Originally Posted by
enzino
True, but I was thinking about the downwash of the airflow caused by the flaps without being re-energised by the slats. Just an academic guess.
Same fantasies here. Slats provide a lot of airflow on the deep end of the wing, or so I was told.
But there was an interesting point upthread, that Airbus may have not flown tested such and such ... got me thinking. There is abundance of F3 in the System Failure tables. So I gather the F3 is actually the good flap. And F4 only provided for a "deep" setting to comply with the Vat limit for 140 kts Category C aeroplane at MLW. Looking at the numbers, especially the non-HGW A321 and the A321NEO, that design goal is rather obvious.
Accepting the idea that loosely speaking F4 is the workaround flap, it would make a lot of sense to see this F3 being used whenever justifiable.