Hundreds of people died after an Air India flight plummeted on 12 June, marking the world’s deadliest air disaster in a decade. But what are investigators looking at now – and how long could the investigation take?

The devastating Air India plane crash which killed 229 passengers and 12 crew has been labelled a “mystery” by experts.

The Gatwick-bound flight, which was carrying 53 Britons, came down just after take-off from Ahmedabad airport in western India on Thursday 12 June, leaving only one survivor and also killing people on the ground.

CCTV footage was captured of the crash, but experts say it has led to more questions than answers.

An ongoing investigation will be reviewing the footage and other key evidence left in the crash’s wake.

But what are investigators looking at – and how long could it take?

What is unusual about the crash?

Air India Flight 171 took off from Ahmedabad Airport at 1.38pm local time on Thursday, and was only in the air for around half a minute.CCTV footage shows the plane struggling to gain altitude and it quickly begins to descend towards buildings, with its wheels still out and its nose raised.

It crashed into a densely populated neighbourhood in Ahmedabad, killing at least 29 other people on the ground.

According to flight-tracking website Flightradar, the aircraft reached a height of just 625ft before crashing.

The plane was a 12-year-old Boeing 787 Dreamliner, of which there are currently around 1,200 in operation worldwide.

While other Boeing planes like the 787 Max have been plagued by high-profile safety incidents, this was the first deadly crash in 16 years of the Dreamliner model’s operation, according to experts.

Boeing under scrutiny after Air India crash

Several experts have told Sky News that they spotted potential anomalies in the footage.

One of them was the landing gear, which appears to remain open throughout the clip.

Former British Airways pilot Alastair Rosenschein and Paul Edwards, an aviation security analyst and fellow at the Royal Aeronautical Society, have told Sky News the landing gear should have been up and that this remains a mystery.

Former pilot analyses footage

Mr Rosenschein is also among the experts who have called into question the aircraft’s flaps, which need to be set correctly as they extend the shape of each wing and create vital extra lift at lower speeds to enable the plane to take-off and climb effectively.

Some have also suggested a bird strike could be linked to the crash, but experts have pointed out that the aircraft has two engines and is designed to be able to fly on one, making this unlikely.

What are investigators looking at?

The investigation is being carried out by India’s Aircraft Accident Investigation Bureau (AAIB) with assistance from the UK, the US and officials from Boeing. Anti-terror squads have also been part of the investigation teams, according to Reuters.

Aurobindo Handa, former director general of AAIB, told Sky News that any theories about what may have caused the incident are at this point purely speculative.

He says the most crucial thing for investigators was to retrieve the black boxes – or Digital Flight Data Recorders (DFDR) – from the crash site.

Black boxes have two components – the flight data recorder and cockpit voice recorder – and provide crucial insights for crash investigators.

File photo of a black box found on planes. Pic: AP
Image:File photo of a black box found on planes. Pic: AP

They include altitude, airspeed, the status of controls and pilot conversations which help determine probable causes of crashes.

They also store essential information about the configuration of the aircraft, including the flaps and landing gear.

Mr Handa said that when there is a crash, the retrieval of the black boxes becomes the “primary focus” straight after first responders have carried out their rescue mission.

It took investigators more than a day to recover the black boxes, longer than it took in many of the investigations Mr Handa oversaw, and he said this would have been because the aircraft was badly charred from fire.

Indian newspaper The Economic Times reported that they were due to be sent to the US so the data could be extracted, as they had suffered fire damage and could not be analysed in India.

The government said in a statement that the AAIB would decide where the recorders would be examined after making a “due assessment” of all technical, safety and security considerations.

Mr Handa said analysing the boxes’ raw data will take some time.

“There are hundreds of components and instruments giving feedback to the DFDRs,” he explained. “All the control services, all the engine parameters – everything goes into this.”

Investigators are also looking at the plane’s remnants from the crash site, which they compare with the flight data when coming to conclusions.

But Mr Handa said the investigators will likely be particularly reliant on the black box data in this instance because so much of the plane will be unrecoverable due to the fires.

Smoke and debris at the crash site. Pic: Reuters
Image:Smoke and debris at the crash site. Pic: Reuters

They will also be scanning CCTV footage of the nearby area and speak with witnesses to get to the root cause of the crash, according to Amit Singh, a former pilot and an aviation expert.

Reuters has reported that investigators are assessing why the landing gear was open, citing a source “with direct knowledge”, and that they are also looking at any possible operational issues with flaps.

A possible bird strike is not a focus of the investigation, Reuters has reported.

How long could the investigation take?

Investigators across the world follow a standard UN-prescribed Manual of Accident Investigation, also called DOC 9756, which outlines detailed procedures to arrive at the most probable cause of a crash.

In the case of a high-profile investigation of a major accident, countries are encouraged by the manual to publish a preliminary report within 30 days of the incident, but this is not mandatory.

The guidance says the state should aim to release its final report “in the shortest possible time and, if possible, within twelve months of the date of the occurrence”.

It adds that if this is not possible, the state should release an interim statement on each anniversary of the incident detailing the progress of the investigation and any safety issues raised.

Mr Handa told Sky News the findings will likely not be published anytime soon, even if the investigators believe they have found the probable cause of the crash early on, because they must examine and analyse every possibility and every piece of data available to them.

He added that it is not just a case of finding the cause of the crash, but also uncovering the chain of events that led to that failure.