Baltic BCS3 near Bordeaux on Feb 12th 2020, uncontained engine failure

Last Update: May 4, 2022 / 19:19:07 GMT/Zulu time

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Incident Facts

Date of incident
Feb 12, 2020

Classification
Incident

Airline
Air Baltic

Flight number
BT-677

Departure
Riga, Latvia

Destination
Malaga, Spain

Aircraft Registration
YL-AAU

Aircraft Type
Airbus A220-300

ICAO Type Designator
BCS3

An Air Baltic Bombardier C-Series CS-300, registration YL-AAU performing flight BT-677 from Riga (Latvia) to Malaga,SP (Spain), was enroute at FL390 about 80nm south of Bordeaux (France) when the left hand engine (PW1500G) suffered an uncontained failure and was shut down. The crew diverted the aircraft to Bordeaux where the aircraft landed safely on runway 23 about 25 minutes after leaving FL390.

The airline reported the captain shut the left hand engine down and diverted to Bordeaux. A replacement aircraft is dispatched to take the passengers to Malaga.

A replacement CS-300 registration YL-AAV was dispatched to Bordeaux and resumed the flight. The aircraft is curently enroute at FL290 and is estimated to reach Malage with a delay of 8 hours.

The French BEA reported the left hand engine suffered a technical failure, the aircraft diverted to Bordeaux. The occurence was rated a serious incident and has been delegated to the US NTSB, who are already investigating three occurrences of the same type that Swiss had suffered in 2019, see Incident: Swiss BCS3 near Paris on Jul 25th 2019, engine shut down in flight, Incident: Swiss BCS3 near Geneva on Sep 16th 2019, uncontained engine failure and Incident: Swiss BCS3 near Paris on Oct 15th 2019, engine shut down in flight.

In all three cases of Swiss the affected engine had suffered the uncontained release of the stage 1 low pressure compressor rotor.

As result of the occurrences in 2019 the FAA had released Air Worthiness Directive 2019-19-11 and Transport Canada had released Emergency Airworthiness Directive (EAD) CF-2019-37, the latter limiting the N1 setting to 94% above FL290, which requires the autothrust to be disengaged in order to respect that limit.

According to Mode-S Data transmitted by the aircraft the aircraft had been enroute for about 4 hours and had done several climbing and descending changes in flight levels. Near Bordeaux the aircraft had been enroute at FL370 then climbed to FL390 and had been level at FL390 for about 2 minutes when the engine failure occurred.

On Mar 31st 2020 the FAA released another Airworthiness Directive AD-2020-07-02 arguing:

The FAA has received reports of four instances of IFSDs occurring on the affected model turbofan engines since 2019.

In response to the two IFSDs that occurred in July and September 2019, and in response to ongoing investigations of these IFSDs, the FAA issued AD 2019-19-11 (84 FR 50719, September 26, 2019), to perform inspections of the LPC R1 to prevent failures. The FAA subsequently superseded AD 2019-19-11, issuing AD 2019-21-11 (84 FR 57813, October 29, 2019) in response to another IFSD and to expand the population of affected engines that needed inspection of the LPC R1. Since the effective date of AD 2019-21-11, another IFSD occurred in February 2020. Analysis by the manufacturer determined that the LPC vane schedules were putting the engine in a condition to experience an acoustic resonance that damages the LPC R1, which then leads to LPC R1 failure. In response, the manufacturer updated the EEC FADEC software to improve vane scheduling to avoid acoustic resonance.

This condition, if not addressed, could result in uncontained release of the LPC R1, damage to the engine, and damage to the airplane. The FAA is issuing this AD to address the unsafe condition on these products.

On May 4th 2022 the NTSB released their final report concluding the probable causes of the incident were:

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.

The NTSB analysed:

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.

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.
Incident Facts

Date of incident
Feb 12, 2020

Classification
Incident

Airline
Air Baltic

Flight number
BT-677

Departure
Riga, Latvia

Destination
Malaga, Spain

Aircraft Registration
YL-AAU

Aircraft Type
Airbus A220-300

ICAO Type Designator
BCS3

This article is published under license from Avherald.com. © of text by Avherald.com.
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