The original departure was scheduled for October 12. A storm front over the Andes forced the aircraft to stop overnight in Mendoza, Argentina, on the Argentine side of the mountains. This delay — a single day — would later take on a kind of grim significance for everyone who studied what happened next. It meant the crossing was attempted in deteriorating conditions on the afternoon of October 13 rather than in better weather on October 12.
The route from Mendoza to Santiago required crossing the Andes through one of the high mountain passes. The standard navigation procedure involved flying to the Planchón pass, then descending to cruise altitude on the Chilean side before turning north toward Santiago. The crew calculated the approach using timing: they knew approximately how long the crossing should take, and when that time elapsed, they radioed Chilean air traffic control that they were over Curicó, a city on the Chilean side of the mountains, and requested permission to begin descent.
The calculation was wrong. The plane was not over Curicó. It was still over the Andes.
The official investigation later concluded that the error was caused by a stronger-than-expected headwind. The aircraft had taken longer to cross the mountains than the crew calculated. When they believed they were on the Chilean side and began descending through the cloud layer toward what they assumed was lowland terrain, they were in fact still over the high Andes. The fuselage struck the crest of a ridge at approximately 3:34 p.m.