Narrative:

Descending into vps area for approach and landing at vps. During descent from 11000 ft to 6000 ft, adu went blank and therefore could not properly control autoplt. Xferred to manual flight control and continued. Captain (PF) got distraction working around thunderstorm cells, and coordinating a runway change (destination airport runway in use with surface wind at edge of tailwind limitations for the aircraft). Flight was operating over military airspace from surface to 5000 ft. In a turn to avoid WX, descended approximately 400 ft below assigned altitude of 6000 ft. First officer and controller saw the error about the same time. PF climbed immediately back to 6000 ft. Queried approach control for possible conflicts. None present. Remainder of flight was uneventful. I believe this was a 'textbook' snowball that could have easily been prevented with better CRM. I (captain) was PF, the first officer was relatively new and low time. I realize that I was trying to do everything -- troubleshoot a system failure that had no set procedure, 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 first officer (he could certainly fly a heading and altitude), and I could have easily handled the other tasks. Lesson learned: CRM.

Related reports:

Google
 

Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.