From what I can remember after reading "Fly the wing" and "Turbine pilot's manual" usually it forms on the wing first(usually). Always wondered where this first happens on the 737 airframe.
With mach trim inoperative our mach number is limited to .74, so I assume its somwhere around .73..(Think I read it somewhere before, maybe even in 737tech site or Eric Parks' notes, just can't be sure enough to confirm).
