。
The airplane had already left the airfield.
Literal
Airplane [topic-は] already airfield [from-を] had-departed.
立つ here is used in the sense of 発つ (たつ, 'to depart'). Modern Japanese typically writes 発つ for departures, making 立っていた slightly old-fashioned. The を marks the departure point, similar to 家を出る ('to leave home'). すでに means 'already,' and ~ていた indicates the departure preceded the narrative reference point.