Narrative:

Operating aircraft X to ZZZ; ATC informed us that our filed altitude would result in a significant delay and asked if we could accept FL360 or FL380. We requested either altitude; and ATC assigned us FL380. The ca (captain) instructed me to accept it in order to avoid turbulence reported at lower altitudes; even though the international relief officer (international relief officer) and I both mentioned it was close to our maximum altitude. After takeoff the international relief officer suggested we request FL370; but due to heavy workload and ATC radio traffic we were unable to attempt the request. Soon after the international relief officer departed the cockpit for his break; ATC assigned us to cross 10 southeast of zzzzz at FL380 and report level. The ca (PF) (pilot flying) selected vs in order to meet the restriction; which caused our airspeed to begin decreasing. It continued to decrease at which point I called out airspeed to bring his attention to the situation and he reduced the vs but not enough to reverse the trend and the airspeed continued to decrease to just above the yellow arc prior to level off. I again stated our airspeed is too low and he again made a small correction but it still did not reverse the trend. We leveled off at FL380; and I reported level; and was immediately handed off. As I began accomplishing those tasks; I was distracted from the aircraft state and by the time I scanned the instruments again we had slowed well into the yellow arc in level flight and continued to decelerate. I stated you need to reduce the pitch and the ca selected a slight descent to rectify the situation as I set the altitude a couple hundred feet lower; but the aircraft continued to slow to just above the red arc. The ca disengaged the autopilot; added full power; reduced the pitch further; and turned off the tracks. I selected FL350; [notified] ATC; requested an offset and a lower altitude. The aircraft began to accelerate as we descended to FL350. After the aircraft accelerated to normal speed; we [let ATC know things were rectified] and continued to ZZZ without further incident.

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Original NASA ASRS Text

Title: B777 First Officer reported an airspeed decay in cruise due to excessive weight for the selected flight level.

Narrative: Operating Aircraft X to ZZZ; ATC informed us that our filed altitude would result in a significant delay and asked if we could accept FL360 or FL380. We requested either altitude; and ATC assigned us FL380. The CA (Captain) instructed me to accept it in order to avoid turbulence reported at lower altitudes; even though the IRO (International Relief Officer) and I both mentioned it was close to our MAX Altitude. After takeoff the IRO suggested we request FL370; but due to heavy workload and ATC radio traffic we were unable to attempt the request. Soon after the IRO departed the cockpit for his break; ATC assigned us to cross 10 SE of ZZZZZ at FL380 and report level. The CA (PF) (Pilot Flying) selected VS in order to meet the restriction; which caused our airspeed to begin decreasing. It continued to decrease at which point I called out airspeed to bring his attention to the situation and he reduced the VS but not enough to reverse the trend and the airspeed continued to decrease to just above the yellow arc prior to level off. I again stated our airspeed is too low and he again made a small correction but it still did not reverse the trend. We leveled off at FL380; and I reported level; and was immediately handed off. As I began accomplishing those tasks; I was distracted from the aircraft state and by the time I scanned the instruments again we had slowed well into the yellow arc in level flight and continued to decelerate. I stated you need to reduce the pitch and the CA selected a slight descent to rectify the situation as I set the altitude a couple hundred feet lower; but the aircraft continued to slow to just above the red arc. The CA disengaged the autopilot; added full power; reduced the pitch further; and turned off the tracks. I selected FL350; [notified] ATC; requested an offset and a lower altitude. The aircraft began to accelerate as we descended to FL350. After the aircraft accelerated to normal speed; we [let ATC know things were rectified] and continued to ZZZ without further incident.

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.