The Federal Tort Claims Act allows citizens who have been injured by the federal government to sue the United States. But there's an important exception. No suit against the government is allowed when the victim is a service member injured by the negligence of the United States military.
The rule protecting the military is called the Feres Doctrine. In aviation accident cases, the doctrine bars service members from suing the government regardless of whether the crash was caused by the negligence of a military mechanic, air traffic controller, dispatcher or pilot.
Not surprisingly, the Feres Doctrine is controversial. It allows the military to avoid responsibility for not just simple negligence, but for gross negligence as well. Because of its unfairness, Congress has repeatedly been asked to abolish the rule, or at least limit it. (Large pdf of Congressional Hearing here.) But the Feres Doctrine remains the law. As long as the victim was an "active" service member, and the injury or death was "incident to service," the military is immune from suit.
That doesn't mean that injured soldiers or their families cannot sue others who may have contributed to a military aircraft accident. For example, if a defect in the design of the aircraft contributed to the crash or to the injuries the crew member received, the crew member can still sue the aircraft's manufacturer. The aircraft manufacturer may be able to assert defenses of its own, such as the "government contractor defense," but not the Feres Doctrine.
The military prepares an investigative report after every accident involving one of its aircraft. The report focuses on the military's role in the accident. It seldom addresses whether a manufacturer or other civilian contractor may have been at fault. In fact, as discussed in this article concerning a military helicopter crash off the California coast, sometimes the report provides no answers at all. Families will often need to enlist an aviation accident attorney to conduct an investigation on their behalf. The attorney may need to file suit against the manufacturers just to obtain access to the evidence bearing on who, other than the military, may be responsible for a service member's injury or death.