PLT OF SMT TWIN ATX FOUND HIMSELF 3500 FT ABOVE ASSIGNED ALT DUE TO A STUCK ALTIMETER.

Narrative

THE ACFT WAS A CESSNA 421 GOLDEN EAGLE. I CLBED UP TO AND LEVELED OFF AT MY REQUESTED AND ASSIGNED ALT OF 12000 FT MSL. AFTER ABOUT 4000 FT MSL ON THE CLB; CONDITIONS BECAME IMC AND REMAINED THAT WAY UP UNTIL THE OCCURRENCE OF THE INCIDENT. I WAS HAND FLYING THE ACFT LEVEL AT 12000 FT MSL AS INDICATED ON THE PLTS ALTIMETER; FOR APPROX 10 MINS; WHEN SUDDENLY THE ALTIMETER STARTED TO RAPIDLY SPIN UPWARD. WHEN IT STOPPED IT WAS INDICATING 15500 FT MSL. VERY SOON AFTER THIS TOOK PLACE; AND WHILE I AM TRYING TO FIGURE OUT WHAT WENT WRONG; THE CTLR ASKED ME ABOUT MY ALT. I TOLD HIM THAT I HAD EXPERIENCED SOME SORT OF ALTIMETER MALFUNCTION AND ASKED AT WHAT ALT HE SHOWED ME. HE SAID 15000 FT MSL WHICH WAS WHAT MY ALTIMETER ALSO WAS NOW INDICATING. AFTER THAT CONFIRMATION; HE ASKED ME WHAT ALT I WANTED TO MAINTAIN. I REQUESTED 14000 FT WHICH HE CLRED ME TO. THAT WAS THE END OF THE DISCUSSION ABOUT THE PROB AT THE TIME. BY NOW I REALIZED WHAT HAD HAPPENED WAS MY ALTIMETER FOR SOME REASON HAD BECOME STUCK SOON AFTER I LEVELED OFF AT 12000 FT MSL. THE ACFT HOWEVER HAD NOT YET COMPLETELY REACHED ITS CRUISING SPD SO IT AGAIN BEGAN TO CLB SLOWLY AND I DID NOT DETECT IT UNTIL THE ALTIMETER FINALLY BECAME UNSTUCK AND IT THEN SPUN ON UP TO MY ACTUAL ALT. I WAS ABLE TO COMPLETE THE FLT WITH NO FURTHER PROB OR A REOCCURRENCE OF THE ALT STICKING. AFTER THE FLT WE CALLED AND TALKED TO THE CTLRS SUPVR AND EXPLAINED THE PROB TO HIM. IT DID NOT CAUSE A LOSS OF SEPARATION AND THE SUPVR SAID THAT THERE WOULD BE NO ACTION TAKE ON IT. I LEARNED SEVERAL THINGS FROM THIS INCIDENT; BUT I BELIEVE THE MOST IMPORTANT ONE AND THE ONLY THING THAT COULD PREVENT SOMETHING LIKE THIS FROM HAPPENING AGAIN IS ONES CONTINUOUS SCAN OF ALL FLT INSTS; INCLUDING AND ANY DUPLICATE INSTS THAT MAY BE PRESENT; SUCH AS THE ALTIMETER ON THE COPLTS SIDE. I ALSO FAILED TO NOTICE THAT MY VSI MUST HAVE BEEN SHOWING MY CLB RATE WHICH SHOULD HAVE ALERTED ME TO THE FACT THAT THINGS WERE NOT ADDING UP AND COULD HAVE HELPED ME REMEDY THE SIT BEFORE IT GOT AS BAD AS IT DID. I WAS FLYING WITHOUT A COPLT AND THIS IS AN INCIDENT WHERE THE PRESENCE OF ONE MIGHT HAVE ALSO HELPED TO PREVENT THIS OCCURRENCE. SINCE MY JOB DOES NOT PROVIDE A COPLT FOR ME I MUST DO HIS JOB AND PERFORM ALL THE FUNCTIONS THAT HE WOULD NORMALLY DO; THAT I AM CAPABLE OF; AND THAT WILL NOW INCLUDE A MORE FREQUENT CHK OF THE INSTS ON HIS SIDE OF THE COCKPIT.