。
The airplane was no longer visible.
Literal
Airplane [possessive-の] figure [topic-は] visible-not became.
姿 (すがた, 'figure, form, silhouette') adds a visual/literary quality compared to simply saying 飛行機は見えなくなった. It's the airplane's silhouette against the sky that vanishes — a slightly more evocative phrasing common in narrative writing.