The deadliest plane crash in aviation history occurred in the Santa Cruz de Tenerife, Canary Islands on March 27, 1977 between two Boeing 747s on the same runway. KLM Flight 4805 collided with Pan Am Flight 1736 on the foggy runway. All 249 passengers on the KLM plane died and 333 of the 394 aboard Pan Am perished. There was a total of 583 fatalities.
This crash happened after a miscommunication between the two flight crews and air traffic control. The KLM aircraft initiated takeoff while the Pan Am flight was taxiing midway down the same runway. As KLM flight began to lift into the air, the lower part of its fuselage struck the upper part of the Pan Am plane's fuselage as it was turning off the runway.
In response to this terrible tragedy, officials required all control towers and flight crews to use standardized English phrases worldwide. In addition, the previous hierarchy in the cockpit was de-emphasized and decision making by mutual agreement became the norm. This allows the crew to know what is happening, and this, which is called crew resource management is now the accepted practice worldwide.
Some of the most deadly crashes in history from PBS.org: NOVA Series