What happened
While performing the Sierra (VOR/DME) approach to runway 02 at Tribhuvan International Airport under instrument weather conditions, the crew of the Boeing 737 experienced a flap malfunction. Following the fault, the crew requested a return to Calcutta to comply with company performance requirements, which require full flaps for the steep approach profile. Although the flap issue was temporarily resolved by cycling the flap lever, the crew determined they could not continue the current straight-in approach due to their position and the required descent angles. They requested a left turn toward the Romeo fix.
In a non-radar environment, the controller cleared the aircraft to proceed with the Sierra approach. The crew reiterated their inability to land and requested a turn back to Romeo to restart the approach. During subsequent communications, the crew began a climb from 10,500 feet toward FL180 while turning right. As the aircraft maneuvered, the crew was also tasked with maintaining FL115 and proceeding to Romeo to contact Area Control.
As the flight progressed, the crew attempted to program new navigational data into the Flight Management System but encountered difficulties. While flying on a heading of 005°, the Ground Proximity Warning System issued terrain alerts. Despite increasing engine thrust and an automated 'Level Change' command, the aircraft struck a 16,000-foot peak at an altitude of approximately 11,500 feet. The impact occurred roughly 23.3 nm from the Kathmandu VOR. The crash resulted in 113 fatalities and the total destruction of the aircraft.
Findings
- The crew was preoccupied with programming navigational information into the Flight Management System during the flight.
- A flap malfunction initially prompted the decision to deviate from the planned approach.
- The aircraft was operating in a non-radar environment, which complicated communication and separation.