Flight Details
Aircraft Type: Bombardier CRJ-900 (CL-600-2D24) Operator: Lufthansa CityLine (on behalf of Lufthansa) Registration: D-ACNX Flight Number: LH1390 Route: Frankfurt am Main Airport (FRA), Germany to Poznań–Ławica Airport (POZ), Poland Date of Incident: 21 April 2025 Total Occupants: Not officially confirmed; typical CRJ-900 configuration carries approximately 76–90 passengers Weather Conditions: No adverse weather reported during climb
Introduction
On 21 April 2025, a Lufthansa CRJ-900 operating flight LH1390 from Frankfurt to Poznań was forced to return to Frankfurt shortly after departure due to pressurisation issues and abnormal noise linked to a forward door. While climbing through FL220, the flight crew reported a suspicion of door seal leakage or misalignment, which led to increased cockpit noise and concerns about cabin integrity.
The crew decided to discontinue the climb, informed air traffic control, and returned for a safe landing on runway 25L approximately 25 minutes after departure. After overnight inspections and attempted return to service, the same aircraft encountered another technical issue on the subsequent flight, prompting a second return.
Sequence of Events
Flight LH1390 had departed Frankfurt’s runway 18 and was conducting a routine climb towards cruising altitude when the crew became aware of:
Unusual high-frequency or whistling noise in the cockpit, commonly indicative of air leakage near a pressurised fuselage seal ECAM alerts or system warnings possibly tied to door monitoring sensors Crew-perceived instability in pressurisation control, though a decompression did not occur
The cockpit crew opted to halt the climb at FL220 and requested a return to Frankfurt. No emergency declaration (MAYDAY or PAN PAN) was made, but the flight was prioritised for approach sequencing due to the nature of the potential fault.
The aircraft conducted a standard approach and landed uneventfully on runway 25L. Emergency services were not required, and the aircraft taxied to the stand under its own power.
Crew & Communication
The flight crew responded promptly and professionally, demonstrating strong Crew Resource Management (CRM) by:
Sharing workload to assess the source of the noise Monitoring cabin pressure and door status through onboard indications Coordinating early with ATC to secure a return slot Communicating with the cabin crew to confirm cabin conditions Advising passengers of the diversion without triggering panic
Cabin crew did not report signs of decompression (e.g. oxygen mask deployment, fogging, or hissing from the cabin). The abnormality remained confined to perceived noise and cockpit indications.
Aircraft Systems & Technical Analysis
The CRJ-900’s pressurisation and door systems are monitored via:
Cabin Pressure Acquisition Modules (CPAM) Door proximity switches and sensors Pressure seal inflators Cabin pressure control valves (outflow and safety valves)
Common causes of pressurisation issues with cockpit or cabin doors include:
Seal damage or improper seating during previous closure Failure of the door sensor circuit leading to false warnings Misalignment from improper handling during turnarounds Noise caused by air leakage across pressurised thresholds
The description of “strong noise in the cockpit” is consistent with a minor air leak, which can become acoustically amplified in small flight deck environments. While not necessarily an immediate hazard, such noise combined with potential sensor discrepancies requires immediate attention.
After landing, the aircraft remained on the ground for approximately nine hours, undergoing:
Inspection of door seals, hinges, and locking cams Re-pressurisation leak testing Functional testing of cockpit aural alerts Evaluation of data from digital flight data and maintenance reporting systems
The aircraft was cleared for a subsequent flight but suffered another technical issue that required another air turnback—details of the second event remain pending. This may indicate an unresolved root cause or intermittent fault missed during the initial repair.
Passenger Experience & Cabin Conditions
No depressurisation occurred, and the cabin remained stable. Passengers likely noticed:
Noise levels inconsistent with typical cabin ambience PA announcements informing them of a technical issue and return Smooth return and landing without inflight upset
There were no reports of injury, oxygen deployment, or crew distress. Passengers were reaccommodated, and the flight to Poznań was later completed using a different aircraft.
Emergency Response & Aftermath
Lufthansa Technik teams at Frankfurt conducted overnight fault-finding, but the recurrence suggests:
A possible sensor wiring or feedback loop problem Issues with door locking indication logic Incomplete seating of the flight deck forward door seal or latch mechanism
Following the second return on the next attempt, the aircraft was removed from service for comprehensive troubleshooting. Boeing and Bombardier (now part of MHI RJ Aviation) may be consulted if the issue proves to involve recurrent design-related anomalies.
Investigation Status
No regulatory investigation is expected, as there were no injuries, damage, or safety-of-flight emergencies. However, Lufthansa’s internal quality assurance and reliability tracking programmes will flag this aircraft for:
Deeper airframe and door system inspection Review of line maintenance documentation Cabin pressurisation leak detection runs Door cycle tracking and component wear logs
Root Cause & Contributing Factors (Preliminary)
Primary Cause:
Suspected improper seal engagement or sensor feedback error on one of the aircraft’s pressurised doors, leading to noise and pressurisation alert.
Contributing Factors:
Possible maintenance oversight during prior door closure or aircraft turnarounds Incomplete detection of the fault during post-return inspection Potential intermittent sensor malfunction or door frame structural alignment issue
Safety Recommendations & Industry Impact
Airlines operating the CRJ series may review:
Seal integrity inspection frequency, especially on older airframes or high-cycle regional jets Additional maintenance training on door locking diagnostics and troubleshooting Use of borescope or dye penetrant inspections to check for door frame deformation after repeated thermal pressurisation cycles
While this incident was minor, it highlights the cascading disruptions caused by overlooked pressurisation anomalies, even in non-catastrophic scenarios.
Conclusion
Lufthansa flight LH1390’s return to Frankfurt due to door-related pressurisation noise was handled proactively and safely by the crew. Though the aircraft passed inspection and attempted to return to service, the recurrence of issues necessitated further withdrawal from operations. The case underscores the importance of robust post-incident diagnostics, particularly when sensor discrepancies and physical anomalies intersect.
Disclaimer
This article is based on publicly available information and reports at the time of writing. While every effort has been made to ensure accuracy, we cannot guarantee the completeness of the information provided.
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