History
On October 23, 2011, about 1415 eastern daylight time, an Atlantic Southeast Airways Bombardier CRJ-700, registration N751EV, powered by two GE CF34-8C5B1 turbofan engines, experienced a No. 1 engine undercowl fire after landing at Hartsfield-Jackson Atlanta International Airport. The flight crew received a left engine fire warning while taxiing to the gate. The pilot shut down the affected engine and the warning ceased. No fire bottles were discharged. The aircraft taxied to the gate and passengers deplaned normally. The airplane was undamaged and no injuries were reported. The flight was a scheduled Part 121 passenger flight from Des Moines, Iowa.
Engine Damage
Examination of the engine (SN 965663) revealed a fractured and separated start OBV fuel return adapter-fitting. The start OBV valve body alignment tab was not fully engaged, and the electrical harness near the OBV was sooted and thermally damaged. No cowling damage was noted. The engine was removed, and the start OBV components were retained for further investigation and sent to Honeywell.
Investigation Findings
Honeywell conducted metallurgical examination. The adapter-fitting fractured through the o-ring groove, with a fatigue region covering about 120° and two transition zones followed by overload. Features were consistent with cyclic loading. The crack initiation zone was identified but exact site could not be determined due to localized damage. Material analysis confirmed specifications.
The return adapter-fitting tube end ball-nose had a dent and material deposits; the fuel return tube conical seat had scratches and deposits. Energy dispersive x-ray showed silicon and oxygen, consistent with sand. Honeywell concluded, with GE's agreement, that the OBV was dropped and impacted a hard surface, possibly concrete. This impact dented and plastically deformed the piston housing conical seat, possibly reducing installation preload and fatigue capability, leading to crack initiation and propagation.
Metal chips were found at the base of the fuel return adapter-fitting port serrations, remnants from broaching. Similar chips in other ports. The chips' effect on installation loads and failure contribution was unknown. Honeywell reviewed and revised manufacturing procedures.
Load analysis by Honeywell and GE showed that adapter-fitting joint capability is affected by variations in preload from torque, friction, primer use, and installation tool. Static loads could create loose contact; vibratory loads could cause wear and eventual fatigue separation of piston housing threads or pull-out. However, no failure mechanism for adapter-fitting fracture from loads alone was found, supporting the impact hypothesis. The joint was under-designed for loads from the attached fuel tube.
Corrective Actions
To address manufacturing and installation issues, Honeywell developed a standalone document consolidating best practices. To address under-design, GE issued service bulletins 75-0012 and 75-0017 to replace Honeywell OBVs with alternate supplier OBVs, with removal schedule based on flight time. On December 13, 2012, the FAA issued a notice of proposed rulemaking for an airworthiness directive requiring removal and replacement per the service bulletins.