On April 5, 2002,at 1715 eastern standard time, a Boeing 767-332ER, N1608, registered to Delta Airlines Inc, operating as flight DAL74, experienced an in-flight separation of the auxiliary power unit door while climbing to cruise altitude near Atlanta, Georgia. The international passenger flight operated under the provisions of Title 14 CFR Part 121 with an instrument flight plan filed. Visual weather conditions prevailed at the time of the separation. The airplane sustained substantial damage. The air transport pilot (Captain), the flight crew, and 187 passengers were not injured. The passenger flight departed Atlanta, Georgia, enroute to Milan, Italy, at 1700. According to the Captain, as the airplane climbed through 15, 000 feet, and accelerated through 320 knots, he heard a loud bang. Shortly after that the pilot felt a severe vibration that shook the entire airplane. The pilot took immediate action and leveled off, he then called for an altitude hold and a reduction in airspeed to 270 knots. There were no warnings, cautions or other abnormal indications in the cockpit. The hydraulic pressure was normal and engine indications were steady with no fluctuations. The Captain directed the First Officer to tell air traffic controller that they had a problem and needed to return to the airport. The flight was issued an instrument clearance to Atlanta. The flight landed in Atlanta without further incident. A review of maintenance records showed that the auxiliary power unit (APU) had been serviced by ground maintenance personnel about one hour before the airplane departed. The examination of the airframe revealed that both APU doors were missing from their normally installed positions. Examination also discovered a 19 inches wide by 26 inches long hole in the left inboard elevator. On the right inboard elevator there was also a hole that measured 14 inches wide by 29 inches long. Damage was also noted on the fuselage structure that enclosed the APU. Prior to the flight the ground personnel had conducted a pre-departure check that included the servicing of the APU. This check was performed one hour before the flight. No other mechanical malfunctions were reported prior to the flight. According to the operators engineering division, there were two possible reasons for the loss of the APU doors: (1). The lack of latch engagement, (2). The failure of the receiving pins. The right door was recovered in three pieces. Only one hinge was recovered for the left door. The forward three latch hooks were intact. The latch for the aft position was fractured. The left door in which the receiving pins were attached was not recovered. Failure examination of the right door structure indicated that a load was applied to the forward lower corner of the door and it failed in a tearing motion. The APU door aft latch hook had been severed. The fracture surface of the aft latch hook indicated a ductile tension failure. The other portions of the recovered door shoe rivet failures and structural failures also suggestive of an overload condition. The other three remaining latches appeared to be functional and were not damaged.