As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING HANDS ON EMERGENCY EQUIPMENT TRAINING, DISCOVERED THAT COCKPIT ESCAPE HATCH WILL NOT OPEN. REMOVED AND REPLACED CREW COMPARTMENT OVERHEAD HATCH IAW B747-400 M/M 52-21-01. CLEARANCES AND RIG HAVE NOT BEEN DISTURBED. OPS AND CHECKS PASS.