On 16 August 1991, the Boeing 737-2A8 registered VT-EFL crashed into the hilly terrain of Thangjing Hill during its descent, killing all 63 passengers and 6 crew members on board.

The crew decided to violate the approach procedure by taking a short cut and eventually slammed the aircraft into the hilly terrain.

The flight was supposed to take about 60 minutes and later on the aircraft would have continued its journey to Dimapur in the neighboring Nagaland state.

Imphal Tower granted their request and later asked the crew to descend to 10,000 feet (3,000 m) and to report back to the ATC if they had reached the VOR for the runway.

Imphal Airport lost contact with the aircraft just after 12:45, at 1,500 metres (4,900 ft) altitude on the Instrument Landing System (ILS).

Due to clouds, the aircraft eventually discontinued the search and immediately flew to its intended destination of Guwahati.

ATC then tried to give the crew of Flight 257 several blind calls by clearing them to land at the runway, but the transmission was met with silence again.

Few hours after the formation of a search and rescue party, several villagers reported that an aircraft had crashed somewhere in the Thangjing Hill range.

Due to the hilly and slushy terrain, added with the heavy rain condition at the crash site, the evacuation of the bodies were hampered and had to be delayed until dawn.

[6] Indian Minister of Civil Aviation Madhavrao Scindia along with senior officials from the ministry visited the crash site on the next day.

[7] On 26 September 1991, the Indian government appointed permanent judge of Calcutta Umesh Chandra Banerjee to hold a formal investigation into the crash.

The flight data recorder had only captured 5 parameters: altitude, heading, airspeed, vertical acceleration and time.

[1] By 12:38 local time, Captain Heldar informed co-pilot Choudury that they were approximately 25 nautical miles (46 km; 29 mi) from Imphal, based on his observation on the geographical features in the area.

The crew then asked permission from the controller to clear them to set a direct outbound track, which was granted by the ATC.

[1] Approximately 14 nautical miles (26 km; 16 mi) southwest from the VOR, the crew reported that they were on the outbound leg, turned the aircraft to the right and announced that they were going to descend to 5,000 feet (1,500 m).

With a distance of approximately 10 nautical miles (12 mi; 19 km) southwest from the airport's VOR, they were somewhere among the hills of Thangjing range.

As the aircraft was nowhere near any kind of navigational devices of the airport, the instruments that were supposed to tell the crew that they were on the right track (i.e. outer marker signal) eventually could not sound.

As there was no more radar coverage from Comilla to Agartala and Imphal, the pilots, according to the investigators, started to innovate their own procedure and new flight plan by cutting corners in several places.

The controller would then assist the pilots to finally reach the airport in a safe manner, away from the mountainous terrain around the city of Imphal.

As a result of this short cut, the crew revised their estimated time for arrival in Imphal for approximately four minutes earlier than the previous ETA.

This time, the crew requested for a direct track to the outbound leg, even though they had not reached the VOR (fix point) for an entry to the airport's holding pattern.

Upon reaching the holding pattern, the crew should proceed to the outbound leg of the teardrop entry for approximately 2 minutes and 30 seconds and descend the aircraft further to 5,000 feet (1,500 m).

By using the VOR, pilots could estimate the exact time that would be needed to conduct a proper entry into the airport's holding pattern.

Signals from the airport's beacon eventually could not be received by the aircraft as it was not flying on the proper ILS track anymore.

According to the investigation, this was also proof that the pilots had actually understood the proper procedure for an approach, however they decided to disobey it.

[1] Local weather station reported that the sky was in overcast condition with a total of 8 okta in the area of Thangjing Hill range, meaning that the visibility would have limited the crew's vision.

Despite being false on their exact position, added with the deteriorating weather condition in the area, the crew decided to continue their flight to Imphal.

The investigation concluded the cause of the crash as follow: The accident occurred by reason of a grave error on the part of the Pilot-in-Command in not adhering to the operational flight plan and ILS let down chart and not realizing that his early descent to 10,000 feet (3,000 m) and turning right for outbound leg without reporting overhead VOR would result in loss of time reference and as such misplace him in the hilly terrain.

The Pilot-in-Command's action may have been influenced by his extreme familiarity with the terrain.As a result of the crash, a total of 50 recommendations were issued by members from the Board of Inquiry, particularly on flight crew training and discipline.

The Board of Inquiry also issued a recommendation for the establishment of an independent investigative body for transport accidents in India.