Flank steak is not one of my first choice meats, but I couldn’t pass up a big discount in the form of “$ off” stickers on the packages. I got 3 pieces and decided to try to figure out how long to cook them.
My BBQ MO is to take my knobby meat hammer and beat it into submission, then rub it and let it sit for 12-24h. Then a quick sear and slice.
With the APC I did all three pieces unseasoned at 130F/54C. I took the first piece out at 18h, the second at 36h and the last at 60h.
That’s an extremely rare temperature. It’s generally accepted as risky business to cook meat below 55F for more than 4 hours. Length of time ceases to be a factor when you have the potential for bacteria growth during a long cook.
If you want tender meat that rare, buy a better cut.
I’d probably suggest a bit higher than 55C for ribs just because of the bone mass which blocks heat. But 54C is where it’s at for boneless cuts. You’re singing to the converted here.