Sentence:  Between world wars, when Douglas Bader was a cocky, teen-age R.A.F. cadet . . . a man could navigate by eye and the nearest railroad track and <WSD> fly by the seat of his pants</WSD>. 
Meaning of the word marked with <WSD> tags:  To pilot an aircraft without the aid of instruments and without a flight plan, using only instinct, visual observation, and practical judgment.