FLC OF AN MLG ACR ACFT MISREAD THEIR ALTIMETER CAUSING A DEV FROM ASSIGNED ALT DURING CRUISE.

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WE DEPARTED COLD BAY AND CLBED TO OUR CRUISING ALT OF 23000 FT. IT WAS THE COPLT'S LEG AND HE WAS HAND FLYING THE ACFT; WHICH IS QUITE COMMON IN OUR OP. THIS PARTICULAR ACFT HAD BEEN EXPERIENCING AN ONGOING PROB WITH THE AUTOPLT. FOR SEVERAL WKS WE HAD BEEN WRITING UP THE AUTOPLT FOR CAUSING PORPOISING WITH EITHER ALT HOLD ON OR OFF. WHEN LEVELING OFF AT 23000 FT THE COPLT ENGAGED THE AUTOPLT. THE AIRPLANE IMMEDIATELY WENT INTO SOME FAIRLY WILD PORPOISING. AT THE SAME TIME THE COPLT'S PAPERWORK AND COFFEE WENT FLYING ALL OVER HIM. I COULD SEE HE WAS PRETTY WELL OVERLOADED SO I GRABBED THE YOKE AND TURNED OFF THE AUTOPLT. I HAND FLEW THE ACFT FOR A SHORT WHILE AND THEN REENGAGED THE AUTOPLT. I DID NOT REENGAGE THE ALT HOLD. I WANTED TO USE THE TRIM WHEEL ON THE AUTOPLT FOR AWHILE TO SEE IF IT WOULD CTL THE PORPOISING. AFTER A FEW MINS THE TRIM WHEEL WAS ABLE TO GET PRETTY GOOD CTL OVER THE PORPOISING. BUT; USING THE TRIM WHEEL REQUIRED CONSTANT ADJUSTMENTS TO MAINTAIN ALT. IN OTHER WORDS; IF LEFT UNATTENDED; THE ACFT COULD JUST WANDER OFF ALT. OF COURSE; THAT IS PRECISELY WHAT HAPPENED. WHEN THE COPLT WAS READY TO TAKE OVER AGAIN; WE EXECUTED ONE OF THE WORST 'TRANSFER OF CTL' I HAVE EVER SEEN. BASICALLY; I JUST SAID 'YOU GOT IT;' AND THAT WAS ABOUT ALL. I NEVER MENTIONED THAT THE ALT HOLD WAS OFF. THE COPLT ASSUMED THAT THE AUTOPLT WAS IN A GOOD MOOD AND EVERYTHING WAS ENGAGED. A SHORT TIME LATER; CTR CALLED US TO INQUIRE ABOUT OUR ASSIGNED ALT. IT WAS QUITE A SHOCK TO FIND THAT WE WERE CRUISING AT 24000 FT INSTEAD OF OUR ASSIGNED ALT OF 23000 FT! I'M NOT EXACTLY SURE WHAT HAPPENED; BUT I THINK IT WAS MY FAULT. I BELIEVE THAT WHEN WE HAD THE ORIGINAL ALT EXCURSION; AND I TOOK OVER CTL; I FLEW THE AIRPLANE TO THE NEAREST THOUSAND FT MARK ON MY ALTIMETER. IN OTHER WORDS; I MISREAD THE ALTIMETER. INSTEAD OF READING THE NUMBERS IN THE WINDOW OF THE ALTIMETER; I JUST LOOKED AT THE POINTER. THE COPLT HAS ONLY A 3-POINTER TYPE ALTIMETER; SO WHEN HE RESUMED CTL OF THE ACFT; IT IS UNDERSTANDABLE THAT HE WOULD ASSUME WE WERE AT THE CORRECT ALT. THIS SIT WOULD MAKE FOR A CLASSIC CASE OF MISREADING THE 3-POINTER TYPE ALTIMETER; WHICH IS FAMOUS FOR THIS PROB. BE THAT AS IT MAY BE; THERE IS NO EXCUSE FOR MY MISREADING THE SINGLE POINTER ALTIMETER ON MY SIDE.