What happened
On January 31, 2015, a Boeing 737-800 was in the process of configuring for landing when the aircraft experienced a flap blockage in position 1. Upon encountering this discrepancy, the crew requested permission from Air Traffic Control to enter a holding pattern. During this period, the crew performed the required "TRAILING EDGE FLAP DISAGREE" checklist.
Following the checklist instructions, the crew proceeded to land with the flaps set to position 15, utilizing the alternative flap extension method. Following the landing, the crew observed significant icing on the aircraft. After de-icing procedures were completed, a functional test of the flap extension and retraction system was performed, which yielded successful results.
The investigation
The investigation focused on identifying the source of the flap disagreement and verifying the integrity of the flight control systems. Subsequent to the incident, a test of the Flaps/Slats Electronic Unit (FSEU) was conducted during maintenance, which showed no system errors. Maintenance personnel also cleaned the connectors within the flap extension and retraction system.
Upon the aircraft's return to its base, the FSEU was replaced in accordance with the Aircraft Maintenance Manual (AMM 27-51-01/401). Following this component replacement, system tests were completed successfully, and the aircraft was cleared for unrestricted service. The malfunction did not recur during subsequent operations.