B777-200
Flight Crew Air Transport Pilot (ATP)
Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Weight And Balance
General None Reported / Taken
Equipment / Tooling
I pulled the flight plan including the takeoff performance in operations (OPS). The flight plan contained a dispatch fuel of 204.7; which included more than 20;000 pounds of ferry fuel; this fuel load gave us a Planned Takeoff Weight (PTOW) of 597.9. The takeoff performance was planned for a takeoff temperature of 34 degrees C. At 34 degrees C; the climb limit weight was 601.1 and the runway limit weight for 18L was 622.2. At 36 degrees C the climb limit weight was 592.5 and the runway limit weight for 18L was 613.1.The temperature at the time the takeoff performance was pulled was 37 C. Since I saw that the ferry fuel would put us overweight (based on climb limit weight) if the temperature at takeoff was above 34 C (and we were 3 degrees warmer than that an hour and 15 minutes prior to departure); I asked if the ferry fuel could be reduced in order to avoid having a delay to wait for either the temperature to drop or to burn off the excess fuel. Dispatch was hesitant but said they would try to reach the fueler if the aircraft had not already been fueled. We arrived at the aircraft 15 minutes later to find the fuel load had not been reduced but was actually slightly higher than planned.With the OAT reported on the current ATIS as 36 C (which by our printed takeoff performance would have put us overweight by 5;000 pounds based on the takeoff performance climb limit weight); I asked the FO to print a takeoff performance via the ACARS for a temp of 35 C (we had a takeoff performance for 34 and 36 degrees; I wanted one for 35 and 37) so I could have a valid takeoff performance for single degree changes.When the ACARS printed the takeoff performance for 35 degrees; there was a significant change in both the climb limit and runway limit weights. Comparing the two takeoff performance printouts (one printed in operations; the other via ACARS) this is what I found (all weights are for flaps 5):OPS takeoff performance at 34 C - climb limit 601.1 Runway 18L limit 622.2ACARS at 35 C --- climb limit 631.2 Runway 18L limit 649.2OPS takeoff performance at 36 C - climb limit 592.5 Runway 18L limit 613.1ACARS at 37 C --- climb limit 621.5 Runway 18L limit 641.5In addition; the OPS takeoff performance for 34 C required max power for takeoff; while the ACARS takeoff performance for 35 C allowed a reduced power takeoff with a 40 C assumed Temperature.We pulled the ACARS takeoff performance 22 minutes before scheduled departure and I made my first phone call to Load Control 3 minutes later. I was put on hold and after 5 minutes on hold; I hung up and called Dispatch; I was transferred back to Load Control and was once again put on hold until a supervisor came on line. The supervisor did not understand what I was trying to convey and put me on hold again while he tried to figure out how to conference Dispatch in on the conversation. After at least 20 more minutes; the Load Supervisor came back on the line and told me he could not make the new phone system work. By this time (now at least 15 minutes after departure time); the OAT had dropped to 34 C - so I had the FO pull an ACARS takeoff performance for 34 C.That new ACARS takeoff performance for 34 C had a climb limit of 600.1 (1;000 pounds less than the OPS takeoff performance for the same temp pulled an hour and a half earlier; but at least close....)With two takeoff performance printouts that were now roughly the same; and with the OAT now low enough to be legal with the more conservative original takeoff performance; I told the Load Supervisor to abandon his unsuccessful attempts to conference Dispatch in on the call and we departed.The Load Supervisor was unable to explain how a takeoff performance generated using a starting temp of 35 degrees would calculate limit weights 30;000 heavier than a takeoff performance generated using a starting temp of 34 degrees.I have no idea which takeoff performance was accurate - but clearly one of them was inerror and did not depart until the OAT and weights were within the limits of the more conservative takeoff performance.I am very concerned that there may be an error in the computer program generating the takeoff performance and hope that this ASAP will result in an investigation to find and correct that error.I have retained the hard copy of all documents referenced in this report and [can] provide them upon request.
B777 Captain reported noticing anomalies in the allowable operating weights generated by his performance computer.