NTSB Factual Report

HISTORY OF FLIGHT

On October 23, 2011, about 1415 eastern daylight time, an Atlantic Southeast Airways (ASA) Bombardier Canadair Regional Jet (CRJ)-700, registration number N751EV, powered by two General Electric (GE) CF34-8C5B1 turbofan engines, experienced a No. 1 (left) engine undercowl fire after landing at the Hartsfield-Jackson Atlanta International Airport (ATL), Atlanta, Georgia. The flight crew reported that they received a left engine fire warning during taxi to the gate. The pilot shut down the affected engine and the fire warning quickly ceased. No fire bottles were discharged. The airplane taxiing to the gate and the passengers deplaned normally. The airplane was undamaged and no injuries were reported. The incident flight was a 14 CFR Part 121 regularly scheduled passenger flight from a Des Moines Airport (DSM), Iowa to ATL.

ENGINE DAMAGE

Examination of the engine, SN 965663, revealed that the Start Operability Bleed Valve (OBV) fuel return adapter-fitting had fractured and separated, the start OBV valve body alignment tab was not fully engaged with the mating notch in the start OBV duct, and the electrical harness in the vicinity of the OBV was sooted and thermally damaged. No cowling damage was noted. The engine was removed from the airplane and the start OBV, start OBV fuel return tube, and pneumatic joint clamps were retained for further investigation and sent to Honeywell, the OBV manufacturer, for evaluation.

TEST AND RESEARCH

Metallurgical Examination

All the metallurgical examinations were conducted by Honeywell at their Phoenix, Arizona facility. The fuel return adapter-fitting was fractured through the o-ring groove and the fracture surfaces exhibited an approximate 120° fatigue region with two transition zones on either side of the fatigue region followed by overload region. The fracture features were consistent with cyclic loading. The crack initiation zone was identified; however, due to localized damage the exact crack initiation site could not be determined. Material analysis and hardness measurements of the OBV piston housing and the fuel return adapter-fitting confirmed them to be consistent with specified AL 6061-T6 and CRES A286 material respectively and were processed in accordance with the manufacturer’s specification.

The return adapter-fitting tube end ball-nose area had a dent and material deposits outboard of the sealing area and axial scrape marks beneath and adjacent to the deposited material. The fuel return tube conical seat exhibited axial scratches and material deposits that corresponded to the same location as the axial scrape marks and material deposit noted on the ball-nose. The valve flow body flange of the OBV also exhibited a dent and material deposit. An energy dispersive x-ray (EDS) of the contaminants embedded in the dent on the ball-nose and the valve flow body flange had elemental peaks of silicon and oxygen and according to Honeywell are elements present in sand. Based on this information, Honeywell concluded, and GE agreed, that the OBV was dropped and impacted a hard surface, possibly a concrete floor. It was hypothsized that the impact was sufficient enough to dent and plastically deform the piston housing fuel return adapter-fitting conical seat (the piston housing is a component of the OBV where the adapter-fitting are installed) possibly reducing the installation preload, which would result in reducing the fatigue capability of the adapter-fitting. The reduced preload could allow the o-ring necked down area of the adapter-fitting to experience a cyclic load that initiated and propagated a fatigue crack resulting in the separation of the adapter-fitting.

Manufacturing Issues

After the separated portion of the fuel return adapter-fitting was removed from the piston housing, metal chips were found at the base of the fuel return adapter-fitting port serrations, which were remnants from the broaching operation used to create the serration into the piston housing that engages the adapter-fitting lockring. The industry design standard for creating the adapter-fitting port provides two options when drilling and machining (broaching) the adapter-fitting port; one with and one without an undercut at the base of the lockring feature. The intent of the undercut is for ease of chip removal after the broaching process. According to the industry standard, whether an undercut is added to the base of the lockring feature or not, removal of the broaching chips is required. Examination of the other two adapter-ports, revealed metal chips at the base of the serrations as well. It is not known to what extent, if any, the metal chips had a direct effect on the installation loads and the load carrying capability of the adapter-fitting joint in this particular case and whether it contributed to the failure of the fuel return adapter-fitting. The metal chips found in the serrations prompted Honeywell to review and revise their sub-tier supplier piston housing manufacturing and adapter-fitting installation procedures.

Start Operability Bleed Valve Adapter-Fitting Load Analysis

Honeywell and GE performed a series of tests and Finite Element Modeling (FEM) to define the installation and applied loads that each of the three adapter-fittings (fuel supply, return and drain) experienced and to access the appropriateness of the adapter-fitting configuration for each application. The conclusion of all the testing and FEM revealed that the capability of the adapter-fitting joint is directly affected by variations in the applied preload caused by differences in instantiation torque, coefficient of friction, application of a primer on the threads, and use of the combination installation tool and that static applied loads were sufficient to create loose contact between the adapter-fitting and the OBV piston housing threads. Vibratory loads can create relative motions between the adapter-fitting and the piston housing threads generating a wear mechanism that can eventually lead to a fatigue separation of the piston housing threads and the pull-out of the adapter-fitting; however, no failure mechanism was found for an adapter-fitting fracture (separation) mechanism from just the applied loads, further supporting the assumption that a hard impact facilitated the fracture sequence for the fuel return adapter-fitting. However, the FEM did show that the capability of the adapter-fitting joint was under designed for the loads that could be applied by the attached fuel tube.

ADDITIONAL INFORMATION

Corrective Actions – Service Bulletins, Airworthiness Directive, and Manufacturing and Installation Improvements

A series of corrective measures were taken to address the manufacturing and installation issues and lack of sufficient load-caring margin for the adapter-fitting on the OBV. To address the manufacturing and installation issues, Honeywell developed a stand-alone document that incorporated and consolidated industries best practices for the installation of the adapter-fitting that would reduce the variations associated with the previous installation procedures. To address the under-designed OBV, GE issued service bulletins (SB) 75-0012 and 75-0017 to replace the Honeywell OBV with the alternate supplier’s OBV and the removal schedule was based on the amount of flight time each OBV had accumulated with the OBV with the most time removed first. On December 13, 2012, the Federal Aviation Administration (FAA) issued a notice of proposed rulemaking (NPRM) titled “Airworthiness Directives; General Electric Company Turbofan Engines,” which was published in 77 Federal Register Volume 77, page 74125. The proposed Airworthiness Directive (AD) would require the affected OBVs be removed from service and replaced with OBVs eligible for installation in accordance with the removal schedules in the GE SBs.

NTSB Probable Cause

The Start Operability Bleed Valve (OBV) fuel return adapter-fitting fractured and separated in cyclic fatigue, which allowed fuel to spill onto a hot engine case that ignited the undercowl fire. The OBV fuel adapter-fitting was dropped and the damage reduced its load carrying and fatigue capability, eventually leading to its failure.