I travelled turkish a lot in the past two years and every time Salah came I went to the back and requested to pray. They always say "it's not in our policy" then they look around and say "okay quickly pray here" and alhamdulillah I prayed very comfortably every time. Also, you need to time the request; try to ask them when most of the air hosts are by the first class seats and there's only one person right at the back (there is a long gap like this on their flights when you can pray peacefully).