Investigation of a fatal accident in May involving a Bell-Boeing MV-22B Osprey tiltrotor transport points to an undiagnosed failure mode in the Osprey’s trouble-prone propulsion system and has resulted in tight restrictions on restricted visibility landing (RVL) operations. The preliminary report findings—disclosed in documents obtained by Aviation Week—also reopen questions about an April 2010 fatal accident involving a U.S. Air Force CV-22B in Afghanistan.