As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE STARTING THE APU WHILE AIRBORNE THERE WAS A LOUD BOOM AND SIMULTANEOUSLY THE APU FIRE ANNUNCIATOR ILLUMINATED MOMENTARILY. THE APU WAS SHUT DOWN AND THE APU FAUL ANNUNCIATOR ILLUMINATED. REMOVED AND REPLACED THE FUEL SHUTOFF VALVE SOLENOID AND FUEL FLOW DIVIDER IAW MM 49-30-54 & 49-30-58.