Indian Airlines Flight 257

There were 69 people on board, consisting of 63 passengers and 6 crew members. According to the manifest, there were 68 adults and 1 infant on board Flight 257. A total of 32 passengers were booked for Imphal and 31 were booked for Dimapur. There were no foreigners on board.[3] Among the passengers were Irengbam Tompok, former Deputy Chief Minister of Manipur and Keisham Bira, former Minister of Education of Manipur.[4]

Of the 6 crew members, 2 were flight crew and the other 4 were cabin crew members. The flight was piloted by Captain Sekhar Heldar and Captain Roy Choudhury. Captain Heldar (37) had a total flying experience of 3,783 hours, of which 2,369 hours were on the Boeing 737 and the other 1,414 hours were on the Fokker F27 aircraft. Captain Choudhury (26) had a total flying experience of 1,647 hours, of which 1,397 hours were on the type. Both pilots were based in Calcutta.[1]

While there was no indication that the flight had been deliberately brought down, authorities decided to investigate on the possibility of a sabotage. According to eyewitnesses, the aircraft had gone down "in a ball of fire" over the nearby Loktak Lake. Approximately one day before the crash, a separatist group based in the neighboring state of Assam had threatened a hijacking of a flight from Calcutta to Jorhat, Assam.[2] Manipur itself was located in Northeast India, where low-level insurgencies were raging across the region at the time.[8][1]

As there was no evidence that the aircraft had been bombed, hijacked, shot down or sabotaged in any kind of way, investigators ruled out sabotage from the possible causes of the crash.[1]

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.[1]

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.[1]

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.[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. About a minute later, the crew told the tower that they were 12 miles (19 km; 10 nmi) inbound, suggesting that they had covered the last 13 miles (21 km; 11 nmi) in about a minute. The crew also reported that they were near the VOR for Imphal. In truth, the aircraft was actually 15 nautical miles (28 km; 17 mi) 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.[1]

A depiction of an offset/teardrop entry (green) to an airport holding pattern (blue). In this image, the entry is from the north. The red dot is the "fix" point. In the case of Flight 257, the entry was from the southwest and the fix point was the VOR for Imphal.

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). 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 nautical miles (12 mi; 19 km) southwest from the airport's VOR, they were somewhere among the hills of Thangjing range.[1]

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 feet (2,100 m). 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.[1]

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.[1]

Investigators did not note any abnormality from the approach of Flight 257 until the flight passed over Comilla Airport. 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 aircraft was supposed to head over to Agartala, but the crew intentionally deviated from its heading. They later gave the controller a false position report by saying that they were over Agartala, even though the aircraft was virtually nowhere near the said location. This would have given a false impression to the ATC that they had followed the flight plan. Fellow pilots who had been interviewed by investigators described the actions of the crew as "total, grossest indiscipline".[1]

Imphal Airport was not equipped with radar. Subsequently, the ATC could not know the exact position of the pilots and could only rely on intermittent positional reports from the pilots. 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. Imphal Airport itself was located in a mountainous area and the weather condition was known for its unpredictability. If the pilots decided to falsify their actual position, ATC could not help much on the matter.[1]

The decision to not head over to Agartala was probably to cut time. 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. The aircraft then inexplicably put into a descent much earlier than their expected time to descend. According to investigators, the crew probably thought that they were already near the VOR even though their actual distance was still too far from the VOR. This was probably influenced by the difference between the flight plan's airspeed and the true airspeed of the aircraft, wherein the actual airspeed was higher than the supposed airspeed, causing the crew to think that they would reach their destination much more earlier than usual.[1]

At 12:39 IST, the crew transmitted another positional report again. 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. Their request however was immediately granted by the ATC. Fellow senior pilots reported that, for an approach to Imphal, it is necessary for pilots to fly above the VOR first (the fix point) before conducting a direct landing.[1]

In accordance with Flight 257's flight plan and the approach chart for Imphal, Flight 257 was expected to arrive at the airport from the southwest, flying with a northeast direction with a heading of 070. It would then reach the VOR and descent from 10,000 to 8,000 feet (3,000 to 2,400 m) in order to enter the holding pattern for Imphal Airport. The VOR for Imphal was located at 24°46′01″N 93°54′28″E. The holding pattern itself was located northeast of the airport, above the city of Imphal. 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). After completing the holding pattern, the aircraft would continue towards a turn (radial) of 217 degree with a speed of no more than 170 knots (310 km/h; 200 mph). Later, the crew should descend to 4,200 feet (1,300 m) to capture the localizer for its inbound leg. The localizer would eventually help the crew to safely land at the airport.[1]

According to the report, the VOR was also crucial for the approach as it was used by passing aircraft as a time reference for the landing. 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. Disobeying the approach procedure by not flying over the VOR would have caused the pilots to lose, not only their time reference but also precise location that had been set by the written procedure, enabling them to fly above the areas that had not been covered by the airport's navigational aids.[1]

At 12:11 IST, the crew stated that they were approaching the VOR of Imphal Airport, when in actuality they were still 15 nautical miles (28 km; 17 mi) south-west from the VOR. The pilots had possessed false knowledge on their exact position. Few minutes later, the aircraft turned to the right and started to descend to 10,000 feet (3,000 m). By doing so, they could not follow the airport's ILS. Flight 257 was already way off course as it was being flown across uncharted territory. 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.[1]

One of the crew later stated "two minutes up", indicating that they had accounted the exact time for the outbound leg. 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. The crew then announced that they were commencing the turn (radial). The crew thought that they were inside the protected area of the holding pattern (the inside of the holding), but the aircraft was not even in the holding pattern. It was actually somewhere between the hills southwest of the airport.[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. The crew had taken a short-cut by deviating from the flight track. There were instances when the pilots had deliberately sent false reports on their actual position. 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.[1]

Checks on the pilots flight history revealed that the pilot flying (PF) of Flight 257, Captain Heldar, had flown for at least 36 times to Imphal. Indian Airlines did not set a limit for pilots to use the same route so occasionally pilots chose to fly the same route over and over again. Over time, Captain Heldar requested the flights to Imphal to be given to him. As a result, Captain Heldar had been familiarized with the route. This eventually led Captain Heldar to be a bit over-confident. In the CVR recording at 12:38 IST, Captain Heldar could be heard saying "this is one hill, that is another hill and in between the two." His familiarity with the surrounding terrain caused him to estimate his calculation and distance on the basis of the features of the terrain rather than on the aircraft's instruments.[1]

While there were clear violations of the proper procedure for an approach to Imphal, Captain Heldar was accompanied by Captain Choudury in the cockpit. Captain Choudury should have corrected Captain Heldar's actions, but the CVR indicated that he did not give any kind of caution, advice or corrective responses to Captain Heldar's actions. Investigators described his demeanour on the entire flight as a "mere passive and mute spectator" in the cockpit. Hence, Captain Choudury was also blamed by the investigators for being equally undisciplined and lacking in professionalism.[1]

The final report was submitted to the Indian government on 30 April 1992, approximately eight months after the accident. 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.

— Board of Inquiry[9]

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.[1]