As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMBOUT, THE CREW FELT A VIBRATION WHICH INCREASE AS SPEED INCREASED. THE CREW CONTACTED AIR TRAFFIC CONTROL AND REQUESTED TO RETURN TO THE FIELD. FLIGHT 4136 LANDED WITHOUT INCIDENT. MAINTENANCE INSPECTED AND FOUND THE COCKPIT CREW ESCAPE HATCH COVER WAS CURLING UP. MAINTENANCE SECURED THE HATCH IN ACCORDANCE WITH DORNIER MAINTENANCE MANUAL 52-00-00. THE AIRCRAFT WAS RELEASED FOR SERVICE.