What happened
On November 23, 2019, a Boeing 737-800 was climbing through 13,968 ft after departing from Antalya (LTAI) when the crew received a 'MASTER CAUTION' alert indicating the fuel filter bypass for engine number 1 had activated. The flight crew implemented the 'FILTER BYPASS' emergency procedures and continued the flight.
As the aircraft climbed to 21,959 ft, the crew reduced engine thrust, which subsequently decreased the fuel flow through the filter and caused the warning to clear. However, after receiving clearance to continue the climb to the assigned cruise altitude of FL360 and subsequently increasing engine thrust, the 'FUEL FILTER BYASS' warning re-activated. The warning disappeared again once the crew reduced thrust during the descent, and no further alerts occurred during the remainder of the flight.
The investigation
The investigation, conducted by the operator, focused on the intermittent nature of the warning in relation to fuel flow changes. Upon landing at Katowice–Pyrzowice, maintenance personnel inspected the fuel filter element. The inspection revealed no contamination or debris within the filter element itself. Following the aircraft's maintenance manual, the technical crew replaced the fuel differential pressure switch and performed all necessary operational tests. The aircraft was subsequently cleared for return to service.
Findings
- The investigation established that the malfunctioning fuel differential pressure switch was the primary cause of the incident, as the component triggered the bypass indication specifically when fuel flow increased.