As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PIT/ERI - FLT 502 - ON DESCENT INTO ERI, THE APU SHUT DOWN AUTOMATICALLY WHEN THE SPEED BRAKE WAS DEPLOYED. A 'LOW AGENT' FOR THE APU FIRE SYSTEM ALSO ILLUMINATED. THE FLIGHT LANDED ERI WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE APU ASSY. US AIRWAYS S/N P108RR. (X)