Sutlej Missing Over Indian Ocean, Part 2

16 Jun 17 4 Comments

Last week, I posted part one of a chapter from my upcoming book, Without a Trace, which explores over two dozen aviation mysteries. Here is the rest of the chapter!

The most recent mysterious disappearance was just one year ago in July 2016, when an Indian Air Force aircraft vanished in the middle of a routine courier flight.

The military flight was flying from Chennai to the Andaman and Nicobar Islands, a federal territory of India.

The aircraft was an Antonov An-32, a Soviet-built twin-engined turboprop military transport plane. These are known as as Sutlej in the Indian Air Force, named after the Sutlej River, the longest of the five rivers that flow through the crossroads region of Punjab in northern India and Pakistan.

The An-32 needs less than 3,000 feet of runway to take off and land (well, two runways or time to go around if you want to do both in rapid succession). It’s easily able to deal with high elevations and hot conditions and so extremely useful for the Indian Air Force. The An-32 is regularly referred to as the “workhorse” of the transport fleet.

However, in an odd foreshadowing of the modern disaster, forty years earlier in 1986, an Indian Air Force An-32 went missing over the Indian Ocean, carrying three crew and four passengers, under inexplicable circumstances when the aircraft fleet was still being delivered.

There are still a hundred of the An-32s in service in the Indian Air Force: mid-life extension and upgrades of the fleet started in 2010 and a further forty upgraded aircraft had arrived from Kiev in the five years before the accident flight. Upgrading the An-32s was a slow and ongoing process.

This particular An-32, callsign AF330, was part of the 33 Squadron in Sulur. On the 22nd of July in 2016, it was flying a routine courier flight, ferrying twenty-three passengers from Tambaram Air Force Station in Chennai to the Veer Savarkar International Airport at Port Blair. It’s just over three hours flight from Tambaram to Port Blair, which has a major Indian naval base as well as sea and air bases of the Indian Coast Guard and Indian Air Force.

The passengers consisted of eleven members of the Indian Air Force, two soldiers from the Indian Army, a member of the Indian Navy and a member of the Indian Coast Guard and eight civilians from Visakhapatnam who worked with the Naval Armament Depot.

The flight departed without incident at 08:30 local time for the flight, which was expected to land at Port Blare at 11:45 after the three hour 15 minute flight. The aircraft carried fuel for four hours flight.

At 09:12, 45 minutes after departing Chennai, the An-32 was 280 km (170 miles) east of Chennai, flying over the Indian Ocean. They were nearing the end of the range of the primary radar coverage from the Indian mainland and, as a military flight, they were apparently not squawking for secondary radar coverage. Another ten minutes flight would have put the aircraft into a known black spot, an area about 150-200 nautical miles (175-230 miles or 280-370 km) with no radar coverage from either Chennai or Port Blair.

But the flight never made it that far. The An-32 was still within primary radar coverage when the flight disappeared off of radar. It seems likely that, because it was near the black spot, there was no real concern about the flight. After all, there were no distress calls or any mention of difficulties. The crew had been in touch with the controller just a few minutes before, simply noting that they were deviating to the right. The controller understood this to be to avoid some weather in view ahead. There was certainly no reason to believe that the aircraft was in danger.

An hour after it had been due to land at Port Blair, there was still no word and An-32 callsign AF330 was officially declared missing.

The aircraft was never heard from again.

The facts were few. Seven minutes before the aircraft disappeared off of radar, the crew told air traffic control: ‘We are deviating to the right’. Then a few minutes later, the aircraft tilted to the left and descended rapidly, losing 23,000 feet in seconds.

The Indian Navy and the Indian Coast Guard launched a search party as soon as it was clear that the aircraft was missing, sending out twelve ships, five aircrafts and a submarine. They searched the area where the radar coverage had been suddenly lost but found no sign of the aircraft.

The sea was very choppy and thick cloud cover in the area hampered the search but aircraft and ships all rushed towards the region in hopes of finding some trace of the aircraft and its passengers. The Air Force put all available assets into the search operation and the Indian Navy and Indian Coast Guard immediately began deploying air and surface vehicles to the region.

The following day, the search continued. By the third day, a further sixteen ships and six aircraft joined the search now spread across the Bay of Bengal – 150 nautical miles east of Chennai.

The An-32 had two emergency locator transmitters but they did not work underwater. None of the An-32 fleet had underwater locator beacons as they weren’t considered ‘primarily a marine/maritime aircraft’. This meant that if the aircraft had crashed into the sea, which seemed likely, there was no signal to pick up.

The aircraft also carried four life rafts, each of which was fitted with a Personal Locator Beacon which would activate upon coming in contact with water. However, they only operate on the surface of the sea so could only be tracked if those on board the aircraft had managed to evacuate and remove the life boats. As it was, no signal from the personal locator beacons was ever picked up.

The military could only hope that they might find traces of the wreckage or at least some debris.

Indian survey ships scoured the seabed along with the submarine but with a water depth of 3,000-4,000 metres (9,800-13,000 feet), specialist equipment was needed to search the area properly.

The Indian Navy, known for their reluctance to ask for foreign support, quietly requested the help of the Russian Navy rescue ship Igor Belousov to help locate the missing aircraft. The brand new ship was designed for search, rescue and salvage of distressed ships and submarines, with submersibles that can sent down to 700 metres (2,300 feet) and lift objects back up to surface. The Russian Navy agreed and diverted the ship, which spent over 12 hours in the surface search zone. They never found any sign of the wreckage and reluctantly left to continue to the ship’s original destination.

The Defence Minister also contacted the US to ask if any of their satellites had captured signals from the An-32 before its sudden disappearance. However, this required a satellite to be crossing the area at the time and even then, with thick cloud cover in the area, the chances were slim. The Indian satellites had not recorded any signals and neither, it seemed, had the US satellites.

With no recorded signals and no underwater locator beacon, the focus had to be to search for visual clues: debris, oil slicks or floating wreckage washed on shore. However, the search area could not be reduced down to a sensible level, which meant that thoroughly searching the area would take months, during which time the visible signs of the crash into the sea would be long gone.

Search area as of 25 July from the Indian Navy website (

The search and rescue operation is to this day India’s biggest search operation at sea. More than 201 search and rescue sorties spent over 500 hours searching over an area of 217,800 square nautical miles, in addition to extensive searches on the coast in hopes of finding some debris washed up from the aircraft.

By the 12th of August, hope was fading. By now, the search aircraft had flown over 1,000 hours and at one stage, 28 ships had been deployed.Although it had been the most intense search ever in Indian waters, no sign of debris had been found. The Navy refocused its efforts from the surface to the seabed, hoping to find the sunken wreckage. They detected two dozen electronic transmissions but, as with the first month of searching for MH370, they were all unrelated. None of the signals were from the missing aircraft or its life rafts.

The National Institute of Ocean Technology and the Geological Survey of India both sent specialised surface vehicles to the point where the An-32 was last seen on radar, using multi-beam echo sounders to profile objects on the sea floor. Both ships also used sonar equipment meant to detect objects on the sea bed. Neither ship saw any sign of anything that could lead them to the final resting place of the An-32. It had vanished.

On the 15th of September, almost two months after the aircraft had disappeared, the search and rescue mission was called off. The Indian Air Force declared that there were no chances of survival and everyone on board was declared dead. The family members were told to sign the ‘certificate of presumption of death’ to start the legal proceedings for compensation.

Once the military gave up on the search, the chances of finding any answers were slim. The only thing they really knew was that for some unknown reason, the aircraft had abruptly turned left without warning before rapidly descending 23,000 feet.

Sabotage was deemed unlikely, with the Defense Minister explaining that everyone on board was from the ‘defence forces’ and had standard operating procedures which had not been initiated. This has not, of course, stopped speculation that the crash was the result of high-tech terrorism.

Another theory was that of faulty maintenance on the aging fleet. Local press claimed that the An-32 was ‘not in the best condition’ when it undertook that flight from Chennai to Port Blair.

The missing An-32 callsign AF330 had received a full overhaul at 1 Base Repair Depot (BRD) in Kanpur less than one year previous and had flown 279 hours since then. However, it had also had a large amount of maintenance in the weeks before its disappearance with logged reports of sluggish throttle movements, a hydraulic leak in the left-hand wing root (where the wing extends from the fuselage), and a pressure leak at the left-hand door.

If the pressure leak in the door had not been properly dealt with, it could have burst suddenly, causing an explosive decompression. Explosive or rapid decompression is caused by a sudden structural failure in a pressurised environment, where the speed of the decompression is faster than air can escape the lungs. In this instance, even if the structural integrity of the aircraft allowed for continued flight, it is quite likely that the passengers and crew would have had no chance to react.

A recurrence of the leak itself could have led to gradual decompression, where the decompression is slow enough that the issue goes unnoticed. In this case, there is much more time to react. If the leak was known, the pilots would have put on their oxygen masks and declared an emergency (in that order!), so that can be discarded as a possibility. Even if they did not know that the leak was slowly depressurising the aircraft, there are instruments in the An-32 to warn that the aircraft is depressurising, but it is possible that the effects of hypoxia were such that the crew and passengers disregarded the warnings, as happened 10 years earlier in the case of Helios Airways Flight 522.

However in the Helios case, the aircraft continued to fly for hours after the flight crew had lost consciousness. If the An-32 had suffered a similar slow failure, there’s no reason why the aircraft itself would not have continued in a straight line for some time, allowing it to come into radar range of Port Blair.

Another possibility is that the hydraulic leak was not correctly dealt with, which would mean that the aircraft became uncontrollable. However, as they were cruising at over 20,000 feet, even if the aircraft hydraulics had failed, there should be plenty of time to make a distress call.

None of these theories answers the question of what happened to AF330 in any satisfactory way.

In India, the focus shifted on beefing up the maintenance, repair and overhaul of the existing fleet. As most of India’s defence equipment is purchased from foreign manufacturers, they have a very clear challenge when it comes to maintaining their fleet.

The Indian Air Force prioritised fitting underwater locator beacons onto the remaining An 32 fleet which was expected to fly over the sea, a project which was already in progress but had only been moving ahead slowly. Originally planned to be fitted as an integral part of the black box, the Ministry of Defense decided to instead push forward with a stop-gap effort, installing stand-alone underwater locator beacons to the aircraft as an emergency measure.

Indian Air Force cargo planes An-32 during the maintenance and equipment upgrade in Ukraine - photograph by dragunov

Unfortunately, the pressure to expedite the upgrading of the fleet has lead to even more disappearances as a result of the conflict in the Ukraine.

Sixty-four An-32s in India have not been able to be upgraded locally as the Ukraine state-owned arms trading company, who had agreed to upgrade the 104 transport aircraft, is no longer supplying the Indian Air Force the engineers to do the work nor parts required to complete them.

Meanwhile, forty An-32s were sent to the Ukraine to have the maintenance done in Kiev. Only 35 were returned.

The remaining five An-32s? They have disappeared.

An Indian Air Force official has admitted that they have become untraceable. A diplomat from the Ukraine Embassy said that the Indian Air Force must resolve the issue of the missing aircraft directly with Antonov and that the Ukrainian government cannot help.

As a result, the Indian Air Force have a lost a total of seven Antonov An-32s. The circumstances are different but the fact remains that all seven have disappeared without a trace.


  • Another strange occurrence.
    If I may speculate:
    Loss of pressurisation should not in itself cause an aircraft to crash.
    I had a rapid decompression twice, once at flight level 370 (37000 feet) and once at FL 390. It was not really a “big deal”. How a crew can miss the unmistakeable warnings that illuminate once the cabin pressure is less than the equivalent of 10.500 feet is something I don’t quite understand. Decompression is at the bottom of my list of possible causes.
    The crew diverted from their track and informed ATC. This suggests that, indeed, they were seeing some weather on their radar that they wanted to avoid.
    Without weather reports of actual thunderstorm activity, or other weather phenomena that could pose a hazard, we are once again speculating.
    What happened? Did the aircraft break up in sudden, violent turbulence? Did the aircraft have a previously unnoticed structural weakness? Did the aircraft suffer a sudden electrical or instrument failure in IMC causing vertigo in the crew?
    We are unlikely to ever know.

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