Narrative:

The crew had started their duty day at XA00 local and completed the day with 12 hours on duty and flew 8.7 hours. On the last of 4 legs the FMS locked up and was unusable for the duration. After completing the flight using VOR to VOR navigation; we passed ZZZ VOR and were trying to get to the field using an NDB just north of ZZZ. The indication from the NDB was not matching up with visual navaids (cities and hwys) so we proceeded in the direction of the field. At that time we were both looking outside the aircraft to navigation to the airport and ATC notified us we were 500 ft below our assigned altitude. The autoplt was engaged and did not capture the altitude. We disengaged the autoplt and climbed to 6000 ft. I believe both pilots focusing their attention outside the cockpit and not monitoring the instruments led to the problem. Additional factors were fatigue and the fact that it was the last leg of the day and were in the local area.

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Original NASA ASRS Text

Title: CE550 FLT CREW DESCENDS BELOW ASSIGNED ALTITUDE WHILE BOTH PILOTS ARE SCANNING OUTSIDE LOOKING FOR THE FIELD. FATIGUE IS CITED AS A FACTOR.

Narrative: THE CREW HAD STARTED THEIR DUTY DAY AT XA00 LCL AND COMPLETED THE DAY WITH 12 HRS ON DUTY AND FLEW 8.7 HRS. ON THE LAST OF 4 LEGS THE FMS LOCKED UP AND WAS UNUSABLE FOR THE DURATION. AFTER COMPLETING THE FLT USING VOR TO VOR NAV; WE PASSED ZZZ VOR AND WERE TRYING TO GET TO THE FIELD USING AN NDB JUST N OF ZZZ. THE INDICATION FROM THE NDB WAS NOT MATCHING UP WITH VISUAL NAVAIDS (CITIES AND HWYS) SO WE PROCEEDED IN THE DIRECTION OF THE FIELD. AT THAT TIME WE WERE BOTH LOOKING OUTSIDE THE ACFT TO NAV TO THE ARPT AND ATC NOTIFIED US WE WERE 500 FT BELOW OUR ASSIGNED ALT. THE AUTOPLT WAS ENGAGED AND DID NOT CAPTURE THE ALT. WE DISENGAGED THE AUTOPLT AND CLBED TO 6000 FT. I BELIEVE BOTH PLTS FOCUSING THEIR ATTN OUTSIDE THE COCKPIT AND NOT MONITORING THE INSTS LED TO THE PROB. ADDITIONAL FACTORS WERE FATIGUE AND THE FACT THAT IT WAS THE LAST LEG OF THE DAY AND WERE IN THE LCL AREA.

Data retrieved from NASA's ASRS site as of January 2009 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.