Date: 201509
Local time of day: 1201-1800
Location: KZAK.ARTCC, HI
Flight conditions: VMC
Light: Daylight
Aircraft: B747 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A KZAK
ATC / advisory: Center KZAK
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Situational Awareness
Second reporter function: Pilot Not Flying; Captain
Anomaly: Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Deviation / Discrepancy - Procedural Weight And Balance
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Human Factors

Narrative: Climbed aircraft from FL350 to FL370 using VNAV. Aircraft leveled off; .85M; shortly afterwards airspeed started to decrease; throttles advanced by auto-throttle; airspeed kept decreasing; manually fire walled throttles. At max EPR; airspeed kept decreasing; as airspeed dropped below min maneuver speed and continued to decrease; I disconnected autopilot and lowered the nose slightly below the horizon. Airspeed stabilized at max thrust and began to increase as we descended. Returned to FL370 at .85M. With autopilot on; aircraft still could not hold airspeed at max EPR. Requested a descent to FL350. Descended and had no problem at FL350.   At the time the aircraft weight was 635.8 VNAV max alt 389; opt alt 358. Flt plan had us going up earlier but delayed an hour due to better winds at 350. Flight conditions were smooth; max deviation from assigned FL370 was 200 feet low; airspeed went 20 knots below min maneuver speed before recovering. At FL350 .85M EPR required to maintain airspeed appeared to be higher than normal for those conditions. 3 flight attendants reported minor injuries. They were in the middle of a crew change when the event occurred; moving about in the crew rest area.Suspect the aircraft was much heavier than the weights given us at departure.


Second narrative: I was the Captain on this flight and had just briefed the relief pilots before retiring to the bunk room for my scheduled rest. Shortly after starting my rest and in the bunk room I heard the engines spool up rapidly and the aircraft seemed to experience a series of mountain wave like pitch excursions. This seemed to be very short lived and I estimate that it went on for about 30 seconds. I assumed that it was associated with turbulence and went back to sleep. Later I found out that upon climbing from FL350 to FL370 on schedule with the flight plan that airspeed dropped and required full power which was insufficient to restore proper airspeed. The flying pilots then requested and received clearance to descend back to FL350. In retrospect we recalled a longer takeoff roll; sluggish climb performance; increase fuel burn; as well as the above event that lead us to suspect that the aircraft weight may have been higher than the flight plan had been constructed. The crew asked that the cargo be weighed on arrival.


Synopsis: B747 flight crew reported an inability to maintain M.85 after climbing to FL370 even though the FMC and flight plan indicated it as acceptable. The crew returned to FL350 and suspected the aircraft may have been substantially heavier than advertised.