The Tenerife airport disaster, which occurred on March 27, 1977, remains the accident with the highest number of airliner passenger fatalities. 583 people died when a KLM Boeing 747 attempted to take off without flight clearance and collided with a taxiing Pan Am 747 at Los Rodeos Airport on the Canary Island of Tenerife, Spain. There were no survivors from the KLM aircraft and only 61 of the 396 passengers and crew on the Pan Am aircraft survived. Pilot error was the primary cause, as the KLM captain began his takeoff run without obtaining air traffic control clearance
Further legitimacy was restored to Juan Carlos's position on 14 May 1977, when his father (whom many monarchists had recognized as the legitimate, exiled King of Spain during the Franco era) formally renounced his claim to the throne and recognized his son as the sole head of the Spanish Royal House, transferring to him the historical heritage of the Spanish monarchy, thus making Juan Carlos both de facto and de jure king in the eyes of the traditional monarchists.
On 20 May 1977, the leader of the only recently legalized Spanish Socialist Workers' Party (PSOE), Felipe González, accompanied by Javier Solana, visited Juan Carlos in the Zarzuela Palace. The event represented a key endorsement of the monarchy from Spain's political left, who had been historically republican. Left-wing support for the monarchy grew when the Communist Party of Spain was legalized shortly thereafter, a move Juan Carlos had pressed for, despite enormous right-wing military opposition at that time, during the Cold War.