On this date in 1977 two Boeing 747’s collided in dense fog at the airport on the island of Tenerife. 583 people perished in what is still the deadliest accident in aviation history.
A KLM 747-206B attempted to take off while a Pan Am 747-121 back taxiing on the same runway. A combination of extremely limited visibility, poor runway/taxiway marking and confusion based on communications problems, including simultaneous transmissions and non-standard radio use, resulted in the fiery crash.
As a consequence of the accident, sweeping changes were made to international airline regulations and to aircraft. Aviation authorities around the world introduced requirements for standard phrases and a greater emphasis on English as a common working language. For example, ICAO calls for the phrase “line up and wait” as an instruction to an aircraft moving into position but not cleared for takeoff. Also several national air safety boards began penalizing pilots for disobeying air traffic controllers’ orders. Air traffic instruction should not be acknowledged solely with a colloquial phrase such as “OK” or even “Roger” (which simply means the last transmission was received), but with a readback of the key parts of the instruction, to show mutual understanding. Additionally, the phrase “takeoff” is spoken only when the actual takeoff clearance is given. Up until that point, both aircrew and controllers should use the phrase “departure” in its place (e.g. “ready for departure”). Cockpit procedures were also changed. Hierarchical relations among crew members were played down. More emphasis was placed on team decision-making by mutual agreement, part of what has become known in the industry as Crew Resource Management.
(with information from Wikipedia)