AN MD88 DSNDING IN ZOB; OH; AIRSPACE OVERSHOT ASSIGNED ALT DUE TO CONFUSION OF CLRED ALT.

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DURING INITIAL DSCNT INTO CLE; WE WERE ASSIGNED FL240; THEN RECLRED TO FL250. AFTER LEVELOFF WE WERE AGAIN CLRED TO FL240 (FL230?). APCHING FL240 I CALLED: FL240 FOR FL230 WHICH WAS IN THE ALT ALERT WINDOW. THE FO SAID: I THINK IT SHOULD BE FL240. WE STARTED TO CONFIRM WITH CTR AS WE PASSED FL240 AND CTR CALLED SAYING THAT WE SHOULD LEVEL AT FL240; AND THEN SAID: LEVEL AT YOUR PRESENT ALT. WE LEVELED AT FL233. AFTER LEVELING; WE WERE RECLRED TO CONTINUE DSCNT WITH NO APPARENT CONFLICT. ANALYSIS: I'M IN THE HABIT OF STATING NEW ALT AND POINTING TO THE ALT ALERT WINDOW WHEN CHANGING NUMBERS. THE FO WAS SURE THE CLRNC WAS TO FL240; NOT FL230 BUT FL230 WAS IN THE WINDOW. AS I POINTED TO THE NUMBER I MAY HAVE NUDGED THE SET KNOB OR THE KNOB MAY HAVE 'BUMPED' TO FL230 AFTER INITIAL SETTING. AS BEST REMEMBERED WE BOTH ACKNOWLEDGED AND SAID THE INITIAL CLRNC. I PROBABLY SHOULD HAVE HIT ALT HOLD AT THE FIRST INDICATION OF DOUBT AND WE WOULD HAVE LEVELED AROUND FL237. NOTE: THERE WERE NO MAJOR DISTRACTIONS HOWEVER WE WERE GETTING ATIS AND BEGINNING TO BRIEF THE APCH INTO CLE. SUPPLEMENTAL INFO FROM ACN 387116: I HEARD THE CAPT STATE 'FL240 FOR FL230' TO WHICH I REPLIED 'I THOUGHT WE WERE CLRED TO FL240.' AT THAT POINT CTR STATED THAT THEY SHOWED US GOING THROUGH FL240 WHICH PROMPTED THE CAPT TO START AN IMMEDIATE LEVELOFF. CTR THEN TOLD US TO LEVEL OFF WHERE WE WERE FOR THE TIME BEING. SUBSEQUENTLY CTR CLRED US TO FL230. WE ENDED UP LEVELING AT APPROX FL233 BY THE TIME CTR TOLD US TO LEVEL OFF; THEN WENT DOWN TO FL230 WHEN CLRED. WE HAD NO MAJOR DISTRACTIONS OF NOTE: THE ONLY OTHER ACTIVITY WAS GETTING ATIS AND BEGINNING TO BRIEF THE APCH.