On June 17, 2024, Cargojet Airways flight 1926 experienced an in-flight separation of the auxiliary power unit (APU) doors while climbing through 13,000 feet near Milan, Indiana. The flight crew did an air turnback to Cincinnati/Northern Kentucky International Airport (CVG), Hebron, Kentucky, and landed without further incident. Post flight inspection of the airplane revealed that the APU access doors had separated from the airplane and substantially damaged the lower surface of the left and right elevators (see figure 1). The flight was operated under Title 14 Code of Federal Regulations (CFR) Part 129 as a non-scheduled international cargo flight from CVG to Vancouver International Airport (YVR), Vancouver, British Columbia, Canada. Figure 1. View of the structural damage to the APU doors and elevators. (Source: Cargojet) The flight was operated by two flight crewmembers that consisted of a captain and a first officer (FO). According to the flight crewmembers, the pre-fight inspection, start-up, taxi, and takeoff were uneventful. There were no unusual noises, vibrations, or Engine Indicating and Crew Alerting System (EICAS) messages observed during the takeoff. After takeoff, the flight was cleared to FL360 by air traffic control (ATC). While the aircraft was passing through 13,000 feet, the flight crew noted an audible “loud bang” followed by airframe vibrations and a FIRE/OVERHEAT SYS EICAS message. Additionally, the lower EICAS displayed an APU FIRE LOOP 1 and 2 status notification. The flight crewmembers requested and were instructed by ATC to level off at 17,000 feet and reduce their airspeed to 280 kts. Because the vibrations had stabilized in intensity, they requested a climb to FL240, and they informed dispatch of their situation through the Aircraft Communications Addressing and Reporting System (ACARS). As the flight climbed through FL210, the intensity of the vibrations began to increase, so they levelled off at FL240 and requested to return to CVG. The autopilot remained engaged throughout the flight and the aircraft handled normally. The landing and subsequent taxi to parking was uneventful, with no abnormal noises or vibrations noted. During a postflight inspection of the airplane, the flight crewmembers observed that the APU access doors were missing and the lower surface of the left and right elevators were damaged. There are two (a left and a right) APU access doors that have four latch assemblies (see Figure 2). The right door contains the latch hooks, and the left door contains the U-bolt and engagement pins. Safety features include a trigger action actuator safety and an overcenter toggle action of the latch handle. The latches are designed so that failure of the spring will not cause the latch to open. The APU access door is opened by pushing the trigger in each latch handle, using moderate thumb pressure, to release the latch safety catch and allow the latch handle and hook to begin the movement required to unlatch the door. Figure 2. APU doors and latch. (Source: Boeing AMM 52-48-13. Copyright © Boeing. Reproduced with permission.) Departed sections of the left hand (LH) APU access door were located and recovered. However, the right hand (RH) APU access door, including all four latch hooks, were not recovered. The damaged sections of the LH and RH elevators were removed from the aircraft along with the remaining portions of the LH and RH APU access doors. The sections of the LH APU access door, a section of the RH APU door, an elevator control rod assembly, an APU door hold open strut, a horizontal stabilizer panel seal, a flexible drive shaft, the LH elevator assembly, and the RH elevator assembly were sent to the Boeing Equipment Quality Analysis (EQA) facility in Seattle, Washington for examination and analysis. According to Cargojet Airways, their review of the flight data recorder (FDR) data showed that the APU access doors likely began to separate at the onset of the APU fire loop fault indication about 04:56:38Z, at an indicated altitude of 12,011 feet MSL, and an indicated airspeed of about 315 kts. A review of the data also revealed no further anomalies that were related to the occurrence. Two sections (an aft section and a forward section) of the recovered LH APU door were visually examined at the Boeing EQA. The aft section measured approximately 21 inches along the latch edge and 27 inches along the torn forward edge (see figure 3). The U-bolt, part of the latching mechanism, was visually examined and was unremarkable and typical of an in-service aircraft. Figure 3. Left photo is of the recovered left hand door aft section and the right photo is a close up of the u-bolt. (Source: Boeing. Photo Copyright © Boeing. Reproduced with permission.) The forward door section measured approximately 38 inches along the forward edge, and 70 inches along the latch edge. The door was distorted and bent, and the skin was wrinkled in multiple areas (see figure 4). The skin was fractured and bent in the outboard direction at the second support rib from the forward edge of the door. Additionally, the skin was separated from the second support rib with portions of the rivet remaining in the second support rib. The three U-bolts on the forward access door section were visually examined with nothing remarkable. Figure 4. Recovered left hand door forward section. (Source: Boeing. Photo Copyright © Boeing. Reproduced with permission.) Cargojet Airways maintenance records showed that during the period of June 15 -17, 2024, the APU oil level was checked as part of a weekly and daily service check. This inspection required the opening and closing of the APU doors. The maintenance technician who conducted the pre-departure check for the accident flight stated that he visually observed the APU doors and that they appeared to be latched and secured. After the event, Cargojet Airways maintenance conducted a fleet campaign to inspect APU latch assemblies on seventeen (17) B757, three (3) B767-200 and twenty-one (21) B767-300’s. The results of the inspection showed that (3) 757 aircraft and (3) B767-300 aircraft needed adjustments to their latch assemblies to bring them within minimum and maximum closure force limits. For the latch assemblies needing adjustment, maintenance attempted, on the ground in a hangar environment, to simulate vibration and external forces to pull open the door. They were unable to duplicate a failure and open the doors when an out of limit latch was correctly closed. Because the RH APU access door and latches were not recovered and examined, the exact reason why the APU doors opened in-flight cannot be determined. However, based on the examination of the LH access door, it is likely that one or more latches were not fully engaged and closed before departure.