Originally Posted by
awair
Severe damage: engine indications may disappear, so TAC is automatically disabled in case this is only an indication failure and not an engine failure.
This was a major concern during TAC development - what we called "Adverse TAC". TAC is based on indicated N1 (since N1 is normally directly related to thrust). There are certain engine failure where N1 and thrust can go opposite ways (bird strike is big one - major fan damage due to a bird strike can actually unload the fan so it speeds up while thrust decreases - another is a sheared fan shaft forward of the N1 speed sensor so fan speed rapidly drops while the unloaded turbine speeds up giving a large increase in indicated N1).
Anyway, adverse TAC - the rudder going the wrong way with an engine failure - is potentially catastrophic. So there is a "real time engine model" constantly running in the Engine Data Interface Unit (EDIU) - if the engine parameter indications don't make sense to the EDIU (indication a faulty indication or severe engine damage) it disables TAC.
Also, TAC rudder authority is limited - for the same basic reason - limiting the ability of Adverse TAC to make a bad situation worse. So even with TAC active you'll likely need to apply some rudder.