On January 18, 2024, Atlas Air flight 3885, a Boeing 747-87UF, N859GT, sustained minor damage as a result of a No. 2 (left inboard) undercowl engine fire after departure from Miami International Airport (MIA), Miami, Florida. The five occupants aboard the airplane were uninjured. The international cargo flight was conducted under Title 14 Code of Federal Regulations Part 121 and was destined for Luis Munoz Marin International Airport (SJU), San Juan, Puerto Rico. The flight crewmembers reported that, after departure, while the airplane was passing through an altitude of about 3,000 ft mean sea level (msl), they received an OVHT ENG 2 NAC [overheat engine No. 2 nacelle] caution message followed by a FIRE ENG 2 warning message on their engine indicating and crew alerting system displays. Simultaneously, the fire warning light illuminated, and the fire bell sounded. The captain declared a mayday with air traffic control, and the flight was cleared back to MIA via radar vectors for a landing on runway 9. The crew completed the electronic non-normal checklist and shut down the No. 2 engine. One fire bottle was discharged, and the fire warning light subsequently extinguished. The airplane made an uneventful three-engine landing, and the airplane was met by firefighting personnel, who subsequently cleared the airplane to taxi under its own power to the parking location. A review of the airplane’s maintenance records revealed that, on January 14, 2024, the airplane underwent a borescope inspection of the No. 2 engine, a General Electric (GE) GEnx-2B67 engine. The inspection, performed by a third-party vendor, required the removal of the port M borescope plug; see figure 1 for the location of port M. Figure 1. Engine diagram with port M depicted in the combustor diffuser nozzle (Source: General Electric). The maintenance work card provided instructions on how to properly reinstall the borescope plug to ensure that the locking feature was engaged. The locking design on the borescope plug incorporated a plug cap that utilizes fingers, detents, an internal spring, and a locking slot, as shown in figure 2. When properly installed with the locking slot engaged on the engine, case friction is generated by the fingers and detents, and the plug would be prevented from backing out. The work card was initialed by the technician who performed the work and an inspector who reviewed the work. Initialing a maintenance item indicated that the task was completed in accordance with the maintenance manual procedure. Figure 2. Exemplar borescope plug (Source: General Electric). A postincident inspection of the No. 2 engine revealed burn through of the thrust reverser fan duct fixed inner wall (see figure 3). The engine cases were intact with no evidence of an uncontained engine failure. The combustor diffuser nozzle (CDN) case port M borescope plug was not secured in the case and was found loose in the engine cowling. The burn-through observed on the thrust reverser wall was directly above the open CDN port M. The engine low pressure spool (N1) and high pressure spool (N2) rotated smoothly with manual rotation. A borescope inspection through CDN port M was performed, and no combustor damage was visible. A review of quick access recorder, flight data recorder, and continuous engine operating data showed no evidence of engine failure or surge/stall. Figure 3. Burned area above borescope port M (Source: Atlas Air). A postincident inspection of the port M borescope plug (see figure 4) revealed general debris and oxidation. The plug’s geometry/features conformed to the drawing requirements with no anomalies noted to the plug’s threads and its locking components. The plug’s materials and coatings were consistent with the design requirements. The plug was threaded into an exemplar engine without binding or stiffness, and the locking feature engaged as designed. Figure 4. Borescope plug recovered from the incident airplane (Source: General Electric).