ALTDEV ALT OVERSHOT -- ACR CLBS ABOVE ITS ASSIGNED ALT WHEN CAPT GETS DISTRACTED.

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WHILE CLBING FROM FL290 TO FL310; I MISTAKENLY THOUGHT I HAD ENGAGED THE ALT HOLD SWITCH AT FL310. THE FO CALLED OUT OF FL300 FOR FL310 AND IN THE INTEREST OF SMOOTHNESS I DECREASED THE RATE OF CLB APPROPRIATELY. I APPARENTLY DIDN'T ENGAGE THE ALT HOLD AT FL310 AND THE ACFT WAS CLBING SO SLOWLY THAT WE DIDN'T NOTICE THE DRIFT UP UNTIL THE FO CALLED OUT WE WERE PASSING FL313. I IMMEDIATELY CORRECTED TO FL310. I BELIEVE THE CONTRIBUTING FACTORS WERE AS FOLLOWS: CHKING A PREVIOUSLY MALFUNCTIONING OMEGA SYS. CREWS SWITCHING BTWN BLOCK 4 (NON-ALT CAPTURE) AND BLOCK 5 (ALT CAPTURE) AUTOPLTS. BLOCK 4 AUTOPLTS HAVE NO DISPLAY ON APCH PROGRESS DISPLAY (APD) ANNUNCIATOR PANEL WHEN ALT HOLD IS ENGAGED. IN THE FUTURE I WILL BE MUCH MORE CAREFUL TO BE AWARE THAT ALT HOLD IS ACTUALLY ENGAGED AT PROPER ALT AND MAKE MY CREW MORE AWARE OF THIS ALSO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT SAID THAT HIS CHIEF PLT TOLD HIM THAT THE B727-200'S WITH THE BLOCK 4 AUTOPLTS HAVE MORE ALT DEVS THAN ALL OF THE REST OF THE FLEET COMBINED. IN THIS CASE THE CAPT SAID THAT HE WAS AWARE OF THE LACK OF AN ALT CAPTURE MODE AND SO HE HAD REDUCED THE CLB RATE TO BE VERY SLOW -- 100 OR 200 FPM HE THOUGHT -- IN ORDER TO BE ABLE TO ENGAGE ALT HOLD MODE ONCE THEY HAD ARRIVE AT FL310. HOWEVER; HE HAD GOTTEN DISTRACTED BY SOMETHING AND THE FO HAD TO ALERT HIM THAT THEY WERE HIGH. THE CTR CTLR ALSO ASKED THEM IF THERE WERE A PROB. THE FO SAID THAT THEY WERE RETURNING TO FL310 AND NOTHING MORE WAS SAID. THE FAA DID NOT CONTACT THE FLC. THE CAPT SAID THAT WHILE THE 2 DIFFERENT AUTOPLTS WERE OF SOME CONCERN EVERYONE WAS AWARE OF THE PROB AND SINCE THE ACFT WERE SCHEDULED TO BE PHASED OUT THEY PROBABLY WILL NEVER BE UPGRADED.