What was found
CREW REPORTED ON CLIMB THROUGH 29,000 FT, 'ALT' COMPARISON MONITOR ILLUMINATED. CREW FOUND THE NR 1 MADC IN ERROR. NR 3 SELECTED, AND CLIMB CONTINUED TO FL400. 'ALT' COMPARISON LIGHT ILLUMINATED AGAIN, & NONE OF MADC'S AGREED. ON DESCENT, ALL MADC'S AGAIN AGREED TO FL80. APPARENT CAUSE WAS ERRODED PITOT/STATIC PROBES. A/C WAS THOROUGHLY INSPECTED PHYSICALLY & ELECTRONICALLY PER GULFSTREAM V MM & ONLY DEFECT DISCOVERED WAS UPPER LT & RT PITOT/STATIC PROBES WERE ERRODED & CAUSING REPORTED ERRORS. PROBES (P/N 0856TK3 AND 0856TK4) WERE REPLACED PER THE GV MM & AAC GV CAMP DOCUMENT & ALL MADC SYSTEMS PERFORMED TO SPECIFICATIONS. A/C WAS RETURNED TOS ERVICE & RETURNED TO HOME BASE WITHOUT INCIDENT. (M)