Ah, fair question....ON the Airbus (319 anyway), the FD are intrinsically linked to the autothrottle. So assuming a high rate climb, and then a Descend TCAS RA, if you ignored the FD on an Airbus, disconnected the AP and pushed down the FD would still be commanding a climb, the autothrust would follow the FD and command CLB thrust and you would dive away from the RA with climb thrust set, the Thrust would Lock to climb (so even closing the thrust levers would not close the thrust). So, there would be a massive risk of an overspeed (despite the aircraft attempting to protect itself it would still overspeed). The only get out woud be to both disconnect the A/T and close them, this is not a normal Airbus manouver as the natural inclination is to leave the A/T on. So with Airbus, switching the FD off removes the link between FD and A/T, so diving away and closing the thrust would work normally. Does that make sense? (I refer only to my explanation and not to why the Airbus works this way
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