B777-200
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface; Confusion; Communication Breakdown; Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter Unstabilized Approach
Flight Crew Executed Go Around / Missed Approach; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors
The autothrottle became disconnected on final several miles from runway but was not recognized till short final. I noticed IAS at REF (142 knots) at 200 feet AGL. This caught my attention so I asked the pilot flying (PF) to add power and realized that the autothrottles were not driving the power (the first Relief Pilot commented at the same time). The PF did not respond immediately (I believe confused as to why the autothrottle was not responding). The IAS then decreased to approximately five knots below REF. I again requested more power and brought the First Officer's attention to the low speed condition more urgently while I began pushing the throttles up myself. At this point the PF reacted with more thrust and the speed immediately jumped to REF +5 but then continued to accelerate beyond REF +10. I felt the approach to be unstable and called for a missed approach at 50-100 AGL. The go-around was completed with no contact with the runway. However; during the missed approach I was distracted with a traffic call from Tower and subsequently lost focus on the procedure as we climbed out. We returned and landed uneventfully in a second approach. At the gate following engine shut down I was informed by the Relief Pilots that we may have oversped the flaps on the climbout after the missed. I entered that into the logbook and debriefed the events during a closed door session with all cockpit crewmembers. More attention to cockpit automation was necessary. I.e. when; how; and why did the autothrottles disconnect? During the missed approach I became distracted by external events and was not able to properly monitor the missed approach procedures. More attention to aircraft configuration and missed approach procedures on my part could have prevented the flap overspeed.
The reporter advised that a later review of the incident revealed that the PF had disconnected the autothrottles at the same time as the autopilot with the intention of hand flying the approach.  By utilizing a 'double click' of the autothrottle disconnect button the autothrottle off aural warning is prevented from sounding assuming the act was intentional; as it was in this case.  The oversight was attributed to fatigue following a more than 12 hour flight.
As they approached 200 AGL on final approach the Captain (pilot not flying) of a B777-200 noticed airspeed was at reference; decreasing; and the autothrottles had somehow been disengaged.  An abrupt power increase resulted in excessive acceleration and the landing was aborted prior to touchdown and a probably flap overspeed was noticed by a Relief Pilot.  A subsequent approach was uneventful. No explanation for the inactive autothrottles was provided.