。
The airplane was on the very verge of taking off.
Literal
Airplane [topic-は] precisely takeoff was-about-to.
せんとする is a classical/literary expression meaning 'about to do,' formed from the classical verb base せ + ん (contraction of the volitional む) + とする. まさに ('precisely') emphasizes the imminence. This is significantly more elevated than the modern ~ようとする and lends a formal, narrative tone.