Chengdu's new airport is now TFU. CTU is largely used for domestic flights now.
With that said, the closest airport that UA flies to is HKG. So if you want to maximize UA metal, then you can fly ORD-SFO-HKG, and then depending on ticketing, a HKG-TFU segment. CA flies HKG-TFU, so you could potentially do it all on *A.