What happened
On the scheduled service traveling from Hong Kong toward Bangkok and eventually Cairo, the Comet aircraft departed at 13:30 UTC. While cruising at 31,000 feet, the crew contacted Bangkok ATC to report that they had entered the Bangkok FIR at 15:08 and passed the Ubol NDB at 15:13. The flight requested a direct route from the Ubol NDB to the Bangkok VOR, which was approved by controllers.
At 15:27, the crew informed air traffic control that they would reach the 100-mile perimeter by 15:30. After stating they were 90 miles from their destination, the crew requested permission to descend. Bangkok control authorized a descent to 4,000 feet on the 073-degree radial of the Bangkok VOR and instructed the flight to contact approach control at 15:39.
Following a transfer to Bangkok approach control at 15:40, the crew reported they were descending from 13,000 feet with an estimated arrival at the Bangkok VOR at 15:44. After receiving instructions to adjust the altimeter setting and clearance for final approach on runway 21R, the aircraft disappeared from radar. The Comet subsequently struck the side of Khao Yai Mountain, located approximately 52 nm northeast of Bangkok.
Findings
Investigations determined that the primary reason for the crash was the pilot's decision to begin the descent prematurely. Although the crew reported being 90 miles from the Bangkok VOR, the aircraft was actually still 137 miles away at the time the descent began. This discrepancy led to the aircraft striking the mountain while still 52 miles from its target. It is believed that the pilot likely estimated their position over several waypoints rather than verifying their actual location, leading to significant errors in distance and time calculations.