On October 18, 2023, at about 16:15 Central Standard Time, a FedEx Boeing 767-300F, registration N287FE, powered by two General Electric CF6-80C2 turbofan engines, experienced a No. 1 (left) engine fire during initial climb from Memphis International Airport, Memphis, Tennessee, USA. Upon landing, airport rescue and firefighting observed damage to the No. 1 engine thrust reverser and core cowling, but no indications of an active fire. No fire suppressant was applied to the engine. Ground inspection of the No. 1 engine with the cowls and thrust reverser opened revealed the engine experienced an undercowl fire.
The main fuel pump’s (MFP) fuel inlet flange was found fractured and still bolted and attached to the engine’s main fuel supply line. The exposed MFP fracture surface appeared melted (Figure 1). Fractographic analysis confirmed melting and resolidification of the fracture surfaces with no evidence of a mechanical failure mode.
Figure 1: Fractured main fuel pump and detached main fuel supply line
A small area of coking was found on the hydromechanical unit’s (HMU) idler adapter pad sealing surface where an upper fuel crossover tube attaches (Figure 2). This crossover tube delivers high pressure fuel from the MFP to the HMU and is secured using four bolts installed into threaded steel inserts within the idler adapter. Two of these threaded inserts were found protruding/pulled out from the adapter pad.
Figure 2: Idler adapter’s crossover tube mount pad surface
Computed tomography scans found three of the four idler adapter bolt holes used to secure the upper fuel crossover tube’s steel inserts had damaged and/or missing threads (Figure 3). The damaged bolt holes did not have sufficient thread engagement to secure the crossover tube to the adapter pad. High-pressure fuel loads likely caused the inserts to partially pull out, resulting in a gap forming between the crossover tube and the idler adapter pad.
Figure 3: Thread engagement of inserts No. 2 (left) and No. 4 (right)
Review of the idler adapter’s manufacturing requirements found the idler adapter’s bolt holes should be drilled through and then have self-tapping, threaded steel inserts installed into the holes. Review of the incident idler adapter’s manufacturing process found its holes were drilled through and then pre-tapped prior to installing the steel inserts. Investigation found multiple insert holes had stripped threads, linear scoring, counterclockwise material smearing, missing surface anodization, cross-threading, and concave hole profiles, all of which were evidence of improperly installed steel inserts into adapter holes with preexisting threads.
Avio Aero initially manufactured the HMU idler adapter at its Brindisi, Italy facility in 1996 and then moved production to their Turin, Italy facility in 2003. According to GE, the idler adapter should have completed a first article inspection (FAI) at the Brindisi facility in 1996, and again at the Turin facility in 2003; however, no FAI documentation from either facility was found.
In early 2023, Avio Aero realized they were pre-tapping other non-GE parts that used self-tapping inserts, which were deviations from those parts’ drawing requirements. The idler adapter was found to be one of these parts and Avio Aero removed the pre-tap operation from the adapter’s manufacturing process in August 2023. Avio Aero then notified GE who determined pre-tapped adapter holes violated the adapter drawing requirements. Avio Aero could not produce any documentation showing who authorized the deviation or why the pre-tap operation was incorporated into their manufacturing process.
As a result of this investigation, GE Aerospace released Service Bulletin (SB) 72-1648 HMU/MEC Idler Adapter Inspection, Revision 00, on June 28, 2024, requiring repetitive, on-wing shim checks at each insert location and a one-time pull check of installed inserts at the next engine shop visit. The FAA was determining the need to issue an Airworthiness Directive mandating compliance with GE SB 72-1638 at the time of this report.
Avio Aero now uses a pneumatic tool to install the adapter’s threaded inserts and added the insert pull-out check to the manufacturing process. GE Aerospace plans to add notes to the next revision of the adapter drawing specifying no prior hole tapping is allowed, pull-out tests must be performed on all installed inserts, and installed inserts must not protrude above the surface.