As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
APU REMOVED FROM AIRCRAFT WHEN APPARENT THERMAL DAMAGE WAS NOTED AT TAIL SECTION OF AIRCRAFT. UPON RECEIPT OF APU AT HONEYWELL REPAIR STATION THE APU WAS EVALUATED IN A TEST CELL. FLAMES OUT THE TAILPIPE WERE NOTED DURING TEST CELL OPERATION OF THE APU. CURRENTLY ROOT CAUSE ANALYSIS IS BEING PERFORMED TO DETERMINE THE FAILRE MODE OF THE APU.