The Bell Boeing V-22 Osprey is an American military tiltrotor aircraft with an accident history that has generated controversy over its perceived safety. The aircraft was developed by Bell Helicopter and Boeing Helicopters; the companies partner in its manufacture and support. The V-22 Osprey has had 13 hull loss accidents that have resulted in a total of 51 fatalities. During testing from 1991 to 2006, there were four crashes resulting in 30 fatalities. Since becoming operational in 2007, the V-22 has had eight crashes, including two combat-zone crashes, and several other accidents and incidents that resulted in a total of 21 fatalities. Crashes and hull–loss accidents June 1991 On 11 June 1991, a miswired flight control system led to two minor injuries when the left nacelle struck the ground while the aircraft was hovering in the air, causing it to bounce and catch fire at the New Castle County Airport in Delaware. The pilot, Grady Wilson, suspected that he may have accidentally set the throttle lever the opposite direction to that intended, exacerbating the crash if not causing it. July 1992 On 20 July 1992, pre-production V-22 #4's right engine failed and caused the aircraft to drop into the Potomac River by Marine Corps Base Quantico with an audience of Department of Defense and industry officials. Flammable liquids collected in the right nacelle and led to an engine fire and subsequent failure. All seven on board were killed and the V-22 fleet was grounded for 11 months following the accident. A titanium firewall now protects the composite propshaft. April 2000 A V-22 loaded with Marines, to simulate a rescue, attempted to land at Marana Northwest Regional Airport in Arizona on 8 April 2000. It descended faster than normal (over ) from an unusually high altitude with a forward speed of under when it suddenly stalled its right rotor at , rolled over, crashed, and exploded, killing all 19 on board. The cause was determined to be vortex ring state (VRS), a fundamental limitation on vertical descent which is common to helicopters. At the time of the mishap, the V-22's flight operations rules restricted the Osprey to a descent rate of at airspeeds below (restrictions typical of helicopters); the crew of the accident aircraft had descended at over twice this rate. Another factor that may have triggered VRS was the operation of multiple aircraft in close proximity, also believed to be