AN MD80 AUTOPILOT SWITCHED TO THE 'VERT SPD' MODE AND DSNDED BELOW THE CLRED ALT WHEN THE CAPT SELECTED 'SPD' ON THE PFD AS THE ACFT WAS LEVELING.

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THIS FMS ACFT FLEW A FLAWLESS VNAV DSCNT FROM FL370 NON-STOP TO IDLE TO 11000 FT. I HAD PROGRAMMED THE FMS TO CROSS ZZZZZ AT 11000 FT AND 310 KTS. THE ACFT CAPTURED 11000 FT AND LEVELED OFF. THIS WAS FOLLOWED IMMEDIATELY BY THE SPD BEING DRIVEN TO 224 KTS WITH THE THROTTLES RETARDING. IN THE INITIAL CONFUSION I SELECTED SPD SELECT AND ROTATED THE SPD BACK TO 310 KTS. A VERY BRIEF DISCUSSION ENSUED AS TO WHY IT DID THAT. FO PULLED UP THE DSCNT PAGE AND VERIFIED THAT IT SAID ZZZZZ AT 11000 FT AND 310 KTS. MY FO THEN CALLED OUT 'ALT!' WE HAD DSNDED TO 10700 FT. I DISENGAGED THE AUTOPLT AND CLBED BACK TO 11000 FT. ATC DID NOT MENTION IT. ALL I CAN THINK THAT HAPPENED WAS THAT WHEN I SELECTED SPD SELECT WE STILL HAD SOME RESIDUAL DOWN VVI AND WERE NOT COMPLETELY IN FMS ALT HOLD THAT CREATED THE INSIDIOUS DSCNT. HAD I SELECTED ALT HOLD PRIOR TO SPD SELECT; AS WE DO EXITING PERFORMANCE IN ANY OTHER AIRPLANES; THIS WOULDN'T HAVE HAPPENED. HOWEVER; I DON'T UNDERSTAND WHY IT FLEW SUCH A PERFECT DSCNT; LEVELED OFF WITH INSTRUCTIONS TO CROSS ZZZZZ AT 11000 FT AND 310 KTS THEN SLOWED THE SPD TO 224 KTS. WHY NOT 240 KTS? AND WHY WOULD IT DO THAT WITHOUT CAPTURED AT THE 11000 FT ALT? FROM A HUMAN FACTORS POINT OF VIEW; IT WAS VERY LATE (FOR ME); DARK; AND THE 'SURPRISE' DISTR OF AN UNCOMMANDED SPD REDUCTION TO UNIQUE SPD (224 KTS VERSUS 240 KTS) WERE JUST ENOUGH TO KEEP ME FROM CATCHING THE SOLID HARD 11000 FT LEVELOFF. SUPPLEMENTAL INFO FROM ACN 778175: REFLECTING BACK ON THE INCIDENT; I WOULD VENTURE TO GUESS THAT THE CAPT MIGHT HAVE PRESSED 'SPD SELECT' AS THE AUTOPLT WAS COMPLETING ITS ALT CAPTURE AT 11000 FT; THUS PUTTING THE ACFT INTO A VERT SPD DSCNT JUST AS IT APCHED 11000 FT. WE BOTH WERE TAKEN BY SURPRISE AS TO HOW IT GOT INTO VERT SPD BUT; REGARDLESS OF THE CAUSE; THE MOST IMPORTANT LESSON LEARNED IS THAT YOU SIMPLY CANNOT TURN YOUR BACK ON THE ACFT AND THE OP OF ITS AUTOMATION EVEN FOR A BRIEF MOMENT.