UK air accident investigators have arrived in India as tributes have been paid to victims of the Air India disaster.

Flight AI171 crashed shortly after take-off from Ahmedabad Airport on Thursday, killing all but one of its 242 passengers.

The London Gatwick-bound plane – which was carrying 53 British passengers, 169 Indian nationals, seven Portuguese and one Canadian – crashed into a medical college and erupted in a huge fireball.

The sole survivor was Briton Vishwash Kumar Ramesh, 40. Around 30 more people on the ground died.

Air India officials and Indian government investigators have been at the crash site since Thursday, and were joined by four investigators from the UK’s Air Accidents Investigation Branch (AAIB) on Saturday. The AAIB said they have expertise in aircraft operations, engineering and recorded data.

A crane removes the wreckage of the Air India Boeing aircraft on Saturday. (AFP via Getty Images)
No initial findings have been disclosed so far, but Reuters reported investigators are looking at several aspects of the crash including issues linked to its engine thrust, flaps and why the landing gear remained open after take-off.

The second black box

India’s aviation ministry said investigators and rescue workers recovered the digital flight data recorder, one of the two black boxes on the plane, from the rooftop of the building on which the jet crashed.

There has been no information on the cockpit voice recorder, the other black box, which is also crucial to the probe.

Commercial aircraft have two black boxes: one that collects sounds from the cockpit and another that collects flight data such as altitude, speed and direction. (Getty)
The black boxes, which are actually coloured orange to make them easier to find in the event of a crash, will be critical in establishing what happened.

They are mandatory on civil flights, with their purpose being to preserve clues from cockpit sounds and data to help prevent future accidents.

According to Airbus, “flight recorders store data (aircraft parameters) and sound (pilot, copilot, radio communications and the cockpit ambient noise). The recording device is crash-protected up to a certain level. It is resistant to fire, explosion, impact and water immersion.”

Was the landing gear down when it shouldn’t have been?

Video of the plane before the crash has shown its landing gear was down. Usually, this folds back into the aircraft immediately after it becomes airborne.

Former British Airways pilot Alastair Rosenschein told Sky News: “It’s clearly got its [landing] gear down and that is not correct… it should have been up.”

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He added: “I cannot understand why the [landing] gear would have been down… [and] left down. That would suggest, perhaps, a hydraulic problem because it’s hydraulics that raise the gear.” However, he made clear this was speculation.

US aerospace safety consultant Anthony Brickhouse also said of the landing gear being down: “If you didn’t know what was happening, you would think that plane was on approach to a runway.”

Steve Scheibner, an American Airlines pilot, also suggested there were abnormalities with the plane’s wing flaps and linked this to the landing gear being down.

The plane crashed shortly after take-off. (Reuters)

Was there a mechanical or power failure?

Pilot Sumeet Sabharwal and his co-pilot Clive Kundar cried “mayday” as the plane lost altitude, saying the engine was “losing power”.

According to reports, Sabharwal, who had more than 8,000 hours of flying experience, said he had “no thrust” and was “unable to lift”.

And the Wall Street Journal reported on Friday, citing unnamed sources, that an investigation into the crash was focusing on “whether the aircraft had a loss or reduction in engine thrust”.

Dr Jason Knight, a senior lecturer in fluid mechanics at the University of Portsmouth, told The Telegraph: “It appears from the video there is a cloud of dust just after take-off.

“I’m not sure, but it appears as though the cloud of dust could be from the engines as they both fail.”

A twin-engine failure is extremely rare.

A bird strike to blame?

Reuters reported a possible bird strike is not a focus of the probe, though the BBC quoted experts familiar with Ahmedabad Airport as saying it is “notorious for birds”, which can cause engine failures.

Dr Knight also said “the most likely [reason] is a bird strike in both engines”.

But Scheibner said he didn’t think a bird strike is a “likely theory”. “We don’t see any birds in the picture. It would have to be a lot of birds to foul out both engines and we don’t see any indications coming out of the back of the engine that that happened: you’d see flames, you’d see sparks.”