Halliday & Matthiessen (2014: 427n):
Certain adjectives can serve as Event in a verbal group in a hypotactic verbal group complex, e.g. (conation: potentiality:) be able/apt/prone/likely → to do; (modulation: time: frequency:) be wont → to do; (modulation: manner: quality:) be wise/right/wrong → to do; (projection:) be willing/keen/eager → to do; be afraid/scared → to do.