Narrative:

Prior to engine start, ground marshaller observed small amount of smoke in right wheel well area near APU exhaust. Shortly thereafter, received fire signal from ground marshaller indicating the APU. Captain directed APU shutdown (accomplished with fire switch) and airport fire/rescue summoned (accomplished by first officer). There was no cockpit APU fire or wheel well fire indication. I went outside to assess situation while captain directed deplaning of passengers via jet bridge. Upon arrival at wheel well, there was still some smoke present and the ground marshaller (who was now standing by with a ramp fire bottle) stated that the flames did not go out when the APU was shut down. I directed the ground marshaller to fire the extinguisher at the flame area. Flames were extinguished, fire retardant cleaned from wheel well, APU MEL'd and flight continued. Callback conversation with reporter revealed the following information: reporter says the fire was a result of deicing fluid being trapped in the APU shroud combined with an exhaust leak. The shroud is double-walled with insulation between the walls. The aircraft hadn't been de-iced for the last 2 legs, so they suspect the fluid leaked into the shroud wall and then if the shroud had a crack, allowing the hot exhaust to come in contact with the glycol-soaked insulation resulting in the fire. Reporter says it is normal to have some smoke coming from the APU after de-icing, but this time when the ground crew checked they saw a detachment ring in the APU area being red hot. The APU was shut down and reporter thinks that put out the fire, but when he looked the ground crew said it did not, so he directed them to fire their ground extinguisher into the area. That resulted in a major clean up of the area because of the highly corrosive extinguishing agent. It required washing down thoroughly and drying with an air hose. After the clean up, the area was inspected, the APU MEL'd and the flight continued.

Related reports:

Google
 

Original NASA ASRS Text

Title: BEFORE ENG START, GND MARSHALLER NOTIFIED FLC OF SMOKE COMING FROM THE APU. NO COCKPIT INDICATIONS OF FIRE, BUT APU WAS SHUT DOWN. WHEN SO WENT TO ASSESS THE SIT, GND CREW SAID FIRE DIDN'T GO OUT WITH INTERNAL APU EXTINGUISHER. SO DIRECTED GND CREW TO FIRE EXTINGUISHER WHICH STOPPED ANY FIRE SMOKE. ANTI-ICE FLUID SUSPECTED.

Narrative: PRIOR TO ENG START, GND MARSHALLER OBSERVED SMALL AMOUNT OF SMOKE IN R WHEEL WELL AREA NEAR APU EXHAUST. SHORTLY THEREAFTER, RECEIVED FIRE SIGNAL FROM GND MARSHALLER INDICATING THE APU. CAPT DIRECTED APU SHUTDOWN (ACCOMPLISHED WITH FIRE SWITCH) AND ARPT FIRE/RESCUE SUMMONED (ACCOMPLISHED BY FO). THERE WAS NO COCKPIT APU FIRE OR WHEEL WELL FIRE INDICATION. I WENT OUTSIDE TO ASSESS SIT WHILE CAPT DIRECTED DEPLANING OF PAXS VIA JET BRIDGE. UPON ARR AT WHEEL WELL, THERE WAS STILL SOME SMOKE PRESENT AND THE GND MARSHALLER (WHO WAS NOW STANDING BY WITH A RAMP FIRE BOTTLE) STATED THAT THE FLAMES DID NOT GO OUT WHEN THE APU WAS SHUT DOWN. I DIRECTED THE GND MARSHALLER TO FIRE THE EXTINGUISHER AT THE FLAME AREA. FLAMES WERE EXTINGUISHED, FIRE RETARDANT CLEANED FROM WHEEL WELL, APU MEL'D AND FLT CONTINUED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAYS THE FIRE WAS A RESULT OF DEICING FLUID BEING TRAPPED IN THE APU SHROUD COMBINED WITH AN EXHAUST LEAK. THE SHROUD IS DOUBLE-WALLED WITH INSULATION BTWN THE WALLS. THE ACFT HADN'T BEEN DE-ICED FOR THE LAST 2 LEGS, SO THEY SUSPECT THE FLUID LEAKED INTO THE SHROUD WALL AND THEN IF THE SHROUD HAD A CRACK, ALLOWING THE HOT EXHAUST TO COME IN CONTACT WITH THE GLYCOL-SOAKED INSULATION RESULTING IN THE FIRE. RPTR SAYS IT IS NORMAL TO HAVE SOME SMOKE COMING FROM THE APU AFTER DE-ICING, BUT THIS TIME WHEN THE GND CREW CHKED THEY SAW A DETACHMENT RING IN THE APU AREA BEING RED HOT. THE APU WAS SHUT DOWN AND RPTR THINKS THAT PUT OUT THE FIRE, BUT WHEN HE LOOKED THE GND CREW SAID IT DID NOT, SO HE DIRECTED THEM TO FIRE THEIR GND EXTINGUISHER INTO THE AREA. THAT RESULTED IN A MAJOR CLEAN UP OF THE AREA BECAUSE OF THE HIGHLY CORROSIVE EXTINGUISHING AGENT. IT REQUIRED WASHING DOWN THOROUGHLY AND DRYING WITH AN AIR HOSE. AFTER THE CLEAN UP, THE AREA WAS INSPECTED, THE APU MEL'D AND THE FLT CONTINUED.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.