B747-400 DEPARTS YSSY WITH ERRONEOUS TKOF DATA. ACARS MESSAGE ALERT RECEIVED DURING LATE STAGES OF TKOF ROLL.

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DURING PREPARATION FOR A NORMAL DEP FROM SYD THE CREW OBTAINED THE USUAL 'PLANNED TKOF DATA' TO INITIALIZE THE WTS IN THE FMC. A DEP DELAY WAS INCURRED WHILE AT THE GATE DUE TO LATE CARGO/BAG LOADING. SHORTLY BEFORE THE LAST CARGO DOOR WAS CLOSED THE CREW RECEIVED OVER THE PRINTER THE 'FINAL' WTS FROM WHICH THE NEW DATA WAS LOADED INTO THE FMC WITH THE APPROPRIATE ADJUSTMENTS MADE TO SPDS; ETC. THE ACFT WAS TAXIED OUT FOR NORMAL TKOF AND RECEIVED TKOF CLRNC. DURING THE TKOF ROLL; AN ACARS MESSAGE DISPLAYED ON THE EICAS. IT WAS NOTED BUT PROMPTLY IGNORED DURING THIS CRITICAL PHASE OF FLT. (AN ACARS MESSAGE WAS NOT A TRIGGER IN MY MIND TO INITIATE A HIGH SPD ABORT. ACARS MESSAGES REQUESTING DELAY RPTS ARE TYPICALLY RECEIVED WITHIN THIS SAME TIME PERIOD.) AFTER SAFELY ESTABLISHED IN THE CLB THE RELIEF PLT REVIEWED THE CONTENTS OF THE ACARS MESSAGE; WHICH INDICATED THAT THE FINAL WTS; PREVIOUSLY SENT TO THE COCKPIT PRINTER; WERE NOW INVALID. THE ACARS MESSAGE READ AS FOLLOWS: 'INVALID WTS. DUE TO CARGO LOAD CHANGE; THE STATION WAS REVISED THEIR FINAL LOAD PLAN. YOUR CURRENT WTS ARE NOW INVALID. PLEASE CONFIRM RECEIPT OF THIS MESSAGE.' SUBSEQUENT 'FINAL' WTS WERE SENT TO THE PRINTER AND WERE FOUND TO BE THE SAME AS THE ORIGINAL. FORTUNATELY THIS FLT WAS WELL BELOW MAX TKOF GROSS WT AND SINCE THERE WAS NO CHANGE IN WT/BAL; THE INCIDENT WAS A NON-EVENT. THE LATE TIMING AND THE CONTENT OF THIS ACARS MESSAGE ARE OF CONCERN AND WAS THE TOPIC OF CREW DISCUSSION DURING THIS FLT. FOR EXAMPLE; THIS FLT IS TYPICALLY PLANNED AT MAX TKOF GROSS WT FOR DEP FROM SYD TO ZZZ. IF THIS MESSAGE WAS TO ALERT US TO A ZERO FUEL WT INCREASE; WE WOULD NOT HAVE LEARNED ABOUT THE OVERWT AND ILLEGAL TKOF UNTIL AFTER THE FACT HAD OCCURRED. WORSE YET; WHAT IF THE CARGO WAS LOADED IMPROPERLY AND THE CTR OF GRAVITY WAS OUT OF LIMITS. EITHER OF THESE COULD HAVE CAUSED CATASTROPHIC EVENTS. ON THE OTHER HAND; INITIATING A HIGH SPD ABORT AT A HIGH GROSS WT FOR A MESSAGE OF WHICH THE CONTENTS ARE NOT KNOWN WOULD ALSO SEEM INAPPROPRIATE. ONCE THE CREW RECEIVES THE 'FINAL' WTS IT IS ASSUMED THAT ALL CALCULATIONS HAVE BEEN MADE AND VERIFIED AND THAT THE ACFT IS SAFE AND LEGAL BEFORE ATTEMPTING A TKOF. IF THERE IS AN OPPORTUNITY TO RESCIND THOSE CALCULATIONS BECAUSE THEY WERE MADE IN ERROR AND THE SAFETY OF FLT MAY BE AT RISK; THERE SHOULD BE A MORE URGENT WAY OF NOTIFYING THE CREW BEFORE THE TKOF IS INITIATED.