A year after Air India Flight 171 to London crashed into a medical college campus moments after take-off from the western Indian city of Ahmedabad, killing 260 people, investigators still cannot say with certainty why one of the world's most advanced passenger jets fell from the sky.

An update released by India's Aircraft Accident Investigation Bureau (AAIB) on the first anniversary of the disaster on Friday offered few new clues, saying only that analysis of flight recorder data, aircraft systems, engine components, maintenance records and human factors remains under way.

A preliminary report published last July found that seconds after take-off, the 12-year-old Boeing 787 Dreamliner's fuel-control switches abruptly moved to the "cut-off" position, starving both engines of fuel and and triggering total power loss.

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Cockpit audio captured one pilot asking the other why he had done it, only to receive the reply: "I did not." Investigators did not identify either voice, though many experts saw the exchange as a possible indication of deliberate action in the cockpit.

The crash remains highly unusual.

While take-off and landing are aviation's riskiest phases, fatal accidents immediately after lift-off are uncommon. Boeing found that just 14% of global jet crashes between 2004 and 2013 occurred during take-off and initial climb; Airbus puts the figure at about 5%.

So what brought down AI171 in 32 seconds?

As the investigation enters its second year, several key questions remain unanswered.

John Cox, a former airline pilot and aviation safety consultant, told the BBC that India's AAIB was entitled under International Civil Aviation Organization (ICAO) rules to take more time if necessary.

"There is intense interest within India as to the cause," he said. "The insinuation that it was a deliberate act by the captain has drawn very strong criticism. The timing of the engine failure is key to determining the cause."

Cox said any report must establish precisely when the engines lost power, when the fuel-control switches moved and whether the aircraft experienced technical issues on the accident flight or preceding sectors. "Those questions must be resolved," he said.

The lack of answers after a year suggests investigators are still weighing multiple possibilities, according to Shawn Pruchnicki, a former accident investigator and aviation expert at Ohio State University.

"Air crash investigations are rarely straightforward," he told the BBC. "If investigators had already established a clear cause, the report would likely be out by now."

The continuing delay, he argues, points to competing hypotheses, unresolved leads and possible mechanical issues that have yet to be fully explained.

Not everyone believes the delay is simply a reflection of investigative complexity.

A veteran Canada-based air accident investigator, speaking anonymously to the BBC, said that final reports are sometimes delayed because their conclusions are "politically or institutionally sensitive".

But he warned that "continued speculation about the cause risks muddying the waters further, making it harder for investigators to complete their work – and for the final report, whenever it arrives, to command public trust".

Air-crash investigations often unfold in stages. The inquiry into Air France 447, which crashed in 2009, released a series of interim findings before a final report was published three years later.

Air-crash investigations are usually dry exercises in fact-finding. The AI171 inquiry has become anything but.

The preliminary report's finding that the fuel-control switches moved shortly before both engines lost power prompted speculation in parts of the foreign media that a pilot's actions lay at the heart of the disaster. (At the time of take-off, the co-pilot was flying the aircraft while the captain was monitoring.)

That has triggered a backlash from pilots' groups, safety campaigners and lawyers for victims' families, who say the focus on the cockpit has raced ahead of the evidence.

Captain CS Randhawa, head of the Federation of Indian Pilots, argues investigators should pay closer attention to the aircraft's technical condition, including "encrypted health-monitoring messages [that routinely transmit data on engines, avionics and other critical system] transmitted before and during the flight".

The preliminary report, however, did not mention any such messages. "The preliminary report is incomplete and full of loopholes," Randhawa told the BBC.

According to the Canada-based investigator who didn't want to be named, the inquiry has become "unusually contentious because so many stakeholders have a vested interest in its outcome".

"Families of the deceased pilots are defending their loved ones' reputations; pilot unions are resisting conclusions they believe unfairly implicate the crew; the airline is keen to demonstrate that safety and maintenance standards were sound; and the Indian authorities have a broader interest in preserving confidence in the country's aviation system."

At the heart of the AI171 mystery are two small cockpit fuel-control switches.

These are no ordinary switches: they are physically latched, protected by locking mechanisms and designed to require deliberate action, precisely to prevent accidental engine shutdown.

They are normally used before engine start, after landing or in serious emergencies – not seconds after take-off.

A competing interpretation has emerged of the preliminary report's most puzzling detail: the brief movement of both fuel-control switches to "cutoff" shortly after take-off.

Cox says accidental switch movement is extraordinarily unlikely.

After reviewing the histories of Boeing's 757, 767, 777, 787 and 737 Max fleets – more than 400 million flight hours – he found no case in which a switch failure shut down an engine.

The chances of two such failures occurring within a second of each other, he says, are "one in a trillion or more".

The Canada-based investigator said the preliminary report made it clear that the crash resulted from "human action in the flight deck, not a mechanical or electrical problem with the aircraft".

But another interpretation has emerged.

Simon Hradecky of The Aviation Herald, an aviation news platform, argues the switches may not have caused the emergency but reflected the crew's response to it.

If the engines were already losing thrust, he says, the pilots may have been carrying out Boeing's dual-engine failure memory procedure, which "requires moving both switches to cutoff and back to run to reset engine controls and attempt a relight".

If so, the switch movements would not be the cause of the crisis but evidence of a last-ditch effort to save the aircraft.

One of the more intriguing questions in the investigation concerns the RAT – the Ram Air Turbine.

Think of it as the aircraft's emergency windmill: if both engines fail, or the plane loses normal electrical or hydraulic power, a small turbine drops into the airstream and generates backup power.

According to the preliminary report, the RAT was providing hydraulic power within about five seconds of the fuel switches being cut off.

But simulator tests cited by the BBC reportedly suggest the process should take 14-18 seconds from fuel cut-off to power delivery.

If that's correct, a puzzle emerges: did the RAT deploy earlier than investigators currently believe – perhaps before the engines lost power?

Hradecky suggests the RAT was, therefore, more likely triggered because both engines had already lost power and fallen below idle speed, rather than by a separate electrical or hydraulic failure.

Nothing of this sort is mentioned or hinted at in the preliminary report.

A key unanswered question remains whether the loss of thrust originated during the take-off roll and continued through lift-off, or whether it developed only after the aircraft had become airborne.

One theory advanced by some safety campaigners is that a major electrical fault triggered a reboot of the aircraft's flight computers seconds after take-off.

According to this hypothesis, the systems briefly misidentified the aircraft as being on the ground, prompting a protection system to interpret high engine thrust as a malfunction and cut fuel to both engines.

Under this scenario, the cockpit fuel switches were never physically moved. Instead, the flight data recorder may have captured an electronic fuel-cutoff command rather than a mechanical switch movement.

The theory has been championed by Indian investigative journalist Rachel Chitra, who has highlighted what she sees as inconsistencies in the preliminary report, including questions surrounding the engines' attempted relight after fuel was restored.

Campaigners have also alleged that the aircraft had experienced previous technical problems, including an in-flight fire, though investigators have not publicly linked any such incidents to the crash.

The preliminary report does not mention fire or technical problems. Instead, it presents a different picture.

It says the 2013-built Boeing 787-8 held a valid certificate of airworthiness, had logged nearly 42,000 flight hours, complied with all mandatory airworthiness directives and service bulletins, and was current on scheduled maintenance.

The aircraft's two GE Aerospace GEnx engines were hardly new: one was built in 2012, the other in 2013, with roughly 28,000 and 33,000 flight hours respectively.

Yet both remained well within the expected service life of modern jet engines.

That is why reports by Reuters and Bloomberg saying that the final report is being delayed by continued engine analysis are intriguing.

Dual-engine failures on modern airliners are exceptionally rare.

When they occur, investigators usually search for a common cause – fuel contamination, a disruption in fuel supply, bird strikes, volcanic ash or some broader system failure. No such cause has yet been publicly identified in the Air India crash.

If fuel starvation caused the engines to lose power, the question is whether the switches explain the entire sequence of events.

Experts like Cox and Hradecky believe one clue may lie in the engines' Exhaust Gas Temperature (EGT), recorded by the flight data recorder and electronic engine-control systems.

By comparing the moment EGT began to fall with the recorded movement of the fuel switches, investigators may be able to establish whether the engines started losing power before or after the switches were moved.

The full cockpit voice transcript may yet hold the key.

"There's likely much more on the cockpit voice recorder than has been released. A single remark – 'why did you cut off the switches?' – isn't enough," Peter Goelz, a former managing director of the US's NTSB, told me last year.

Only when the cockpit conversation is matched against the aircraft's final seconds of recorded data may a clearer picture emerge of what brought down the jet.

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