Victor 398

Victor 398 was a UNSC Pelican dropship which crashed into the mountain gorge that is the Halo 3 multiplayer level, Valhalla. The crash, which the crew barely survived, also scattered the Pelican's contents throughout the valley. V-398's rear hatch is sealed, presumably by its surviving crew, which denies access to the bird's interior.

Since Valhalla is located on the Ark, it can be assumed that the Pelican crashed their during one of the UNSC's operations there in Halo 3.