mensaboy, well said.
If I may add......
The fate of this flight was sealed the moment the laptop was put away.
Many people do not understand 4 engine take-off performance and that is why they reason "could the pilot not feel there was a problem". On day 1 of my A340 conversion, I was told "You are going to see a lot more of the departure end of the runway" and this is a fact. A correctly calculated takeof performance will optimally utilise every inch of runway and is very often TORA limited rather than the 2nd segment climb limits that usually dog the twins. In the MEL case the guys would have run most of the runway and then close to the end would have rotated at the calculated Vr(albeit an incorrect one).......why would the pilot have thought anything was amiss until the airplane did not lift off.
In terms of knowing the numbers....well, I don't think I have done a 350 ton take off in a 340 since my CCQ and I have been here a while(I have seen it done many time from the blind seat in the rear right corner). This is an academic point because do you really want to rely on the pilot operating on the wrong side of the clock to sense an error, or would you rather see more robust SOP's in place.
I also don't believe that these guys just ignored SOP.....I have never flown with a captain that did not do the performance check. The question should rather be did they follow the
correct procedure(training) and if so, how did the error slip through. I personally have had to sit down and have a good review of the procedure. I don't know why the FCOM3 brings the qrh into the greeddot check. I think we should simply have a dispatch calculated greendot next to the Estimated TOW(on the LIDO flightplan)and next to this we write the laptop GD and the MCDU GD. If these 3 are close together you cannot have a gross error in TOW.
I might also add that the pre-departure workflow is total chaos. This has been identified as a problem but not addressed. It is like a ping pong ball in a jacuzzi.....all over the place. It is highly probable that the performance check was interrupted by some obscure requirement like security demanding their checklist be signed.......
Anyway, just my tuppence worth.......