#1: The Tenerife Airport Disaster Heavy fog, an issue with transmitters, and miscommunication between the pilots and the control center resulted in the KLM 4805 taking off without permission, while the Pan Am 1736 was still on the runway, at the wrong exit because the pilot couldn't read the signs in the fog.

The Tenerife Airport Disaster, which occurred on March 27, 1977, remains the deadliest aviation accident in history and is widely attributed to a series of critical pilot errors. The disaster involved two Boeing 747s—KLM Flight 4805 and Pan Am Flight 1736—which collided on a fog-shrouded runway at Los Rodeos Airport. The primary error was committed by the KLM Captain, Jacob Veldhuyzen van Zanten, who initiated takeoff without a clear clearance from Air Traffic Control while the Pan Am aircraft was still taxiing on the same runway. A combination of heavy fog, ambiguous radio terminology (the use of "at takeoff" vs. "cleared for takeoff"), and "cockpit gradient" issues—where the co-pilot did not sufficiently challenge the senior captain's decision—led to the tragedy. The collision and subsequent fire killed 583 people. This accident fundamentally changed global aviation, leading to the standardized use of English in all international radio communications and the implementation of Crew Resource Management (CRM), which encourages all crew members to speak up if they perceive a safety risk, regardless of their rank or seniority.