Flight crew reported trailing edge flap problem resulted in fuel dumping and a precautionary landing at departure airport.
Synopsis
Flight crew reported trailing edge flap problem resulted in fuel dumping and a precautionary landing at departure airport.
Narrative
After take-off; we received a level 2 'FLAP DISAG' alert approximately 20 seconds after the flap handle was moved to the '0/EXT' detent. There was no accompanying rolling motion and the configuration page showed the flaps symmetrically extended at 17 degrees. In accordance with the QRH; we matched the flap handle with the indicated flap position as the flaps continued to retract uncommanded with normal hydraulic systems. Eventually the flaps returned to the 0 degree position uncommanded and we matched that position with the flap handle and we left the slats extended while we coordinated with ATC; Dispatcher; and Maintenance. After the decision was made to return to ZZZ; we coordinated with ATC for a fuel dump location and altitude. We were cleared to dump while established in holding at 230 knots southeast of the ZZZ1 120 Radial; 20 NM Legs; Right hand turns at 10;000 ft. We commenced the dump at XA:13 and terminated at XA:27 for a total dump of 80;000 pounds. In preparation for landing; while still established in holding; we attempted to extend the flaps to 'Flaps 15' and the flaps did not move. We also attempted to extend the flaps to 'Flaps 28' and again nothing happened. We returned the flap handle to the '0/Ext' detent and decided to coordinate with ATC for an additional 35;000 pounds of fuel to dump due to our impending 'NO FLAP/SLAT EXTENDED LANDING' procedure. The second dump was initiated at XA:40 and terminated at XA:45. After coordinating for Runway XXC and [requesting priority handling]; ATC advised us that the glide slope for Runway XXC was out of service and we conducted a visual / 'prof to MDA' approach. Landing Data summary showed a MAX VERT G of 1.30 and a MIN VERT G of 0.77. After clearing the runway we waited for approximately 15 minutes for the brakes to cool down from a peak temperature of 472 before taxiing to parking. Over that period with the flap/slat handle in the 'FLAP UP/SLAT RET' position; the flaps began an uncommanded extension with normal hydraulics. When we reached parking; the flaps were indicating 10 degrees extension with the slats retracted. This position was visually verified from outside the aircraft.In accordance with Aircraft Maintenance History; the faulty component was a 'flap control valve'.
Second reporter narrative
On departure; flaps would not retract as commanded. After approximately 20 minutes; flaps retracted to 0 degrees despite commanded inputs. We held. We had an excess of 55;000 pounds of fuel which allowed us to extensively use the CRM/HF process to reach the best decision possible. Captain Name's knowledge gathering process was exemplary. We spent over an hour exploring all options inside and outside the cockpit to build a lock tight shared mental model. After thorough consideration; Captain Name announced his decision demonstrating sound leadership and safety consciousness. Proceeding with the plan; we dumped 80;000 pounds to get to max landing weight. Location was ZZZ1 120 radial 20 NM legs at 10;000 ft. We slowed to configure and could not extend the flaps. We then elected for a 0 flap / Slats extended landing. We elected to dump another 35;000 pounds in order to reduce speed and landing distance upon landing. We flew an extended approach to XXC using Heads-up Display guidance; executed by the Captain. Landing was uneventful and [priority] terminated after pausing taxi to ensure brake energy and temperatures were safe.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.