What happened
On the morning of the accident, Iberia Flight 610 departed Madrid at 08:47, bound for Bilbao. The aircraft, a Boeing 727 identified as "Alhambra de Granada", initially climbed to a cruising altitude of FL260. During the descent phase, the crew received instructions to descend to FL100 and were subsequently cleared by Bilbao Tower for an ILS approach to runway 30.
While the controller offered a direct clearance to the approach fix, the captain chose to follow the standard approach procedure. By 09:22, the aircraft was over the Bilbao VOR at 7,000 feet. As the flight continued its descent to 5,000 feet, the crew adjusted the Altitude Alert System to a preset altitude of 4,300 feet, which was below the minimum sector altitude of 4,354 feet.
As the plane descended, the altitude alert horn activated at 4,040 feet. The crew mistakenly believed this sound was an approach mode alert rather than a deviation warning. Shortly after passing through the minimum sector altitude, the aircraft struck the base of antenna structures situated near the peak of Mount Oiz. The impact caused the left wing to separate from the fuselage, and the remaining wreckage crashed into the hillside, disintegrating upon impact.
Findings
All 148 individuals on board were killed in the accident. Investigations concluded that the crew's misinterpretation of the altitude alert system warnings, combined with a potential error in reading the altimeter and over-reliance on the automatic capture features, led them to fly below the safe minimum sector altitude.