FLC OF AN MLG OVERSHOT DSCNT XING RESTR DUE TO MISSET AUTOPLT MODE CTL.

Narrative

DURING DSCNT WE WERE GIVEN AN ADDITIONAL RESTR TO CROSS HITOP INTXN AT AND MAINTAIN 17000 FT. THIS WAS ENTERED IN LEGS PAGE. I DID NOT; I BELIEVE; RESET MCP ALT TO 17000 FT SINCE I WANTED TO LEVEL AT FL210 TILL WE NEEDED TO GO DOWN TO 17000 FT. WE WERE DSNDING IN VNAV ALL THE TIME WITH THE FOLLOWING ADDITIONAL WAYPOINT RESTRS IN FMC: 1) WHETO AT 12000 FT; AND 2) PSP AT 7000 FT. AS DSCNT CONTINUED; WE ADDED SPD RESTRS TO WHETO AND PSP TO GET SLOWED FOR THE FINAL DSCNT DURING THE EXPECTED VISUAL APCH. AS WE CONTINUED; I CAN'T SAY WHY; BUT I FELT SOMETHING WAS AMISS BUT DID NOT KNOW WHAT AT FIRST AND I STATED THIS TO THE CAPT. THEN I REALIZED WE WERE NEARING HITOP (DON'T RECALL HOW CLOSE WE WERE) AND WERE STILL AT FL210. IT (HITOP) WAS STILL DEFINITELY AHEAD OF US ON MAP DISPLAY. I IMMEDIATELY STATED WE MIGHT NOT MAKE HITOP AT 17000 FT TO THE CAPT; EXTENDED THE SPD BRAKES; WENT TO IDLE THRUST AND MANUALLY DSNDED THE AIRPLANE AS FAST AS POSSIBLE WITHOUT UPSETTING THE PAX. ATC THEN ASKED IF WE'D MAKE HITOP AT 17000 FT; CAPT SAID YES; I BELIEVE. I DON'T RECALL THE ALT AT HITOP BUT WE LEVELED AT 17000 FT PROBABLY 1-2 MI BEYOND. WE WERE; I PRESUME; CLOSE ENOUGH; BUT THE SIT COULD HAVE BEEN WORSE. IN THE END; I THINK OUR DOWNSTREAM WAYPOINT CHANGES CAUSED US TO DROP OUT OF VNAV AND INTO ALT HOLD AT FL210. THIS WOULD EXPLAIN THE LACK OF A 'RESET MCP ALT' MESSAGE PRIOR TO HITOP. THE MORAL OF THE STORY IS TO RECHK AND MONITOR FLT MODE ANNUNCIATIONS AND NEVER ASSUME THAT IT WON'T CHANGE MODES ON YOU WHEN YOU MAKE ALT RESTR MODIFICATIONS. IN THE END; THIS IS SOMETHING I'M QUITE MAD AT MYSELF FOR; FOR NOT MONITORING THE AUTOFLT SYS MORE CLOSELY.