OAKLAND, CA, USA
N12318
BOEING 737-300
THE CREW WAS NOT AWARE AN APU FIRE HAD OCCURRED AT THE GATE PRIOR TO STARTING THE MAIN ENGINES. AFTER DEPARTURE THE AIRPLANE EXPERIENCED SEVERE VIBRATIONS; AN EMERGENCY WAS DECLARED, AND THE FLIGHT LANDED WITHOUT INCIDENT. ABOUT 1/2 OF THE UNDERSIDE OF THE ELEVATOR & TRIM TAB WERE SEVERELY FIRE DAMAGED. THE TAIL CONE WAS CHARRED & BLISTERED OVER A 3 FT BY 3 FT AREA. FOUR GROUND SERVICE PERSONNEL HANDLED THE AIRCRAFT DURING PUSHBACK. THE APU COMBUSTOR CAP DRAIN LINE WAS FOUND CLOGGED WITH SEVERAL PIECES OF ROOM TEMPERATURE VULCANIZING (RTV) MATERIAL (POTTING COMPOUND) USED IN THE APU BUILDUP PROCESS. THE APU MANUFACTURER'S OVERHAUL MANUAL CAUTIONS OPERATORS TO USE RTV SPARINGLY, AND TO REMOVE ANY EXCESS PRIOR TO INSTALLING EXHAUST PIPE SHIMS TO PREVENT PLUGGED FUEL DRAIN FITTINGS AND TORCHING STARTS. IN ADDITION, THE ATOMIZER ASSEMBLY SHOWED STREAKING IN THE PRIMARY ORIFICES, AND EXAMINATION OF THE OIL PUMP REVEALED SLOW SEQUENCING (IGNITION AND FUEL INITIATION).
IGNITION OF PUDDLED FUEL FROM A BLOCKED COMBUSTOR DRAIN LINE, AND A SLOW START DURING WHICH FUEL WAS MISTED AND BLOWN OUT THE TAIL PIPE. AN EXISTING TAILWIND AT THE GATE ALLOWED THE FUEL TO WET THE TAIL SURFACES AND BE IGNITED DURING THE APU START. THE OPERATOR CONFIRMED USING RTV MATERIAL IN THE APU BUILDUP PROCESS AT THEIR FACILITY.
Source: NTSB Aviation Accident Database
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