Indian Airlines Flight 257

Indian Airlines Flight 257 was an Indian Airlines domestic passenger flight operating on the Calcutta–Imphal–Dimapur route. On 16 August 1991, the Boeing 737-200 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 crash was attributed to the pilot's non-adherence to the written procedure for an approach to Imphal. The crew decided to violate the approach procedure by taking a short cut and eventually slammed the aircraft into the hilly terrain. During the approach, the crew did not adequately monitor their instruments, despite the deteriorating weather condition at the time. Their actions were influenced by the pilot's decision to solely rely on his extreme familiarity with the terrain.

Accident

Flight 257 took off at 11:54 a.m from Calcutta to Imphal, the capital of the Indian state of Manipur in the Northeast. 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. Captain Sekhar Heldar was the pilot flying (PF) and Captain Roy Choudhury was his co-pilot. For a flight to Manipur, the aircraft was cleared to fly at 29,000 ft.

At 12:34 p.m, after contacting Chittagong's Comilla Airport and Tripura's Agartala, the crew was asked to contact Imphal Tower for a descent clearance. Imphal Tower granted their request and later asked the crew to descend to 10,000 ft and to report back to the ATC if they had reached the VOR for the runway. Before Flight 257 had reached the VOR, Imphal Tower asked for another position report from Flight 257. The crew then requested to set course for a preparation to land at Imphal, which was granted by ATC.

The crew reported that they were above the supposed VOR. After reporting back on the VOR, they were asked to inform the tower on a procedural turn towards the airport and later were asked to descend to 5,000 ft. After the transmission, another Indian Airlines was about to take off from Imphal. Flight 257 then reported that they were commencing the turn. ATC later warned the crew that there was rain on the eastern area of the airport, which was acknowledged by Flight 257. The ATC then turned their attention to the departing Indian Airlines flight. Following the departure of the said flight from Imphal, the ATC then contacted the crew of Flight 257 again.

This time, there was no response. Imphal Airport lost contact with the aircraft just after 12:45, at 1500 m altitude on the Instrument Landing System (ILS). Several other flights then tried to contact Flight 257 as well, but there was still no response from the crew. Firefighting personnel were alerted and crew from other Indian Airlines were asked to contact on their company channel. The departing Indian Airlines aircraft was later asked to fly to the area where Flight 257 was last contacted. 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.

At 12:55 p.m, a full emergency was declared on the missing aircraft. Airport personnel immediately contacted Imphal's airport and city firefighters and ambulance services to assist with the search and rescue effort. The search and rescue area was concentrated within 5 - 15 mi southwest of the airport. Another Indian Airlines aircraft in the vicinity was later asked to look for the missing aircraft, but due to deteriorating weather condition the aircraft eventually discontinued the search. A 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.

The search and rescue team eventually reached the site about 5–6 hours later. It was located in Churachandpur district at an altitude of 6,700 ft, just 300 ft below the highest peak of the area, about 37 km south-west of Imphal Airport. The aircraft was destroyed as it had slammed head-on with the slope of the hill. Bodies could be seen around the wreckage. There were no survivors. All 69 passengers and crews on board Flight 257 were killed. Due to the hilly and slushy terrain, added with the heavy rain condition at the crash site, the evacuation of the bodies was hampered and had to be delayed until dawn. By 19 August, all of the bodies had been recovered and identified.

Flight recordings

Both flight recorders were recovered from the crash site in damaged condition. The data inside both recorders however was usable. Both recorders were sent to New Delhi for data extraction. The flight data recorder had only captured 5 parameters: altitude, heading, airspeed, vertical acceleration and time. The data of vertical acceleration was described as "limited" as the device had only recorded the aircraft's vertical acceleration during its take-off from Calcutta. Meanwhile, the cockpit voice recorder managed to obtain good quality of the intra-cockpit conversations.

The flight was uneventful throughout the take-off phase. After crossing Comilla Airport in Bangladesh, Flight 257 began to deviate to heading 053, significantly off to the right from its supposed heading of 007, by-passing the planned waypoint of Agartala. Instead of going over Agartala, the crew decided to fly nearly direct towards Imphal. The crew, however, reported to the controllers that they were above Agartala, even though in reality they were nowhere near the said location. The heading that had been put by the crew was maintained for approximately 12 minutes. The crew then turned the aircraft further right to heading 081 for another 9 minutes, before turning left a bit to around heading 070.

After executing the short cut, the crew reported to Imphal about the revised estimated time of arrival (ETA) of 12:42 IST, 4 minutes ahead of its previous ETA. As per the given flight plan, Flight 257 was expected to start its descent about 45 minutes after its departure from Calcutta. By taking the revised ETA into account, the aircraft should start its descent at 12:35 local time. The crew, however, decided to descend early, approximately 6 minutes short of the supposed time to commence descent. If the revised ETA was not taken into account, the aircraft had actually descended 10 minutes short of the flight plan's expected time to descend.

By 12:38 local time, Captain Heldar informed co-pilot Roy Choudhury that they were approximately 25 nmi from Imphal, based on his observation on the geographical features in the area. About a minute later, the crew told the tower that they were 12 mi inbound, suggesting that they had covered the last 13 mi in about a minute. The crew also reported that they were near the VOR for Imphal. In truth, the aircraft was actually 15 nmi from the VOR. The crew then asked permission from the controller to clear them to set a direct outbound track, which was granted by the ATC. According to the CVR, the crew was trying to attempt an offset entry for the airport's holding pattern.

Approximately 14 nmi 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 ft. The aircraft started to descend and the turn was eventually completed approximately two minutes later. By this time, the aircraft was still too far away from the airport and was nowhere near the holding pattern. With a distance of approximately 10 nmi southwest from the airport's VOR, they were somewhere among the hills of Thangjing range.

The aircraft then changed its heading from 196 to 210 for about a minute and 35 seconds. It had descended to an altitude of 6,880 ft. At 12:44 p.m, one of the crew exclaimed "Two minutes up", referring to the usual expected time for an outbound leg towards the airport. In reality, the crew had only taken about one minute and 35 seconds. The crew then initiated the procedural turn to the left. 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.

Flight 257 had gone off course from its track and eventually flew onto areas that had not been charted by the airport. Six seconds before impact, the GPWS warning repeatedly sounded. For a split second, the crew attempted to put the aircraft into a climb but due to insufficient altitude and distance between the aircraft and the terrain, collision was unavoidable.