Ah si en fait, il faut rester appuyer un peu plus longtemps pour qu'il tienne

EDIT :* EAC switch is made solenoid-held
Apparemment il faut attendre la fin de l'alignement INS pour pouvoir activer l'EAC.This. I'm not sure many people realize what a full align means. It's not when the CDU says "INS Nav Ready", but it's when the T= #.# #.# line's second value is below 1.0. Usually T= 3.7 0.8 for example. When that value drops below 1.0 the "INS Nav Ready" will start to flash and you're fully aligned.
At that point you're clear to enter Nav mode which activates the TAD Nav data and EAC ready ability.