Narrative:

After takeoff we attempted to navigate direct to a fix. The flight directors biased out of view and we got momentary airspeed flags. The first officer noticed that all the performance data had dropped from the fmcs. The flight plan was still there; but the activate prompt was showing. The first officer reloaded the perf data; activated and executed the fp and we reengaged the fds. I turned on the autopilot. We attempted to navigate direct again and after a few minutes of normal operation; once again the perf data dumped and the autopilot disconnected. As there were three pilots in the cockpit; I assigned the international relief officer to help the first officer get the data reloaded while I hand flew and worked the radios. We continued climbing and continuing on course as I was confident that we would shortly solve the problem. Once we reloaded; I engaged the B autopilot to see if the problem followed the autopilot. After about two or three minutes the problem came again. At this time; I directed the international relief officer to establish a phone patch with dispatch and maintenance as we had already tried using ACARS and I knew that typing back and forth would be too time consuming. We initially had some trouble getting through but finally got through to the company. We explained the problem and waited to see if they had any ideas. While we waited; we got an right FMC fail message and the first officer lost his presentation. For this we have a checklist and I called for the QRH. We followed the procedure and recovered the fos presentation using the altitude switch; slaving his side to mine. I thought that perhaps this would fix the problem as we not had a hard fault previously. We advised the company to stand by while we reloaded to see if that had recovered VNAV and LNAV. Meanwhile; I asked the international relief officer to look to see if we were legal to go ETOPS on one FMC. By this time; the first officer was flying the airplane and working the radios while the international relief officer and I worked the problem and communicated to company. We reloaded the perf data and reengaged the VNAV and LNAV and autopilot. It took the data and held us on course. We were able to get fuel and ETA predictions now and I thought that we had solved the problem. To make sure; I directed the first officer to build a waypoint on our present course and go direct to it to see if that had an effect. It did; the system dumped again and the autopilot kicked off. We had advised ATC some time back that we were having a problem and talking to company about a possible return to [departure airport]. At this point; all of us were out of ideas. Maintenance; dispatch; and all three pilots agreed that we should not go oceanic with this problem and that we should return for an overweight landing.we advised ATC that our navigational capability was degraded; that we would be making an overweight landing; that we were declaring an emergency; wanted the equipment standing by for our landing; and required vectors to a landing on the longest runway. We ran the QRH for overweight landing. We estimated that we would be about 225;000 lbs for landing. We requested and flew at FL220 to burn more fuel on the way; but we would still be about 20;000 lbs overweight. With the first officer calling out radio altimeter readings to touchdown and the international relief officer calling sink rates; we landed normally with the fire equipment standing by in case we blew a tire or something. They followed us to the gate where we shut down (after waiting about 5 minutes for someone to show up to park us--embarrassing).

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Original NASA ASRS Text

Title: B757-200 Captain reported returning to the departure airport for maintenance attention after encountering autoflight difficulties early in a transoceanic international flight.

Narrative: After takeoff we attempted to navigate direct to a fix. The flight directors biased out of view and we got momentary airspeed flags. The FO noticed that all the performance data had dropped from the FMCs. The flight plan was still there; but the ACTIVATE prompt was showing. The FO reloaded the perf data; activated and executed the FP and we reengaged the FDs. I turned on the autopilot. We attempted to navigate direct again and after a few minutes of normal operation; once again the perf data dumped and the autopilot disconnected. As there were three pilots in the cockpit; I assigned the IRO to help the FO get the data reloaded while I hand flew and worked the radios. We continued climbing and continuing on course as I was confident that we would shortly solve the problem. Once we reloaded; I engaged the B autopilot to see if the problem followed the autopilot. After about two or three minutes the problem came again. At this time; I directed the IRO to establish a phone patch with dispatch and maintenance as we had already tried using ACARS and I knew that typing back and forth would be too time consuming. We initially had some trouble getting through but finally got through to the company. We explained the problem and waited to see if they had any ideas. While we waited; we got an R FMC fail message and the FO lost his presentation. For this we have a checklist and I called for the QRH. We followed the procedure and recovered the FOs presentation using the ALT switch; slaving his side to mine. I thought that perhaps this would fix the problem as we not had a hard fault previously. We advised the company to stand by while we reloaded to see if that had recovered VNAV and LNAV. Meanwhile; I asked the IRO to look to see if we were legal to go ETOPS on one FMC. By this time; the FO was flying the airplane and working the radios while the IRO and I worked the problem and communicated to company. We reloaded the perf data and reengaged the VNAV and LNAV and autopilot. It took the data and held us on course. We were able to get fuel and ETA predictions now and I thought that we had solved the problem. To make sure; I directed the FO to build a waypoint on our present course and go direct to it to see if that had an effect. It did; the system dumped again and the autopilot kicked off. We had advised ATC some time back that we were having a problem and talking to company about a possible return to [departure airport]. At this point; all of us were out of ideas. Maintenance; Dispatch; and all three pilots agreed that we should not go oceanic with this problem and that we should return for an overweight landing.We advised ATC that our navigational capability was degraded; that we would be making an overweight landing; that we were declaring an emergency; wanted the equipment standing by for our landing; and required vectors to a landing on the longest runway. We ran the QRH for Overweight Landing. We estimated that we would be about 225;000 lbs for landing. We requested and flew at FL220 to burn more fuel on the way; but we would still be about 20;000 lbs overweight. With the FO calling out radio altimeter readings to touchdown and the IRO calling sink rates; we landed normally with the fire equipment standing by in case we blew a tire or something. They followed us to the gate where we shut down (after waiting about 5 minutes for someone to show up to park us--embarrassing).

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.