In A320 procedure of switching of both FDs does not come from systemic deficiency but rather to take advantage from a smart feature that puts the ATHR in speed mode without any other action on FCU. Heaven is not going to fall if you kept one FD on but you need to ensure proper mode transition by doing other things. It is matter of developing correct and safe operating habits because when under pressure pilots can get confused and make mistakes executing nonstandard procedures. In SFO accident when the crew switched off FDs in B777 the auto throttle went in throttle hold mode which was at idle then and the crew failed to monitor speed. Airbus FD logic in this case is definitely safer and better. FAA has asked Boeing to do something about it. On the other hand the first fatal crash of A320 in 1982 India was similar because the crew switched only one FD OFF and remained in OPDES and imagined they were in speed mode. Had they switched off the other it would have switched to speed mode.
Last edited by vilas; 15th Aug 2015 at 05:39.