II.in [美 ɪn, 英 ɪn]副in often appears as the second element of certain verb structures in English (close in, hand in, stand in, etc). For translations, see the relevant verb entry (close, hand, stand, etc).
During the run-in, the aircraft was vulnerable to defensive anti-aircraft fire, and it took courage to fly through it with no chance of evasive manoeuvres.