April 21

In the spring of 1946, the U.S. Weather Bureau recorded a massive tornado striking Timber Lake, South Dakota. With a width measured at approximately four miles, it stood as one of the widest tornadoes known to science. The storm uprooted vegetation, damaged structures, and challenged contemporary measurement techniques. This record-breaking event spurred advances in tornado observation and the study of supercell thunderstorms. Meteorologists later used the data to improve warning systems and safety protocols for extreme weather. The Timber Lake tornado remains a benchmark in the history of severe storm research.

On April 21, 1958, United Air Lines Flight 736 was en route over Nevada when it collided with a United States Air Force F-100 Super Sabre fighter jet. The accident occurred near Arden, in what is now Enterprise, Nevada. All passengers and crew aboard both aircraft perished in the collision. This tragedy remains one of the deadliest mid-air collisions in U.S. aviation history. The incident prompted a comprehensive review of military and civilian airspace coordination. Subsequent changes were implemented to improve communication and separation protocols. The legacy of Flight 736 influenced modern air traffic control systems to prevent future airborne disasters.

On April 21, 1964, the U.S. Navy launched the Transit-5bn navigation satellite from Vandenberg Air Force Base. The mission failed to achieve orbit, causing the spacecraft to re-enter the atmosphere and disperse its SNAP RTG power source. Approximately 0.95 kg of radioactive plutonium was released into the environment over the Pacific Ocean. This accident raised concerns about the safety of nuclear power sources in space missions. Investigations led to improvements in launch procedures and containment of radioisotope generators. The incident influenced future satellite design and policy on spaceborne nuclear materials.

On April 21, 2012, two passenger trains collided head-on near Sloterdijk station on the outskirts of Amsterdam, causing one fatality and injuring 116 passengers. The collision occurred during the evening rush hour when one train entered the wrong track, leading to a high-speed impact. Emergency services responded swiftly, providing medical care and evacuating the injured to nearby hospitals. Dutch rail authorities suspended services in the area and deployed replacement buses to transport stranded commuters. An investigation by the Dutch Safety Board examined signaling systems, track assignments, and operational protocols to determine the cause. The crash reignited debates over railway safety standards and accelerated plans to implement advanced train protection systems. In its aftermath, enhancements were made to signaling protocols and staff training to improve safety on the Dutch rail network.

On April 21, 2021, the 44-year-old diesel-electric submarine KRI Nanggala (402) went missing during a torpedo-firing exercise north of Bali. A multinational search operation involving Indonesian, Singaporean, Australian, and Malaysian vessels located the submarine on the seafloor at a depth exceeding its crush limit. All 53 sailors aboard were declared dead, marking one of the worst peacetime losses in the Indonesian Navy's history. Investigations pointed to a mechanical failure and loss of power that prevented the crew from surfacing. The tragedy sparked public grief and renewed calls for modernization of Indonesia's aging naval fleet.