE 3 PLTS. I; AS PIC; SHOULD HAVE SET A POLICY OF 'ONLY PLTS' IN THE FRONT SEATS. I SHOULD HAVE INSISTED; AT THE TIME I WAS CALLED FORWARD; THAT THE PLT SIT BACK DOWN AND FLY; WHILE I REPLACED THE MECH IN THE R SEAT. THEN THE PLT COULD HAVE GOTTEN UP TO BE REPLACED BY THE RELIEF PLT.