FLC DSNDS THROUGH ASSIGNED ALT.

Narrative

ON JUL/XA/95 I WAS THE FO OF SVC FROM DFW TO HOU (AT72 ACFT). MY POS WAS THAT OF PNF FOR THIS LEG. CAPT WAS IN COMMAND AS PIC AS WELL AS THE PF. THIS WAS OUR SECOND LEG COMING OFF A REDUCED REST PERIOD -- WITH EVEN LESS REST AFTER THE HOTEL WAS 45 MINS LATE PICKING US UP AT THE ARPT THE NIGHT BEFORE. OUR POS WAS APPROX 60 NM N OF NAVASOTA VOR (TNV) AT 15000 FT. ZHU CLRED US TO CROSS MARIT INTXN AT 9000 FT AT OUR DISCRETION. 6 NM PAST TNV VOR ON THE STRUK 8 ARR. AT 15 DME N OF TNV VOR; I ASKED CAPT MAYBE HE WOULD WANT TO BEGIN HIS DSCNT (TO MAKE THE RESTR). UPON REALIZING HOW CLOSE WE WERE (ABOUT 21 NM) HE QUICKLY AGREED AND PUT THE AUTOPLT INTO A DSCNT MODE. AT THIS TIME I BEGAN TO GET BUSY WITH PNF DUTIES OF GETTING ATIS; CALLING INBOUND COMPANY; REVIEWING THE ARR RTE. DSNDING THROUGH 10000 FT I ALSO SWITCHED ON LNDG LIGHT AND STERILE COCKPIT LIGHT (NORMAL PROC) AND PROCEEDED TO REMOVE THE APCH PLATE FOR VOR DME RWY 22 AT HOU. ZHU THEN SWITCHED US TO HOUSTON APCH. I BEGAN TO CHK IN WITH APCH BY SAYING 'HOUSTON APCH; X WITH YOU DSNDING THROUGH 7.5...' I PAUSED KNOWING SOMETHING WAS VERY WRONG. AS I SCANNED THE ENTIRE INST PANEL TO ASSESS THE SIT I REALIZED CAPT HAD US IN A 3000 FPM DSCNT WITHOUT RESETTING THE ALT SELECTOR. IT STILL READ 15000 FT. THE AUTOPLT HAD BEEN PUT INTO A 3000 FPM DSCNT WITH NO PRE- SELECTED ALT. 'WHERE THE HELL ARE WE GOING...?' WERE MY NEXT WORDS. HE IMMEDIATELY SELECTED THE AUTOPLT OFF; ADDED PWR AND PROCEEDED TO CLB BACK TO OUR ASSIGNED ALT. I CONVEYED THE FACT THAT WE WERE ASSIGNED 9000 FT. HE ASKED ME TO VERIFY THE ALT -- I DID WITH APCH. AFTER ABOUT 2 MINS HOUSTON APCH SAID...'X YOU'RE SUPPOSED TO BE AT 9000 FT.' BY THIS TIME CAPT HAD RECOVERED AND LEVELED OFF AT 9000 FT SO MY RESPONSE WAS; 'ROGER; WE ARE LEVEL AT 9000 FT.' THE FLT CONTINUED WITHOUT FURTHER INCIDENT OR EVENT.