Flight D8-5351 | Registration: SE-RRO
Date: 7 April 2025 | Location: Enroute over the North Sea, diverted to Copenhagen (EKCH)
Flight Details
• Aircraft Type: Boeing 737-800
• Engines: 2 × CFM56-7B26
• Operator: Norwegian Air Sweden
• Registration: SE-RRO
• Flight Number: D8-5351
• Callsign: NSZ5351
• Route: Stockholm Arlanda (ESSA), Sweden to Alicante-Elche Airport (LEAL), Spain
• Date of Incident: 7 April 2025
• Total Occupants: Estimated 185 (including crew)
• Weather Conditions: No adverse weather reported in the area
Introduction
On 7 April 2025, a Norwegian Air Sweden Boeing 737-800 operating flight D8-5351 from Stockholm to Alicante diverted to Copenhagen following an engine-related alert while cruising at FL370. The crew reported a low oil quantity indication on the right-hand engine (CFM56) and, while initially maintaining cruising altitude, made the decision to divert to the nearest suitable airport. The aircraft landed safely on runway 04R at Copenhagen approximately 45 minutes after the alert. No injuries or further technical issues were reported.
Sequence of Events
The aircraft departed Stockholm Arlanda as scheduled and was enroute over Dutch airspace at FL370, approximately 270 nautical miles southwest of Copenhagen. During routine monitoring, the crew observed a low oil quantity indication on the No. 2 (right-hand) engine.
No associated vibration or temperature exceedances were reported, and the engine continued operating within acceptable limits. However, as oil quantity is a critical parameter for continued engine lubrication and cooling, the crew decided to divert to Copenhagen as a precautionary measure.
The aircraft remained at FL370 briefly before commencing a controlled descent and diversion. The crew coordinated with Dutch and Danish ATC, who facilitated priority routing to Copenhagen.
Flight D8-5351 landed safely on runway 04R without the need for emergency services. The aircraft taxied to the gate under its own power. The flight was cancelled, and maintenance investigations began shortly after.
Crew & Communication
The flight crew responded appropriately to the oil quantity warning:
• Assessed engine parameters (oil pressure, vibration, EGT, N1/N2)
• Confirmed no secondary indications of oil loss or performance degradation
• Advised ATC of the abnormal indication and intention to divert
• Did not declare an emergency, as engine function remained stable
Clear communication with passengers was maintained, and the cabin crew prepared the cabin for precautionary landing procedures. No emergency PA or rapid descent was necessary.
The aircraft remained fully controllable, and the descent profile was standard.
Aircraft Systems & Technical Analysis
The CFM56-7B26 engine fitted to the 737-800 includes dual oil quantity sensors and associated alerts via the EICAS (Engine Indicating and Crew Alerting System). A low oil quantity alert typically precedes:
• Oil reservoir depletion
• Internal leakage
• Oil tank sensor failure
• Seal or gasket malfunction
• Improper oil servicing
Oil quantity loss does not always lead to immediate engine shutdown but may progress rapidly if associated with oil pressure loss or overheating. In this case:
• The alert was isolated to the oil quantity reading
• Engine parameters remained normal
• No EGT rise or fuel flow anomalies were observed
• The crew opted for early diversion before pressure dropped
Post-landing, maintenance teams began detailed inspection involving:
• Oil quantity measurement and top-up check
• Oil filter examination for metal particles
• Borescope inspection of oil pump and bearings
• Review of maintenance logs for recent engine servicing or deferred defects
The aircraft has remained grounded for more than 27 hours, indicating further investigation or parts replacement may be required.
Passenger Experience & Cabin Conditions
The diversion was managed calmly, with no in-flight disturbance or discomfort. Passengers were informed of a technical issue requiring precautionary diversion and were rebooked onto alternate flights from Copenhagen.
Cabin conditions remained stable, with no depressurisation, vibration, or indications of power loss. The aircraft landed without incident, and passengers disembarked normally at the gate.
Norwegian Air Sweden issued a statement confirming the technical issue and praised the flight crew for their cautious and professional response.
Emergency Response & Aftermath
Copenhagen Airport’s emergency services were notified in advance but were not required to attend the aircraft directly on landing. The aircraft did not require towing and shut down normally at the stand.
The flight was cancelled, and maintenance teams began inspection. As of 8 April 2025, the aircraft (SE-RRO) remains on the ground pending further technical assessment.
Passengers continued their journeys via rebooked connections or overnight accommodations where necessary.
Investigation Status
This event is being treated as an operational incident and not a reportable accident or serious incident under EASA or ICAO criteria.
Norwegian’s engineering team is conducting:
• Oil system integrity checks
• Component teardown if internal leakage is suspected
• Data analysis of the aircraft’s ACMS (Aircraft Condition Monitoring System) and CMC (Central Maintenance Computer)
The Swedish and Danish aviation authorities may request a summary report, especially if any systemic issues are found with the oil system or maintenance procedures.
Root Cause & Contributing Factors
Probable Root Cause:
Low oil quantity indication on the right engine, likely due to actual oil depletion, system leak, or faulty sensor.
Contributing Factors:
• Possibly recent engine service activity
• Undetected leak during pre-flight inspection
• Sensor drift or miscalibration
• Ambient conditions or vibration affecting sensor performance
The crew’s decision to divert avoided potential escalation into a pressure loss or in-flight engine shutdown scenario.
Safety Recommendations & Industry Impact
Though this was a contained incident, it reinforces key safety principles:
• The importance of continuous engine parameter monitoring during cruise
• Early decision-making in response to deteriorating but non-critical alerts
• The reliability limits of quantity sensors and the importance of confirming through pressure trends
• The value of conservative diversion policies, particularly over maritime or sparsely populated areas
Norwegian Air Sweden may review its oil servicing records for SE-RRO and consider fleet-wide checks for other aircraft fitted with similar sensor types or oil system configurations.
Conclusion
Flight D8-5351’s diversion due to a low oil quantity indication represents a textbook response to a potentially serious system alert. The absence of engine performance degradation or cabin disruption highlights the benefits of pre-emptive diversion and crew vigilance.
Although the engine continued functioning, the correct decision was made to avoid any risk of in-flight shutdown or damage escalation.
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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