Occurrence
During pre-flight preparation on June 5, 2018, at EPWA, the flight crew of a Boeing 737-800 detected an abnormal position of slot No. 3 and a 'Leading Edge flaps transit' indication. Hydraulic pump activation did not clear the fault. Maintenance diagnosed a sensor issue on slot No. 3 and replaced it, causing a two-hour delay. The aircraft was then cleared for service.
On approach for landing, after the approach briefing, the crew decided to land with flaps 40. When selecting flaps 25, the 'Leading Edge flaps transit' warning reappeared, accompanied by unnatural speed indications and low-speed buffets. Due to the slot failure warning, the crew changed the flap selection to 30 degrees, but the problem persisted. There was also a flap overspeed of about two knots for six seconds with flaps at 30 degrees. The crew then moved the flap lever to 25 degrees and maintained current speed to avoid a stall warning (low buffet speed increased significantly). At 744 feet, they initiated a go-around. During holding, they performed the 'Non-normal checklist: Leading Edge flaps transit' and configured the aircraft for landing with flaps 15, per the checklist. The landing was uneventful. After landing, the fault was logged, and maintenance performed tests that passed, clearing the aircraft for further service.
On a subsequent flight sector, the crew prepared for an approach with flaps 30 and a contingency plan for flaps 15 if the problem recurred. During approach, after selecting flaps 10, the 'Leading Edge flaps transit' warning returned. Following their plan, the crew configured for flaps 15 landing per the non-normal checklist. The landing was conducted with flaps 15 in turbulent conditions. In the final approach phase, the aircraft deviated above the glide path (about 1.5 dots). After landing and taxiing to the stand, the captain logged the fault. Maintenance performed operational tests and identified a faulty 'Leading Edge Flaps' block, which was replaced. The aircraft was then returned to service.
Cause
The cause of the incident was a failure of the 'LEADING EDGE FLAPS' block.