The dependence of the air-sea momentum flux on surface wave fields is investigated at very high winds under tropical cyclones. A coupled wave-wind model is applied to estimate the momentum flux under ten hurricanes in the western Atlantic Ocean during 1998--2003. The model explicitly calculates the wave-induced stress vector and the total wind stress vector from a given wind speed vector and a calculated wave spectrum. It is found that the neutral drag coefficient levels off at high wind speeds under tropical cyclones, being consistent with recent observations and previous modeling studies. The most important finding of this study is that the Charnock coefficient is mainly determined by two parameters: the input wave age (wave age determined by the peak frequency of wind energy input) and the wind speed, regardless of the complexity of the wave field under a real hurricane, and that the Charnock coefficient increases with the input wave age at very high winds.