What must an aviation attorney prove to win a negligence lawsuit against someone who he believes responsible for the accident that injured his client? Two things. First, the aviation attorney must prove that the entity was "negligent." Second, the attorney must prove that the defendant's negligence was a "cause" of the accident or of the injury.
Negligence Defined. Someone is "negligent" if he was not "reasonably careful" under the circumstances. It is not enough for the attorney to prove simply that defendant could have avoided the accident by doing something differently. No mechanic, pilot, or other defendant is expected to be perfect. He is, however, expected to be as careful as others would have been in the same situation. If he was not, then he was negligent.
Violation of Regulations. Sometimes, it turns out that the manufacturer, mechanic, or other defendant violated a federal aviation regulation. In some states, one who violates a regulation is automatically considered negligent, or "negligent per se." In other states, one who violates a regulation isn't automatically negligent, but the violation is something the jury is allowed to consider when deciding the question.
Compliance with Regulations. Proving that the defendant violated a regulation goes a long way towards proving that the defendant was negligent. But what if the defendant proves he complied with all the regulations? Is he off the hook? No. One who complies with every regulation can still be found to be negligent. That's because the aviation regulations are minimum safety standards only. Presumably, those involved in aviation hold themselves to a higher standard of care. In other words, reasonable mechanics or manufacturers are expected to go above and beyond the regulations.
Causation. To win the negligence lawsuit, the victim's attorney must also prove that the defendant's negligence was a cause of the accident or the injury. So, for example, it's not enough for the victim's attorney to prove that the mechanic was not reasonably careful. Sure, the mechanic may have been negligent for failing to tighten the nuts to the correct torque value. But to win the case, the victim's attorney must prove that the failure to tighten the nuts was one of the causes of the accident or injury.