The airplane was at the mercy of the strong wind.

Literal

Airplane [topic-は] strong-wind [possessive-の] doing-as-pleases was.

なすがまま (為すが儘) is a literary expression meaning 'at the mercy of' or 'left to do as it pleases.' The structure Xのなすがまま frames X as an uncontrollable agent. A dramatic, somewhat literary phrasing more common in written narrative than everyday speech.