History of Flight
On June 10, 2017, at about 1750 central daylight time, a SkyWest Airlines Bombardier CRJ700, registration N715SK, experienced a No. 1 (left) engine failure during takeoff from Chicago O'Hare International Airport (ORD), Chicago, Illinois. The crew reported hearing a loud bang, followed by a reduction in engine power and a No. 1 engine fire warning. The crew declared an emergency, shut down the No. 1 engine, and returned to ORD for an uneventful single-engine landing. Aircraft Rescue and Firefighting (ARFF) crews met the airplane on the runway and did not observe fire or smoke from the No. 1 engine, so the airplane was cleared to taxi to the gate. No passenger or crew injuries were reported. The flight was operated under 14 Code of Federal Regulations Part 121 as a regularly scheduled flight from ORD to Bishop International Airport (FNT), Flint, Michigan.
Damage to the Airplane
The airplane fuselage had multiple small impact marks aft of the No. 1 engine. The impacts did not penetrate the fuselage skin and were repairable per the Bombardier structural repair manual. The No. 1 engine aft core cowl had a hole at the 10 o'clock position, four inches aft of the cowl forward flange, measuring approximately 7.5 inches circumferentially by four inches axially.
Test and Research
Engine Examination and Disassembly
A visual examination of the engine by maintenance crews at ORD revealed a hole in the engine cowl, damage to visible low pressure turbine (LPT) blades, and small impacts on the fuselage aft of the No. 1 engine. The engine was removed and shipped to SkyWest's facility in Salt Lake City for preliminary borescope and component removal, then to StandardAero in Winnipeg, Manitoba, Canada for further examination and disassembly. Party members from SkyWest, General Electric Aviation (GE), Bombardier, the FAA, and the NTSB met at StandardAero from July 11-13, 2017.
The No. 1 engine LPT case had a hole at the 10 o'clock position near the forward flange, about six inches circumferentially by four inches axially. The high pressure turbine (HPT) 2nd stage nozzle assembly was missing a vane segment at the 10 o'clock position, and the HPT inner case assembly had a burn-through hole in the same location. The HPT 2nd stage rotor blades exhibited thermal and impact damage. All four LPT stages showed severe impact damage and missing material. The combustion outer liner had a 2.5-inch axial crack from the aft flange forward, connected to a 12-inch-long circumferential crack from the 10 o'clock to 12:30 positions between cooling air holes. At the intersection of the cracks, the liner surface was displaced about 1.5 inches radially inward.
Fuel Nozzle Testing
All 18 fuel nozzles were flow tested at Woodward in Zeeland, Michigan in as-received "dirty condition." No anomalies were noted in spray pressure or spray angle, but 16 of 18 nozzles failed spray quality testing due to minor streaking. According to Woodward engineering, the patterns were consistent with high-time service run hardware.
Main Fuel Pump and Fuel Meter Unit Testing
The main fuel pump was tested at Triumph Accessory Services and met specifications with no faults. The fuel metering unit was tested at Woodward in Rockford, Illinois; multiple flow test points were below minimum for a new unit but within expected range for high-time service run components, per Woodward and GE engineering.
Additional Information
Customer Notification Reports and Maintenance
GE performed engine trending/health monitoring for SkyWest. Four high-priority customer notification reports (CNRs) were sent to SkyWest due to low or decreasing takeoff exhaust gas temperature hot day margin (EGTHDM) combined with low or decreasing delta core speed. The notifications were sent in February 2016, October 2016, March 2017, and May 2017. The probable cause listed on all four CNRs was compressor efficiency. The first recommended action was to review maintenance records; the second was to perform a core engine water wash. SkyWest completed water washes on March 7, 2016, November 2, 2016, April 1, 2017, and May 6, 2017. The first wash restored margin; the second was minimally effective; the third and fourth were ineffective. On June 6, 2017, a high pressure compressor borescope inspection showed no anomalies. On June 9, 2017, the engine was scheduled for removal on June 14, 2017, but failed on June 10.
Interturbine Temperature Exceedances
Electronic engine control and flight data recorder data showed interturbine temperature (ITT) exceedances during the seventh and fifth flights prior to the incident (June 8 and 9, 2017). Both exceedances were within "Area A" per the Bombardier CRJ700/900/1000 AMM, requiring visual inspection of cases and visible blades through the exhaust nozzle. No anomalies were noted, and the airplane returned to service. Only the June 8 exceedance was recorded in the SkyWest maintenance log.
Combustion Liner Failure History
The NTSB is aware of five other similar combustion outer liner buckling occurrences. Two resulted in in-flight events, one causing an engine fire warning, shutdown, and air turn back. Other cracks/buckling were found during inspection or repair. No combustion outer liner failure has occurred on a liner with fewer than 18,000 hours and/or 12,000 cycles. The NTSB also investigated a CF34-8C combustion inner liner burn-through failure under investigation number ENG14IA029.
Corrective Actions
GE released service bulletins SB 72-A0326 (CF34-8C) and SB 72-A0211 (CF34-8E) on October 17, 2018, recommending a one-time borescope inspection of the combustion outer liner aft panel (P4) on liners with over 15,000 flight hours time since new or repair. Compliance was recommended as soon as possible but before 1,000 flight hours.
Following the incident, GE developed a new CNR monitoring parameter for combustor distress, tracking sharp decreases in takeoff EGTHDM with shifts in takeoff delta fuel flow. Several CNR parameters were modified to include a borescope inspection of the combustion liner. A new Core Health Indicator (CHI) parameter was developed in April 2018, using core speed, fuel flow, and compressor discharge pressure during takeoff to calculate predicted EGTHDM, providing supplemental trend data.