What was found
APU FAILED TO START ON ONE ATTEMPT. MULTIPLE PEOPLE OUTSIDE THE AIRCRAFT REPORTED A 6-8' FLAME FROM THE APU EXHAUST. WHEN THE MECHANIC OPENED THE CLAMSHELL, A SMALL PUDDLE OF RAW FUEL DROPPED TO THE GROUND. REMOVED FROM SERVICE. CLEANED RESIDUAL FUEL FROM APU DOORS/COMPARTMENT. PERFORMED APU START 2X PER Q400 AMM TASK 49-00-00-869-801. PERFORMED GVI OF APU FUEL LINES/COMPONENTS WHILE RUNNING, NO ACTIVE LEAKS OBSERVED. APU STARTS AND SHUTS DOWN NORMALLY. RETURNED TO SERVICE. NO PARTS REPLACED.