The V-22 Osprey is a tiltrotor vertical/short take-off and landing (VSTOL), multi-mission aircraft developed to fill multi-Service combat operational requirements. The V-22 is being developed by Boeing and Bell to meet the provisions of the April 1995 Joint Multi-Mission Vertical Lift Aircraft (JMVX) Operational Requirements Document (ORD) for an advanced vertical lift aircraft. The JMVX ORD calls for an aircraft that would provide the Marine Corps and Air Force the ability to conduct assault support and long-range, high-speed missions requiring vertical take-off and landing capabilities. In US Marine Corps service it replaces primarily the ageing CH-46E. The first Full Scale Development (FSD) V-22 flew March 19, 1989. Low rate initial production (LRIP) was authorised 1997 and the first of these were delivered November 1999. During development the engine nacelle proved vulnerable and a new aerodynamic phenomena was found: asymmetric Vortex Ring State (VRS). Four crashed during V-22 development resulted in 30 fatalities, almost resulting in cancellation of the program. The accidents resulted in a major redesign of engine nacelle and its components. Also flight control software was rewritten to prevent asymmetric VRS conditions and a warning system was added that alerts pilots to possible VRS conditions. On 8 December 2005, the US Marine Corps accepted the delivery of the first fleet of MV-22s. In September 2007, the MV-22 was deployed operationally for the first time.