Airfoilmod,
...Alt mode is locked out at advanced levers, to prevent over speed, No? Once throttles are retarded and Alt mode select is done, then truly manual Fuel control is obtained.(FAMEC?)
There are no 'lockouts' that I'm aware of.
Selecting alternate mode makes N1 the controlling parameter and this is referenced to the thrust lever position. This means it is possible to exceed the rated thrust of the engine in certain conditions but not overspeed anything; also, the system is designed so that when you switch, you get at least the thrust you had before, possibly more. If you had firewalled before selecting alt. mode, it's very likely one of the the RPM limits would come into play; at much less than TO/GA to begin with you're unlikely to bust anything.
The manual also says: "If the EECs are in alternate mode, advancing the thrust levers full forward provides some overboost and should be considered only during emergency situations when all other available actions have been taken and terrain contact is imminent." Good one to have up your sleeve if you ever need it...
Green-dot,
Could very well be that they perceived engine surging to be their problem at hand at the time.
I really can't see that: all engine parameters were stable and within limits, just not where they wanted them! An engine surge(ing) is difficult to overlook, even from a mile away... Two very different scenarios, IMHO.
snanceki,
...as a professional engineer I find it difficult to come to any other conclusion (ON THE INFORMATION AVAILABLE TO US) other than some aspect of the FUEL, conditioned by ENVIRONMENT, and FUEL SYSTEM DESIGN caused a condition that has not been previously considered.
I think your comment is pretty much the 'state of the art' at the moment. There are so many possibilities but, like you, I think the probabilities lie here...