Flight History
Flight UA869 departed Hong Kong for Bangkok, an intermediate stop on a scheduled service to Cairo, at 13:30 UTC. The flight climbed to its selected cruising altitude of 31,000 feet. At 15:14, the crew advised Bangkok ATC that they had crossed the Bangkok FIR boundary at 15:08 and passed over Ubol NDB at 15:13, requesting permission to fly direct from Ubol NDB to Bangkok VOR. Bangkok ATC granted the request. The flight reported an estimated time of arrival at Bangkok VOR of 15:47. At 15:27, the crew advised that they would be over the 100-mile perimeter at 15:30. After reporting being 90 miles out, the crew requested descent clearance. Bangkok control cleared the flight to descend to 4,000 feet on the Bangkok VOR radial of 073 degrees, with instructions to report when commencing descent from 31,000 feet. The flight was instructed to contact Bangkok approach control at 15:39. At 15:35, the flight was cleared to 3,000 feet and informed that the altimeter setting was 1007.8 mb. At 15:40, the flight transferred to Bangkok approach control and reported descending from 13,000 feet, estimating arrival at Bangkok VOR at 15:44. Approach control advised the altimeter setting and cleared the flight to cross Bangkok VOR for final approach on runway 21R, with instructions to report immediately when descending from 3,000 feet. This was the last contact with the flight.
Accident
The Comet flew into the side of Khao Yai Mountain, located 52 nautical miles northeast of Bangkok.
Investigation
The official investigation determined the probable cause of the accident. The principal cause was the pilot's action in commencing descent at 15:30 when the aircraft was 137 miles, not the 90 miles reported to Bangkok Control, from the Bangkok VOR. Consequently, the aircraft collided with a mountain at a point 52 miles distant. It is probable that the pilot-in-command did not actually pass over the points he reported to the flight control units but only estimated his position, resulting in grave errors in time and distance computations. It is also probable that the pilot-in-command had been too self-confident, leading to actions not in accordance with fundamental principles of air navigation.