Simple, Boeing says NO,not allowed.I guess its because you will use performance restrictions to calculate your vspeeds and you dont have ass temp tables provided for that condition.
The question still is why. I get the theory behind no ass. temp. with contaminated runway or anti skid inop. Here you have a restriction due to a longer ASD, but I still don't get the EEC in alternate.
You dont take off using soft alternate mode,you will be using the Hard Alt mode following your DDG.
N1 may increase during roll out but yet again if you follow your DDG, a weight penalty will occur,reducing your N1 for the weight,keeping it below your max thrust rating N1 limit,even with 7 perc roll increase.(26k for actual conditions)
Of course you're right regarding hard alternate. I was a bit quick when writing there. Thanks for the workthrough!
Can the reason for the restriction be because of the lower thrust setting in the DDG? If you factor in a roll increase to maintain N1 within engine limits you couldn't count on those extra N1 figures for performance reasons..