Pilot Debrief

AIRBUS A220 near Perrigny-sur-Armançon — 2019-07-25

Final reportENG19IA029
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Date
2019-07-25
Location
Perrigny-sur-Armançon, FR
Airport
CDG
Aircraft
AIRBUS A220
Registration
HB-JCM
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Climb

Probable cause

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.

NTSB narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Crew could have opted for an earlier engine shutdown procedure.

NTSB coding

  • Powerplant sys/comp malf/fail · Enroute-climb to cruise

Evidence available

  • FDR / data
  • 4 docket documents
View NTSB final reportView NTSB docket

Docket documents4

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