NTSB Identification: ENG12IA004
Scheduled 14 CFR Part 121: Air Carrier operation of ATLANTIC SOUTHEAST AIRLINES INC
Incident occurred Sunday, October 23, 2011 in Atlanta, GA
Probable Cause Approval Date: 05/13/2013
Aircraft: BOMBARDIER CL600 2C10, registration: N751EV
NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.
An Atlantic Southeast Airways (ASA) Bombardier Canadair Regional Jet (CRJ)-700 experienced an undercowl engine fire after landing at Hartsfield-Jackson Atlanta International Airport (ATL), Atlanta, Georgia. The pilot shut down the affected engine and the fire warning quickly ceased. No fire bottles were discharged. The airplane taxied 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, Iowa, to ATL.
Examination of the engine revealed that the Start Operability Bleed Valve (OBV) fuel return adapter-fitting had fractured and the electrical harness in the vicinity of the OBV was sooted and thermally damaged. No cowling damage was noted. Metallurgical examination of the fractured fuel return adapter-fitting revealed that it had failed in fatigue due to cyclic loading. The return adapter-fitting tube end ball-nose area had a dent. Material deposits were found outboard of the sealing area and axial scrape marks were found beneath and adjacent to the deposited material. The valve flow body flange of the OBV also exhibited a dent and material deposit. An analysis of the deposited material revealed elemental peaks of silicon and oxygen, elements present in sand and not in the return adapter-fitting alloy composition. The presence of silicone and the dent is consistent with the OBV most likely having been dropped and impacted a hard surface that damaged the fuel return adapter-fitting before it was reinstalled on the engine. The impact damage to the return adapter-fitting reduced its loading-carrying and fatigue capability. The OBV fuel return adapter-fitting eventually fractured and separated, which allowed fuel to spill onto a hot engine case that ignited the undercowl fire.
Although not related to the incident, testing and analytical modeling of the OBV adapter-fitting joint revealed that, as designed, the joint is directly affected by variations in the applied preload caused by differences in installation torque, coefficient of friction, application of a primer on the serrations, 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 during normal engine operation can create relative motion 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 and subsequent fuel leaks that could lead to a fire. To address this design deficiency, service bulletins were issued to replace the affected OBV with an OBV from alternate manufacturer, and the removal schedule was based on the amount of flight time each OBV had accumulated. The FAA issued an Airworthiness Directive mandating the service bulletins.
The National Transportation Safety Board determines the probable cause(s) of this incident to be: 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. Full narrative available
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