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Thai Airways International Flight 311 was an Airbus A310-304, registration HS-TID, which crashed on approach to Kathmandu's Tribhuvan International Airport on July 31, 1992. All 111 on board were killed.

Flight 311 departed Bangkok, Thailand's Don Muang Airport at 10:30 AM local time. After crossing into Nepalese airspace the pilots contacted air traffic control and were cleared to an instrument approach from the south called the "Sierra VOR Circling Approach" for Runway 20. Nepalese ATC at the time was not equipped with radar.

Shortly after reporting the Sierra fix ten miles south of the Kathmandu VOR, the aircraft called ATC asking for a diversion to Calcutta, India because of a "technical problem"; before ATC could reply, the flight rescinded their previous transmission. The flight was then cleared for a straight-in Sierra approach to Runway 02 and told to report leaving 9,500 feet. The captain asked numerous times for the winds and visibility at the airport but ATC merely told him that Runway 02 was available.

A number of frustrating and misleading communications (due partly to language problems and partly to the inexperience of the air traffic controller, who was a trainee with only nine months on the job) ensued between air traffic control and the pilots regarding Flight 311's altitude and distance from the airport. The captain asked four times for permission to turn left, but after receiving no firm reply to his requests he announced that he was turning right and climbed the aircraft to flight level 200. The controller handling Flight 311 assumed from the flight's transmissions that the aircraft had called off the approach and was turning to the south, and he therefore cleared the aircraft to 11,500 feet, an altitude that would have been safe in the area south of the airport. The flight descended back to 11,500 feet, went through a 360 degree turn, passed over the airport northbound, and crashed on a steep rock face in a remote area of the Langtang National Park at an altitude of 11,500 feet.

Investigators from the Nepalese aviation authority, Airbus Industrie, and the Transportation Safety Board of Canada (which assisted with technical details) determined that the aircraft had experienced a minor fault in the workings of the inboard trailing flaps just after the aircraft reached the Sierra reporting fix. Concerned that the complex approach into Kathmandu in instrument conditions would be difficult with malfunctioning flaps and frustrated by ATC and his first officer's inconclusive and weak answers to his questions, the captain decided to divert to Calcutta. The flaps suddenly began to work properly, but the captain was forced to resolve more aspects of the difficult approach himself due to his first officer's lack of initiative. Only after numerous extremely frustrating exchanges with ATC was the captain able to obtain adequate weather information for the airport, but by that time he had overflown Kathmandu and the aircraft was headed towards the Himalayas.

The Nepalese authorities found that the probable causes of the accident were the captain's and controller's loss of situational awareness; language and technical problems causing the captain to experience frustration and a high workload; the first officer's lack of initiative and inconclusive answers to the captain's questions; the air traffic controller's inexperience, poor grasp of English, and reluctance to interfere with what he saw as piloting matters such as terrain separation; poor supervision of the inexperienced air traffic controller; Thai Airways International's failure to provide simulator training for the complex Kathmandu approach to its pilots; and improper use of the aircraft's flight management system.

This accident took place on the same date that China General Aviation Flight 7552 crashed out of control at Nanjing, China, killing 108 out of 126, and only 59 days before PIA Flight 268 crashed just south of Kathmandu, killing 167.

The remains of the aircraft can still be seen in Langtang National Park, and have been featured in Lonely Planet guidebooks as a point of interest on the trek from Ghopte to the Tharepati Pass.

• Lists of accidents and incidents on commercial airliners
• Air safety
• Controlled flight into terrain

• Flight 311 at the Aviation Safety Network Database
• Air Disaster, Vol. 3, by MacArthur Job, Aerospace Publications Pty. Ltd. (Australia), 1998 ISBN 1-875671-34-X, pp. 98-115.