As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PHL FLT 798 ENROUTE PHL TO AMS. THE STANDBY ATTITUDE INDICATOR FAILED ABOUT 45 MINUTES AFTER DEPARTURE. THE DECISION WAS MADE TO RETURN TO PHL. THE FLIGHT RETURNED WITHOUT FURTHER INCIDENT. MX REPLACED THE STANDBY HORIZON INVERTER. OPS CHECKS OK.