飛行機が見えなくなるまで見ていた。

。

I watched until the airplane was no longer visible.

Literal

Airplane [subject-が] visible-not become [until-まで] was-watching.

~まで marks the temporal endpoint: 'until X.' 見えなくなる ('cease to be visible') is the endpoint, and 見ていた ('was watching') is the ongoing action up to that point. A poetic image of watching something recede.