What happened
On September 9, 2017, a Royal Air Maroc Boeing 737-800, registration CN-RNW, was operating a commercial flight from Lisbon, Portugal, to Casablanca, Morocco. During the climb phase, while passing through FL140, the crew received an "OIL FILTER BYPASS" alert for the number one engine. Although all other engine parameters remained within normal limits, the warning persisted despite the crew attempting to stabilize the engine by adjusting the thrust levers to a "Safe Power" setting.
To prevent potential engine degradation, the crew performed a controlled shutdown of the number one engine. The flight continued as a single-engine operation at a reduced cruise altitude of FL180. The aircraft eventually landed safely at Casablanca/Mohammed V Airport without further incident.
The investigation
The Moroccan BEAM investigation examined flight data recorder (FDR) and cockpit voice recorder (CVR) information, crew interviews, and physical testing of the engine components. Investigators analyzed the lubrication system, including the oil pump, filters, and the bypass valve mechanism. Tests were also conducted on the harness responsible for transmitting the alert signal.
Technical analysis of the filter revealed that the bypass alert was legitimate and triggered by a clog in the oil filter. The investigation focused on identifying the source of the contaminant within the lubrication circuit. Maintenance records were reviewed, and no discrepancies were found in the servicing of the engine's lubrication system.
Findings
- The number one engine parameters were normal at takeoff.
- The "OIL FILTER BYPASS" alert activated at approximately five minutes and 33 seconds after takeoff.
- The crew correctly applied the appropriate checklists for an engine shutdown following a filter clog.
- Excessive presence of silicone in the oil filter caused the clogging of the filter.
- The source of the silicone contamination was not definitively identified, though possibilities included external infiltration (such as sand), internal degradation of a filter component, or degradation of a lubrication system component.