Safety was a major concern in the fuel system design. An onboard inert gas generating system (OBIGGS) supplies nitrogen-rich air to the wing and sponson tanks as fuel is depleted. The inert gas displaces fuel vapor and reduces the possibility of fire. All tanks are self-sealing to a 12.7 mm AP threat and meet a drop test requirement of 65 feet when filled with water. Self-sealing breakaway valves are located at each line-to-tank connection. For ballistic and crash protection, a suction system is used for normal sponson tank to feed tank transfers.
The bladder-type wing fuel cells are crashworthy and (except for the access covers) fully self-sealing to fully tumbled .50 cal. APM2 projectiles. The transfer lines and primary feed lines are not self-sealing. These lines contain fuel and, if severed, the fuel could feed a fire. However, they are designed to be under suction (negative pressure) and, normally, the fuel will be sucked clear. Ballistic penetration of one transfer line will result in loss of fuel transfer capability to one feed cell. Fuel remaining in the wing auxiliary and sponson tanks on the side where the transfer line is severed becomes unusable to either engine.
Bladder-type fuel cells in the sponsons and wing auxiliary cells are crashworthy and partially self-sealing in accordance with MIL-T-27422, Type 1, protection Level B. Material used in these cells is self-sealing to .50 cal. APM2 fully-tumbled projectiles. The wing auxiliary cells contain self-sealing material in the lower one-third of the cell. The sponson cells currently contain self-sealing material on the lower one-third of the cell and full self-sealing on the inboard, forward and aft vertical walls.
All fuel cell ullages are protected from explosion due to ballistic impacts by use of onboard inert gas generating system (OBIGGS), which provides inerting nitrogen-enriched gas to the feed cells and sponsons. The enriched nitrogen environment displaces the air in the cells through the vent system, thereby reducing vulnerability to fire and explosions after ballistic impacts and crash landings. The OBIGGS provides the nitrogen-enriched gas at a pressure of 1-2 psig during the entire V-22 mission. Dry bay fire protection in the wing and sponson is also provided.
Voids between the engine feed tank and surrounding structure are filled with ballistic foam between stringers above and below the tank, and between the feed tank and adjacent ribs. Aluminum oxide powder-filled panels are bonded to the forward face of the forward spar sections that lie in front of the engine feed tanks. These panels are designed to suppress incendiary flash and provide fire protection in the small dry bay between the forward spar and the leading edge of the wing skin. Detector/suppressor units provide dry bay protection in the aft wing cove area and in the dry bays immediately inboard and outboard of the engine feed tank. These units employ non-discriminating detectors (e.g., no differentiation between an incendiary flash and a fire) and solid chemical gas generator suppressors, which use non-ozone depleting agents.
Fire protection in the sponson dry bays is accomplished with the use of approximately 0.8 inches of rigid foam sandwiched between each fuel cell and the stiffeners of the forward and aft bulkhead. This provides protection from fire in voids, as well as the adjacent dry bay. This foam is also being reevaluated. Rigid foam (3 inches) is also installed under the cabin floor, next to the fuel cell structure.