# NTSB ENG20LA015: Airbus A220, Bordeaux, France, February 12, 2020

- Type: NTSB incident investigation
- Classification: Incident · No injuries · Class 4 · Limited · Final report
- Date: February 12, 2020, 09:00 local (09:00 UTC)
- Place: Bordeaux, France · Engine failure occurred during cruise, location is an estimate.
- Coordinates: 44.83077, -0.577698 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=44.8308&lon=-0.5777&mi=10
- Phase of flight: [Enroute](https://openairdocs.org/ntsb-phases/400) (OpenAirDocs' grouping)
- Defining event: [Enroute-cruise: Powerplant sys/comp malf/fail](https://openairdocs.org/ntsb-events/340)
- Highest injury: No injuries
- Injuries: 154 uninjured
- Status: Completed
- Report: Approved by Office Director
- Investigation class: Class 4 - Basic report with no Prelim publish
- NTSB office: Aviation Engineering Division
- Parties: Airbus Canada, Transport Accident Incident Investigation Bureau Republic of Latvia, Bureau d'Enquêtes et d'Analyses pour la Sécurité de l'Aviation Civile (BEA), FAA, TSB- Canada, Transport Canada, Pratt & Whitney
- NTSB case key (mKey): 100936
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=ENG20LA015
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/100936/pdf
- NTSB docket: https://data.ntsb.gov/Docket?ProjectID=100936
- Page: https://openairdocs.org/ntsb/ENG20LA015

## 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 because 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.

## Findings

NTSB's coded findings; those marked are part of the probable cause.

- [Engine (turbine/turboprop): compressor section](https://openairdocs.org/ntsb-findings/01057230): Aircraft › Aircraft power plant · Failure (probable cause)
- [Engine (turbine/turboprop): compressor section](https://openairdocs.org/ntsb-findings/01057230): Aircraft › Aircraft power plant · Design (probable cause)

## Analysis

The No. 1 engine low pressure compressor (LPC) stage 1 integrally bladed rotor (IBR) failure was caused by a stage 1 IBR high cycle fatigue crack that originated at the runout of an airfoil leading edge root radius. Multiple analytical methods, including two-dimensional (2D) computational fluid dynamics (CFD), acoustic testing, and instrumented flight testing identified a coupled LPC stage 3 and stage 1 IBR instability caused by an acoustic coincidence with the 2.5 bleed valve duct cavity. At high engine N1 speeds, the stage 3 IBR blade tips generate vortices/turbulent airflow and given the right conditions, the turbulent airflow can create an acoustic tone as it passes over the 2.5 bleed valve duct cavity, located immediately aft of the LPC stage 3 IBR (Figure 1). The acoustic tone drove a LPC stage 3 IBR blade 1st bending mode excitation that was then mechanically transferred through the LPC module and excited a LPC stage 1 IBR stiffwise bending mode that was present at the same approximate frequency. 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 IBR 3 / 2.5 Bleed Valve Acoustic Interaction Factors that contributed to the LPC stage 3 and stage 1 IBR acoustic coincidence and blade excitation within the engine operating range were: installation of electronic engine control (EEC) Software V2.11.7.2 and the low time rub in period on the LPC IBR blade tip clearances. EEC Software V2.11.7.2 changed the LPC inlet guide vane schedule to rotate the IGV’s in the closed direction at specific high power engine conditions to improve engine stall/surge margin. The revised vane schedule inadvertently created conditions that were favorable for generation of the 2.5 bleed valve duct cavity acoustic tone and IBR mode excitation. New engines have tighter clearances between the LPC IBR blade tips and the outer air seals. The reduced clearance created unsteady loading at the blade tip region and resulted in stronger acoustic coupling/flutter response. After a rub in period, the clearance increases, and the occurrence of flutter onset is reduced. There were three PW1524G-3 and one PW1521G-3 LPC stage 1 IBR separation events between July 25, 2019 and February 12, 2020 that occurred on multiple operators. The incident detailed in this investigation was the fourth PW1500G series LPC stage 1 IBR failure. The engine parameters at the time of each of the events and the resulting engine damage were consistent. Additional information about the failure mode and investigation process are available in the investigation dockets for the first two incidents, NTSB investigation numbers ENG19IA029 and ENG19IA034.

## Sequence of events

The events NTSB coded, in order, by phase of flight. The defining event best describes what happened.

- [Enroute-cruise: Powerplant sys/comp malf/fail](https://openairdocs.org/ntsb-events/340) (defining event)

## Aircraft

- Registration: YL-AAU (not matched to a registry record)
- Reported make and model: AIRBUS A220 (300)
- Category: Airplane
- Serial number: 55064
- Year built: 2019
- Engines: Pratt & Whitney PW1521G-3 21970 lbs thrust turbo fan; Pratt & Whitney PW1521G-3 21970 lbs thrust turbo fan
- Damage: No damage
- Flight: Non-U.S., commercial
- Operator: airBaltic
- Registered owner: airBaltic
- Airworthiness certificate: Standard, Transport
- Departed: EVRA, Riga, OF
- Destination: LEMG, Malaga, OF
- Equipment: Unknown

## Weather

- Conditions: Day

## Docket (2 files)

The files NTSB released with the investigation on May 3, 2022, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=100936. 80.9 MB in all.

- Flight data recorder - specialists' factual report (item 1, PDF, 26 pages, 24 photos, 5.9 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11828241&FileExtension=pdf&FileName=A220%20Engine%20Events%20FDR%20Report-Rel.pdf
- Flight data recorder - attachment 4 (FDR tabular data for event flight) (item 2, CSV, 75.0 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=11827606&FileExtension=csv&FileName=Attachment%204-Rel.csv

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

---
OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.