Introduction
This statement reflects the opinion of the Danish Accident Investigation Board regarding the circumstances of the occurrence and its causes and consequences.
In accordance with the provisions of EU Regulation 996/2010, the Danish Air Navigation Act and pursuant to Annex 13 of the International Civil Aviation Convention, the safety investigation is of an exclusively technical and operational nature, and its objective is not the assignment of blame or liability.
The safety investigation was carried out without having necessarily used legal evidence procedures and with no other basic aim than preventing future accidents and serious incidents.
Consequently, any use of this statement for purposes other than preventing future accidents and serious incidents may lead to erroneous or misleading interpretations.
A reprint with source reference may be published without separate permit.
General
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State file number: |
2025-307 |
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UTC date: |
15-6-2025 |
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UTC time: |
14:37 |
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Occurrence class: |
Serious incident |
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Location: |
Copenhagen (EKCH) |
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Injury level: |
None |
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Aircraft registration: |
EI-GEC |
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Aircraft make/model: |
MHI RJ Aviation CRJ 900 Series |
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Current flight rules: |
Instrument Flight Rules (IFR) |
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Operation type: |
Scheduled |
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Operated by: |
CityJet |
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Flight phase: |
Approach |
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Aircraft category: |
Fixed wing |
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Last departure point: |
Palanga (EYPA) |
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Planned destination: |
EKCH |
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Aircraft damage: |
None |
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Engine make/model: |
2 x General Electric CF34-8C5 |
Notification
All time references in this statement are Coordinated Universal Time (UTC).
The Aviation Unit of the Danish Accident Investigation Board (AIB) was notified of the serious incident by Naviair (the Danish Air Navigation Service Provider (ANSP)) on 15-6-2025 at 14:49 hours (hrs).
The AIB notified the Danish Civil Aviation and Railway Authority (DCARA), the Transportation Safety Board of Canada (TSB), the Irish Air Accident Investigating Unit (AAIU), the European Aviation Safety Agency (EASA), the Directorate-General for Mobility and Transport (DG MOVE) and the International Civil Aviation Organization (ICAO) on 18-6-2025 at 07:40 hrs.
During the investigation the Dutch Safety Board (DSB) and the French Bureau d’Enquete et d’Analyses (BEA) were notified.
Factual information
History of flight
The flight was a scheduled passenger flight from Palanga (EYPA) to Copenhagen (EKCH).
On the approach to runway 22L the First Officer (FO) identified fumes (initially recognized as electrical). The Flight Commander (FC) could not smell any fumes.
Shortly after smoke was noticed from the side panel at the FC side.
The FC and the FO donned their full-face oxygen masks. They noticed that their vision through the visor of the oxygen masks was impaired. The visors were dusty and blurred.
Three to four minutes before landing, the flight crew declared an emergency (Mayday) and decided to continue the approach and land.
After landing, the aircraft vacated the runway, stopped on the taxiway and shut down both engines.
The smoke had evaporated and the FC and the FO removed their oxygen masks and noticed a “L PACK AUTOFAIL” caution displayed on the EICAS (Engine Indication and Crew Alerting System) display.
The fire brigade inspected the aircraft from the outside and could not see any sign of heat, smoke or fire.
The flight crew informed the cabin crew of the situation. The cabin crew informed that smoke had been visible in the forward part of the passenger cabin.
The passengers, the FO and the cabin crew disembarked the aircraft normally.
The fire brigade inspected the passenger cabin and flight deck and could not find any traces of heat or smoke.
The serious incident occurred in daylight and under Visual Meteorological Conditions (VMC).
Injuries to persons
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Injuries |
Crew |
Passengers |
Others |
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Fatal |
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|
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Serious |
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|
|
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None |
4 |
69 |
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Damage to aircraft
Internal Air Cycle Machine (ACM) damage.
Other damage
None.
Meteorological information
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METAR EKCH |
151420Z AUTO 15008KT 120V180 9999 NCD 291850Z 21/14 Q1016 NOSIG= |
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METAR EKCH |
151450Z AUTO 17007KT 9999 NCD 21/13 Q1016 NOSIG= |
Aircraft information
General information
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Manufacturer: |
Mitsubishi Heavy Industries (MHI) RJ Aviation |
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Type: |
CRJ900 Series |
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Serial number: |
15246 |
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Airworthiness review certificate: |
Valid until 28-1-2026 |
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Engine manufacturer: |
General Electric |
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Engine type: |
CF34-8C5 |
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ACM part number: |
GG670-95009-5 |
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ACM serial number: |
01296 |
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ACM installation date: |
3-5-2017 |
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ACM Time Since New (TSN): |
33,158 hours (hrs) |
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ACM Cycles Since New (CSN): |
28,802 |
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ACM Time Since Overhaul (TSO): |
9,538 hrs |
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ACM Cycles Since Overhaul (CSO): |
9,583 |
Air Condition Unit (ACU)
The aircraft had two ACUs installed located in the unpressurized aft equipment compartment of the aircraft. The ACUs cooled the engine and Auxiliary Power Unit (APU) bleed air for the flight and passenger compartments.
Each ACU consisted of heat exchangers, condenser/reheater unit, water extractor, pressure sensors, temperature sensors and an ACM.
Bleed air from the engines and APU flowed through the primary heat exchanger and entered the centrifugal compressor in the ACM, where the air temperature and pressure were increased.
The air then passed through heat exchangers and a water extraction loop before it entered the turbine of the ACM. The air reacted with the turbine blades and caused the turbine to rotate together with the centrifugal compressor and ACM fan. The extraction of energy from the air through the turbine section caused the temperature of the air to drop.
The cooled air flowed through a condenser into a mixing manifold before entering the flight deck and passenger compartments.
See Appendix 1 for location and schematic of the ACU.
If the ACU discharged air temperature was too high, a PACK AUTOFAIL fault message would be displayed on the EICAS display in the cockpit. A failure of the ACM could cause this message.
Air Cycle Machine (ACM)
The compressor wheel and the shaft were manufactured in one piece. A turbine wheel was installed at one end of the shaft and a fan wheel at the opposite end of the shaft.
The shaft was radially supported by two journal air (foil) bearings, one at the turbine and one at the fan, and axially supported by two thrust air bearings near the turbine.
No lubrication was used for the air bearings. Instead, the air bearings operated on a continuous film of pressurized air, which eliminated component to component contact.
The film of air in the air bearing could be destroyed by moisture/water or oil ingress resulting in contact between the components.
The thrust bearings were coated with Teflon and the mating stop had a hard chrome coating, to reduce friction during start up and shut down of the ACM.
See Appendix 2 and Appendix 3 for cross section of the ACM.
Crew full-face oxygen mask
There was a full-face oxygen mask stored in a stowage box beside each pilot seat and beside the cockpit jump seat.
See Appendix 4 for a photo of full-face oxygen mask, similar to the full-face oxygen masks used.
Technical safety investigation
Onsite investigation
Shortly after the serious incident maintenance personnel carried out trouble shooting according to the Fault Isolation Manual (FIM) and the Aircraft Maintenance Manual (AMM).
During the operational test of the air condition distribution system a rapid increasing ACU discharge temperature was observed with no airflow from the Left Hand (LH) ACU. The FIM suggested failure of the LH ACU ACM.
Further investigation
ACM
The ACM was shipped to the Original Equipment Manufacturer (OEM) for a tear down inspection.
An accredited representative from the BEA participated in the tear down inspection at the OEM on behalf of the AIB.
Findings:
- The compressor/turbine shaft could not be rotated.
- Compressor/turbine shaft thrust air bearing (turbine side) found completely damaged by friction and heat with signs of whitish deposit marks most probably from moisture ingress.
- Compressor/turbine shaft thrust air bearing (compressor side) found degraded by friction and heat.
- Turbine wheel had rubbed hard against the turbine shroud with damages to the turbine wheel and shroud.
- Eight Parts Manufacturer Approval (PMA) parts were found. Further testing of the PMA parts by the OEM, at an external laboratory, confirmed that six of the PMA parts did not comply to the OEM specifications relating to different material alloys, the mechanical properties and the manufacturing processes.
- General degradation of the ACM due to environmental conditions, including the accumulation of ingressed liquids (water, de-icing fluid), combined with normal wear.
- Missing warranty seal (for tracking of last repair shop).
- Non approved modifications and maintenance practices.
See Appendix 5 for damages to thrust air bearings, turbine wheel and turbine wheel shroud.
The teardown inspection of the ACM concluded that the thrust air bearings most probably failed due to accumulated moisture ingress over time through the air supply combined with general degradation of certain parts. The moisture degraded the load‑carrying capability of the thrust air bearing causing contact between the air bearing and stop, resulting in excessive axial movement of the compressor/turbine shaft. This axial movement caused the turbine wheel to contact the turbine shroud leading to seizure of the ACM rotating shaft.
The fumes and visible smoke in the flight compartment were most likely caused by the degradation of the thrust air bearings, and by the turbine wheel contact with the shroud.
The fumes and visible smoke would not develop any further after the ACM rotating shaft had seized.
Mitigation action by the OEM:
- In 2016 the OEM introduced Service Bulletin SB GG670-95009-21-03. This SB introduced a modification, where the air for the air bearings was taken from the turbine inlet instead of the compressor outlet reducing the content of moisture in the air to the air bearings. The modification had not been embodied on this specific ACM at the time of the serious incident.
- The OEM reported in the European Coordination Centre for Accident and Incident Reporting Systems (ECCAIRS) the use of PMA parts which did not meet the OEM specifications and the non-approved modifications and maintenance practices.
Additional information
Full-face oxygen mask
During the flight, the FC and the FO noted that the visor of both full-face oxygen masks were dusty and blurred, which impaired the vision during the approach and landing.
There was no specific full-face oxygen mask cleaning requirement/task in the aircraft maintenance program.
There was an operational test requirement/task of the full-face oxygen mask every 8,000 Flight Hours (FH) and a restoration overhaul requirement of the full-face oxygen mask every 72 months.
Following the serious incident, the operator issued an Engineering Order (EO) for a fleet inspection and cleaning of all full-face oxygen masks installed including the visor.
The inspection and cleaning repeat interval for the EO would be determined depending on the findings during the fleet inspections.
Appendix
Appendix 1
Appendix 2
Appendix 3
Appendix 4
Return to Crew full-face oxygen mask
Appendix 5
Return to ACM
Damaged thrust air bearing at the turbine side. Bumps and air holes worn away.
Damaged thrust air bearing at the compressor side. Bumps and air holes still visible, but damaged.
Turbine wheel damages.
Turbine shroud damages.
AIB safety investigation process
Contact the Accident Investigation Board
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