Making the battlefield a safer place has become more difficult as more weapons are used, and these weapons are made more deadly. The army, navy and air force have also found that each has quite different dangers. For the army, fragments are the biggest danger; be they bullets or the more common shell fragments. The air force has to worry most about fire. Combat aircraft are largely, by weight, engines and highly flammable fuel. So it was the air force that pioneered the development of flame resistant flight suits. These Nomex suits are about three times as expensive (currently about $150-$200) as clothing made from more conventional materials. The army eventually adopted Nomex suits for its helicopter crews. In the 1980s, tank crews began to wear Nomex suits, it having been noticed that flames were a major source of injury when a tank was hit. The tankers suit had one additional feature, a "casualty strap." This was a sturdy strap along the top rear of the jumpsuit, so that injured men could be more easily hauled out the narrow hatch of an armored vehicle. The navy has also found that fire is the major cause of casualty. Some navies issue Nomex suits that include Nomex balaclavas (to cover the head and neck) and gloves. The navy also has to worry about suffocation from smoke or simply lack of oxygen while firing fires below decks. So the navy provides a lot of breathing gear for emergencies.