History of Flight
On July 25, 2019, about 1305 local time, a Swiss International Air Lines (SWISS) Airbus A220-300, registration HB-JCM, equipped with two Pratt & Whitney PW1524G-3 geared turbofan engines experienced a failure of the No. 1 engine while climbing through flight level 320 over Perrigny-sur-Armançon, France. The flight crew followed quick reference handbook procedures and attempted to shut down the No. 1 engine, but the engine had already been shut down by the Electronic Engine Control (EEC). The crew diverted to Charles de Gaulle International Airport (CDG) in Paris, France, and made an uneventful single-engine landing. No passenger or crew injuries were reported. The regularly scheduled passenger flight was operating from Geneva Airport (GVA), Switzerland, to London Heathrow Airport (LHR), United Kingdom.
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 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 Pratt & Whitney Columbus Engine Center in Columbus, Georgia, USA, for examination and disassembly. The separated LPC stage 1 IBR penetrated the forward and mid LPC cases, creating 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 traveled aft through the bypass duct, exiting 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 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 were 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 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 shipped to the Pratt & Whitney 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, notifying local citizens of the event, providing information on the search area, and requesting 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 as of the date of the report, the LPC stage 1 IBR had 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 event. 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, including two-dimensional 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 the LPC stage 3 IBR, which 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, which 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, resulting in leading edge blade root crack formation and subsequent IBR fracture. Factors that contributed to the onset of 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 Pratt & Whitney, the Federal Aviation Administration (FAA), Airbus Canada, and Transport Canada to reduce the likelihood of additional events. A recurrent borescope inspection (BSI) 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 schedule 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 Pratt & Whitney, redesigned hardware was scheduled to be available to the fleet by the fourth quarter of 2021.