What was found
ALTITUDE REPORTING SYS DISPLAYED ALTITUDES IN EXCESS OF 20,000 FT AT AN ACTUAL ACFT ALTITUDE OF 1,000 FEET ASL. THE PROBLEM WAS CAUSED BY AN INTERNAL FAILURE IN THE ALTITUDE ENCODER. THE PROBLEM WAS WAS SUBSEQUENTLY DUPLICATED ON THE GROUND. THE DEFECTIVE ENCODER WAS REPLACED WITH A NEW ALTITUDE ENCODER, USING AN ADAPTOR MOUNTING PLATE. THE ALTITUDE REPORTING SYS WAS TESTED IAW 14CFR43, APPENDIX E, AND FOUND TO MEET THE REQUIREMENTS OF 14CFR91.411.