Background
Thai Airways Flight 311 was a scheduled passenger flight operating an approach to Runway 02 at Tribhuvan International Airport in Kathmandu, Nepal. The flight was conducting the Sierra VOR/DME approach in instrument meteorological conditions. The aircraft was equipped with a Flight Management System (FMS) and a Ground Proximity Warning System (GPWS).
Sequence of Events
During the approach, a flap fault occurred. In accordance with company and performance requirements that mandate full flaps for the steep final approach, the crew requested clearance to return to Calcutta, India. Approximately 21 seconds after this request, while about 12 nautical miles from the Kathmandu VOR, the flap fault was rectified by retracting and then reselecting the flaps.
The crew determined that continuing the straight-in approach was not feasible due to the steep descent angles and the aircraft's position. They informed the control tower of their desire to restart the approach and requested a left turn to the Romeo fix, located 41 nautical miles south-southwest (202 radial) of the Kathmandu VOR.
The controller, operating in a non-radar environment, cleared the flight to make the Sierra approach, which begins at the 202 radial and 16 nautical miles from the VOR. The crew responded that they could not land at that moment and reiterated their request to turn left to Romeo.
After further dialogue, the crew unilaterally initiated a right turn from a heading of 025° and began climbing from 10,500 feet to flight level 180 (FL180), when the aircraft was about 7 nautical miles south of the Kathmandu VOR. The controller instructed them to report at 16 nautical miles for the Sierra approach.
During the turn, the controller established that the aircraft should maintain FL115 and proceed to Romeo, contacting the Area Control Center (ACC). The flight completed a 360° turn, momentarily rolling out on headings of 045° and 340°, before again proceeding north on a heading of 025°.
When about 5 nautical miles southwest of the VOR, the crew contacted ACC stating they were heading 025° and wished to proceed to Romeo to restart the approach, adding they had technical problems. The ACC confirmed the flight was to proceed to Romeo, and the crew agreed to report over Romeo. The CVR later revealed the crew was entering Romeo and related navigational information into the FMS but experiencing difficulties.
The flight continued north on heading 025°, then altered left to 005° at about 16 nautical miles north. Approximately one minute later, the GPWS sounded "terrain, terrain" followed by "whoop whoop pull-up." The aural warning continued for about 16 seconds until impact. Engine thrust was increasing, and "Level Change" was announced on the cockpit just before impact. The accident site was at the 11,500-foot level of a 16,000-foot peak, on the 015 radial at 23.3 nautical miles from the Kathmandu VOR. The aircraft was destroyed, and all 113 occupants were killed.
Probable Cause
The official investigation determined the probable cause as: TG311 flight crew's management of the aircraft flight path wherein the flight proceeded in a northerly direction which was opposite to the cleared point Romeo to the South; ineffective radio communication between the area control centre and the TG311 flight crew which allowed the flight to continue in the wrong direction, in that the TG311 crew never provided the aircraft's VOR radial when stating DME and the controller never solicited this information and thus the aircraft's position was not transmitted at any time; and ineffective cockpit crew coordination by the TG311 crew in conducting flight navigation duties. Contributing factors were: the misleading depiction of Romeo on the operator's approach chart used by the flight crew; a flap fault, although corrected, required that the initial approach be discontinued; radio communication difficulties between the TG311 crew and the air traffic controllers that stemmed from language difficulties and ineffective discussion of apparent unresolved problems.