Can flexible duct be used as the plenum attached to the air handler. I have never seen this before.
Not a good choice but if its not leaking air nothing says it cannot. How close to the heat source are you I would be very cautious about flex if attached to the top directly. Do you have a better pic I can not tell much from your pic. I am probally in left field you stated air handler are we talking furnace air handler.
Unfortunately, that is the only picture I could. Not much of an angle there. One question brought up is - we have a round duct on a square hole at the top of the air handler. There was no way to get my hand up there to even see how it was installed.
Im sure that piece of flex is tied into a piece of ductboard or possibly a piece of sheet metal, but from the picture cant be certain. As Charley stated if it aint leaking it is ok, tacky but ok.