Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 200; Flight Crew Total 24150; Flight Crew Type 8200
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Fuel Issue
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
Climb out of normal. Cleared to climb to FL340. Oceanic clearance received was for NAT B crossing at FL330. At 110 mi. E of DOGAL; EINN cleared us to FL330. MCP altitude was set and I pushed the Altitude knob to initiate descent. The VNAV cruise page was set for FL330; with a hard tuned speed of M.83 (the assigned speed in the oceanic clearance.) The legs page was also programmed for M.83/FL330 at DOGAL fix. The plane descended to FL330 normally; but we did not notice that at some point the aircraft had slowed to 250 kts. As we crossed DOGAL; we were busy accomplishing the Class II fix procedures; as well as receiving and accomplishing radio communications switch to HF. At the same time; an 'Insufficient Fuel' message displayed in the CDU. It took a moment or two to investigate the reason for the fuel message. We noticed that the Prog Page showed a landing fuel of 3.6 and the landing time had increased about 20-25 minutes over what it initially was. We eventually found that the VNAV cruise page had completely blanked; and all the entered information was now blank. At this time we noticed that the plane had slowed in cruise to 250 kts. (about 40 kt decrease from what it was.) It is unknown when; or how long the 250 kt speed occurred. After reprogramming the VNAV cruise page; the plane accelerated back to M.83; and the Prog Page now showed normal arrival fuel and time. I believe we were not at 250 kts for more than 5 or possibly 10 minutes; but we did apparently cross DOGAL below the assigned M.83 speed. We do not know how or why the VNAV cruise page deleted its entered information.
B777 Captain reported his speed dropped to 250 KTS in cruise flight at FL330 when the FMC dumped the cruise speed settings.