Incident Overview
On 23 November 2019, a Boeing 737-800 departed Antalya Airport (LTAI) and during climb at 13,968 feet, the master caution activated accompanied by a fuel filter bypass indication for engine 1. The flight crew executed the filter bypass procedure and continued the flight.
Sequence of Events
At 21,959 feet, after reducing engine thrust, the filter bypass warning extinguished. Following clearance to continue climbing to cruising altitude, the crew increased thrust, which caused the filter bypass warning to illuminate again. Before reaching flight level 360, the crew reduced thrust and the warning turned off, remaining off for the remainder of the flight.
Post-Flight Maintenance
After landing at Katowice-Pyrzowice Airport, maintenance personnel replaced the fuel filter element. As no contamination was found on the removed filter, and following the aircraft maintenance manual, the differential pressure switch was replaced. Necessary tests were conducted, and the aircraft was returned to service.
Investigation Findings
The probable cause of the incident was a malfunctioning differential pressure switch that triggered the fuel filter bypass indication when fuel flow increased.