A CPR COMMERCIAL RATED PLT FLYING A WESTWIND 1124 FROM LAX TO MMTJ DEVIATED FROM HIS ASSIGNED ALT BY ALLOWING HIS ACFT TO DSND 400 FT.

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IN SUM: ON JAN/XA/99 ON AN IFR FLT PLAN; WHILE ON A FERRY FLT FROM THE MAINT FACILITY AT LAX TO HOME BASE ARPT MMTJ; APPROX 20 MI NW OF MZB VOR; I WAS CLR TO DSND FROM 17000 FT TO 9000 FT AND DIRECT TO THE MMTJ ARPT. CONDITIONS WERE CAVU AND WE WERE TALKING ON SOCAL APCH FREQ. IN THE VICINITY OF THE MZB VOR; WE WERE FURTHER CLRED TO 7000 FT. APCHING PGY VOR; WE HEARD OVER THE FREQ WE WERE MONITORING THAT APCH WAS GIVING OUR POS TO OTHER TFC THAT WAS FLYING E AND THAT WE (ACFT X) WERE AT HIS 11 O'CLOCK POS. AIRPLANE Y RPTED HAVING US IN SIGHT. APCH FURTHER CLRED US TO DSND AND MAINTAIN 6000 FT AND SOCAL TOLD US WE HAD TFC AT 2 O'CLOCK MAINTAINING 5000 FT. AT THIS TIME I STARTED MY DSCNT TO 6000 FT. DUE TO THE CLOSE PROX TO OUR DEST ARPT (MMTJ); I DISCONNECTED THE AUTOPLT AND BEGAN HAND FLYING THE AIRPLANE; WITH BOTH MYSELF AND THE FO LOOKING FOR THE TFC. A FEW SECONDS WENT BY AND I INADVERTENTLY DSNDED BELOW MY ASSIGNED ALT BY 400 FT; AT WHICH TIME MY ALT ALERT WARNED US AND I IMMEDIATELY BEGAN TO RECOVER; WHILE AT THE SAME TIME WE SAW THE TFC BECOMING FROM 3 O'CLOCK TO 4 O'CLOCK AND 4-5 MI. SOON AFTER THAT; WE RECEIVED A RADIO CALL FROM SOCAL APCH INSTRUCTING US TO CLB BACK TO OUR ASSIGNED ALT (6000 FT); TELLING US THAT WE HAD DSNDED BELOW OUR ASSIGNED ALT AND TO CONTACT MMTJ APCH. I CONTACTED MMTJ APCH AND RECEIVED CLRNC TO DSND IN VFR CONDITIONS TO PATTERN ALT. AFTER LNDG IN MMTJ WE WERE INSTRUCTED TO CONTACT SOCAL VIA THE TELEPHONE. I CONTACTED THEM ON THE PHONE AND THEY EXPLAINED WHAT HAPPENED. THEY ALSO GOT MY NAME AND ADDRESS. I WISH TO INFORM YOU THAT THIS INVOLUNTARY ERROR WAS NOT HAPPENED BEFORE IN MY 42 YR FLYING CAREER FLYING IN USA AND IN ALL TYPE OF AIRSPACE. ONE OF US SHOULD HAVE BEEN MINDING THE STORE WHILE THE OTHER LOOKED FOR TFC!