FLC DSNDS THROUGH ASSIGNED ALT.

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WE WERE CLRED TO FL230; TO BE LEVEL BY GIBER; WHICH WAS APPROX 20 AHEAD AND WE WERE AT FL280. THE FO WAS FLYING; AFTER THE DSCNT STARTED AND THE ALT CAPTURE MODE ARMED; I TOLD THE FO AND FE I WOULD MAKE THE LAST PA TO THE PAX. WHEN I GOT DONE WITH THE PA WE WERE DSNDING THROUGH FL240 AND THE FO TOLD ME WE WERE CLRED DIRECT TO POLAR. WE WERE USING THE #1 INS AND #1 AUTOPLT BECAUSE THE #2 AUTOPLT WAS INOP. SO I HAD TO VERIFY THE POLAR COORDINATES ON THE #1 INS. I WAS DOING THIS AND NOTICED WE WERE GOING THROUGH FL230. I INQUIRED ABOUT THE ALT; THINKING WE MAY HAVE BEEN CLRED LOWER THAN FL230. THE FO SAID NO; IT'S FL230 AND TOOK THE AUTOPLT OFF AT ABOUT FL225 AND BOTTOMED OUT AT FL223. CLBED BACK UP TO FL230. THERE WAS NO CONFLICT WITH ANY OTHER TFC AND CTR DIDN'T MENTION IT AT ALL. DURING THE TIME WE WERE GOING THROUGH FL230 THERE WAS A FREQ CHANGE ALSO. THE REASON WE WENT THROUGH THE SET ALT; I BELIEVE; IS BECAUSE WE DSNDED AT A HIGH RATE OF DSCNT; 3500 FPM; TO MAKE THE RESTR. I THINK THE FO CHANGED THE VERT SPD TO HELP THE AUTOPLT LEVEL OFF BUT IT HAD CAPTURED THE ALT JUST PRIOR TO HIM DOING THAT SO WHEN HE CHANGED THE VERT SPD THAT PUT THE AUTOPLT IN A VERT SPD MODE AND JUST CONTINUED DOWN AT A LESSER RATE OF DSCNT.