United Airlines Flight 232 (1989)
After this McDonnell Douglas DC-10 crashed in Iowa, aircraft engineers had to improve emergency procedures and make improvements to aircraft systems all around. This aircraft suffered an engine failure and crash-landed in Sioux City. The number two engine practically tore itself into two pieces, which prompted pilots to control an uncontrollable airplane. According to a report, this crash happened because engineers failed to catch a crack in the engine that was slowly becoming bigger over time.
They wrote, “The hard material inside the nitrogen impurity was knocked loose, creating a void which acted as a weak point in the otherwise uniform titanium alloy. The enormous rotational stresses that should have been distributed evenly over the rapidly spinning disk now ran up against a tiny spot that absorbed stress differently from the rest of the material. Over time, the disk began to suffer from metal fatigue as a tiny crack grew imperceptibly outward from the cavity throughout every flight.” Because of the incident, 112 people died, though there were miraculously quite a few survivors (Admiral Cloudberg).