HB-JCM
AIRBUS A220
The No. 1 (left) engine low pressure compressor (LPC) stage 1 integrally bladed rotor (IBR) separation was caused by a high cycle fatigue (HCF) crack that originated at the runout of an airfoil leading edge root radius. Acoustic tests, instrumented flight tests, and multiple analytical methods were completed and identified a mechanically coupled LPC stage 3 and stage 1 IBR mode excitation driven by an acoustic coincidence with the 2.5 bleed valve duct cavity. At high engine speeds in specific flight conditions, unsteady or turbulent airflow generated by the LPC IBR blade tips created an acoustic tone as it passed over the 2.5 bleed valve duct cavity, located immediately aft of the LPC (Figure 1). The acoustic tone excited a LPC stage 3 IBR blade 1st bending mode that was mechanically transferred through the LPC module to the LPC stage 1 IBR where a stiffwise bending mode was excited. The resultant stresses on the LPC stage 1 IBR blades exceeded material limits and subsequently led to crack formation and eventual progression to overload failure. Figure 1- Engine LPC Diagram Identifying the stage 3 IBR / 2.5 Bleed Valve Acoustic Interaction Factors that contributed to the LPC acoustic coincidence within the engine operating range were installation of electronic engine control (EEC) Software V2.11.7.2 and low engine operating hours. EEC Software V2.11.7.2 modified the LPC inlet guide vane (IGV) schedule to improve engine surge/stall margin in high power flight phases such as top of climb, by rotating the vanes in the closed direction. The revised vane schedule inadvertently created conditions that were favorable for LPC IBR 3 and IBR 1 flutter onset. New engines have tighter clearances between the LPC IBR blade tips and the outer air seals. The tighter clearances created unsteady loading at the blade tip region and resulted in stronger acoustic coupling and flutter response. Flight test data confirmed that acoustic coincidence and flutter response within the engine operating range was significantly reduced following an engine rub in period. The Powerplant Group Chairman’s Factual Report available in the investigation docket includes additional information on the LPC acoustic coincidence and the testing and analytical methods used to identify the failure mode.
HISTORY OF FLIGHT On July 25, 2019, about 1305 local time, a Swiss International Air Lines (SWISS) Airbus A220-300, HB-JCM, equipped with two Pratt &Whitney (P&W) PW1524G-3 geared turbofan engines experienced a No. 1 engine failure while climbing through flight level 320 over Perrigny-sur-Armançon, France. The flight crew followed quick reference handbook procedures and attempted to shutdown the No. 1 engine, but the engine had already been shutdown by the EEC. The crew diverted to Charles de Gaulle International Airport (CDG), Paris, France and made an uneventful single engine landing. A post flight examination of the engine revealed a hole in the LPC case and the LPC stage 1 IBR had separated and was missing. There were no passenger or crew injuries reported. The regularly scheduled passenger flight was operating from Geneva Airport (GVA), Geneva, Switzerland to London Heathrow Airport (LHR), London, United Kingdom. In accordance with ICAO Annex 13 guidelines, the National Transportation Safety Board (NTSB) accepted the delegation of this investigation from the French Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA), who appointed an Accredited Representative to assist with the investigation. DAMAGE TO THE AIRPLANE Airplane structural damage was limited to the No. 1 engine thrust reverser (TR) assembly. The TR assembly exhibited impact damage on the outer barrel, most concentrated on the left side. The left TR sleeve outer skin had two penetrations, at about the 8:30 and 9 o’clock positions. The larger penetration located at about the 8:30 position was an approximately 1 inch long slice, 6 inches from the sleeve trailing edge. The penetration was approximately 60 inches axially aft of the LPC stage 1 IBR plane of rotation. There was no evidence of high energy radial engine fragment penetration through the TR sleeve. TEST AND RESEARCH Engine Examination and Disassembly The incident engine, serial number P736090, was shipped from CDG to the P&W Columbus Engine Center in Columbus, Georgia, USA for examination and disassembly. The separated LPC stage 1 IBR penetrated the forward and mid LPC cases and created a hole from the 9 to 1 o’clock positions radially and between the forward flange of the forward LPC case and the aft flange of the mid LPC case axially. The separated LPC stage 1 IBR was contained by the nacelle and travelled aft through the bypass duct and was liberated out the back of the engine. The remaining LPC stages sustained secondary impact damage. Uncontained engine fragments also caused secondary impact damage to the trailing edge of all the fan blades and the trailing edge of eight consecutive fan exit guide vanes (FEGV) between the 10 o’clock and 11:30 positions. Thermal damage and material loss was observed in stages 5 through 8 of the high pressure compressor (HPC) and stage 2 of the high pressure turbine (HPT) during a borescope inspection (BSI) of the engine core. The low pressure turbine (LPT) stage 2 and 3 rotor blades were all broken, and the fracture surfaces were thermally distressed. The LPT stage 2 and 3 stator vanes exhibited coincident 360 degree impact damage. Metallurgy Multiple loose engine fragments, including a section of the separated LPC stage 1 IBR, were recovered from the nacelle and were shipped to the P&W Materials and Processes Engineering (MPE) Laboratory in East Hartford, Connecticut for examination. The fracture surface features on all recovered fragments were consistent with tensile-shear overload and a primary fracture surface was not identified. LPC Stage 1 IBR Recovery Efforts The NTSB and BEA collaborated to identify a search area where the separated LPC stage 1 IBR may have landed in rural France. The BEA issued a “Call For Witnesses” press release in August 2019 that notified local citizens of the event, provided information on the search area, and requested assistance in locating the missing LPC stage 1 IBR. In November 2019, the BEA coordinated and led a series of large search efforts in France. Several small engine fragments were recovered during the searches, but to date the LPC stage 1 IBR has not been located. Additional LPC Stage 1 IBR Fractures and Metallurgy Findings There were three PW1524G-3 and one PW1521G-3 LPC stage 1 IBR separations that occurred on multiple operators between July 25, 2019 and February 12, 2020. The incident detailed in this investigation was the first of the four PW1500G series LPC stage 1 IBR failures. The engine parameters at the time of the four LPC stage 1 IBR failures and the resulting engine damage were consistent in each of the events. Following the second LPC stage 1 IBR failure on September 16, 2019, a majority of the stage 1 IBR was recovered from the bypass duct/thrust reverser structure. An examination of the recovered LPC stage 1 IBR primary fracture surface revealed a high cycle fatigue crack that originated at the runout of an airfoil leading edge root radius. Scanning electron microscopy (SEM) analysis identified the fracture origin approximately 0.0033 inch (0.084 mm) beneath the material surface. There was no evidence of material or processing anomalies near the crack origin and the fracture surface showed no evidence of cyclic markers or arrest lines. Testing and Analysis to Identify Failure Mode Computer modeling, which included, two-dimensional (2D) computational fluid dynamics (CFD), acoustic testing, component testing, and instrumented flight tests were conducted to identify the root cause of the failures. The testing identified that at specific engine operating conditions an acoustic tone was generated by the 2.5 bleed valve duct cavity. The duct cavity is located immediately aft of LPC stage 3 IBR that excited a coupled LPC stage 3 and stage 1 IBR rotor modes. The sound waves from the acoustic tone excited a LPC stage 3 IBR 1st bending mode that then mechanically transferred through the LPC module and excited a coupled LPC stage 1 IBR stiffwise bending mode. The acoustic coincidence and blade flutter response created LPC stage 1 IBR blade stress levels that exceeded material limits and resulted in leading edge blade root crack formation and subsequent IBR fracture. Factors that contributed to the onset acoustic coincidence included an electronic engine control (EEC) software revision, V2.11.7.2, that altered the LPC variable inlet guide vane (IGV) schedule and tighter LPC IBR blade tip clearance due to low time on the engine. Corrective Actions Several corrective actions were taken by P&W, Federal Aviation Administration (FAA), Airbus Canada, and Transport Canada to reduce the likelihood of additional events. A recurrent BSI inspection of the LPC stage 1 IBR was mandated, and the inspections identified two additional LPC stage 1 IBR crack findings in the fleet. In addition to the recurrent BSI, an N1 speed restriction above FL290 was implemented to reduce the likelihood of mode excitation and an EEC software update, V2.11.9, was released to revert the LPC vane schedule back to the original vane schedule that was installed prior to the LPC stage 1 IBR failures. Finally, redesign efforts are underway to modify the 2.5 bleed valve duct geometry to increase the frequency margin and eliminate the resonant response within the engine operating range. According to P&W, redesigned hardware is scheduled to be available to the fleet by the fourth quarter of 2021.
A No. 1 (left) engine low pressure compressor (LPC) stage 1 integrally bladed rotor (IBR) separation due to a high cycle fatigue crack (HCF) that originated at the runout of an airfoil leading edge root radius. The HCF crack developed as a result of a mechanically coupled LPC stage 3 and stage 1 IBR mode excitation and blade flutter response. The excitation was driven by an acoustic tone generated by turbulent airflow passing over the 2.5 bleed valve duct cavity while the engine was operating at high speeds in specific flight conditions. A primary contributor to the failure mode was an electronic engine control (EEC) software update that changed the LPC vane schedule and increased the likelihood of LPC stage 1 IBR blade flutter onset within the engine operating range.
Source: NTSB Aviation Accident Database
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