However, ultra-high bypass ratio turbofans have a very low design fan pressure ratio (e.g. 1.2, on the bypass section). Consequently, even at cruise flight speeds, the cold (or mixed final) propelling nozzle may only be choked at high throttle settings. The fan working lines become more curved and migrate quickly towards surge as flight Mach number decreases. As a result, the static working line can be well into surge, particularly at low throttle settings.