ATR72 FLC DSNDS BELOW ASSIGNED ALT WHILE APCHING VPS.

Narrative

DSNDING INTO VPS AREA FOR APCH AND LNDG AT VPS. DURING DSCNT FROM 11000 FT TO 6000 FT; ADU WENT BLANK AND THEREFORE COULD NOT PROPERLY CTL AUTOPLT. XFERRED TO MANUAL FLT CTL AND CONTINUED. CAPT (PF) GOT DISTR WORKING AROUND TSTM CELLS; AND COORDINATING A RWY CHANGE (DEST ARPT RWY IN USE WITH SURFACE WIND AT EDGE OF TAILWIND LIMITATIONS FOR THE ACFT). FLT WAS OPERATING OVER MIL AIRSPACE FROM SURFACE TO 5000 FT. IN A TURN TO AVOID WX; DSNDED APPROX 400 FT BELOW ASSIGNED ALT OF 6000 FT. FO AND CTLR SAW THE ERROR ABOUT THE SAME TIME. PF CLBED IMMEDIATELY BACK TO 6000 FT. QUERIED APCH CTL FOR POSSIBLE CONFLICTS. NONE PRESENT. REMAINDER OF FLT WAS UNEVENTFUL. I BELIEVE THIS WAS A 'TEXTBOOK' SNOWBALL THAT COULD HAVE EASILY BEEN PREVENTED WITH BETTER CRM. I (CAPT) WAS PF; THE FO WAS RELATIVELY NEW AND LOW TIME. I REALIZE THAT I WAS TRYING TO DO EVERYTHING -- TROUBLESHOOT A SYS FAILURE THAT HAD NO SET PROC; READ THE RADAR; AVOID THE WX; NEGOTIATE WITH ATC (HAVING TO TELL THE NEW GUY EXACTLY WHAT TO SAY); AND FLY THE AIRPLANE MANUALLY. CRM. ALL I HAD TO DO WAS GIVE THE MANUAL FLYING TASK TO THE FO (HE COULD CERTAINLY FLY A HDG AND ALT); AND I COULD HAVE EASILY HANDLED THE OTHER TASKS. LESSON LEARNED: CRM.