As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: AFTER #1 ENGINE START, STRONG SMELL OF DIRTY SOCKS THROUGHOUT FLIGHT DECK AND CABIN. DIDN'T DISSIPATE UNTIL THROUGH 10,000 FEET, RETURNED ON LANDING. MAINTENANCE HAS ISOLATED THE SOURCE OF THE ODOR TO THE APU DURING INITIAL TROUBLESHOOTING. **SUPPLEMENTAL REPORT WILL BE SUBMITTED. (OPEN)