Russian plane crash: What we know Published duration 29 December 2016

image copyright AFP image caption A Tu-154 jet like this one crashed into the sea

More details are emerging about the air disaster in the Black Sea involving a famous Russian military choir.

An ageing Tu-154 airliner came down off the Russian coast with the loss of all 92 passengers and crew.

This was no ordinary flight. The plane belonged to the Russian defence ministry and was en route from Moscow to Syria, where the Alexandrov Ensemble was due to perform for soldiers stationed overseas.

As the official investigation gets under way and the flight recorders are analysed, here are some of the facts and theories.

What happened?

At 05:23 (02:23 GMT) on Sunday, 25 December, the jet took off in good weather from Adler airport near the city of Sochi, where it had been refuelling, for the next leg of its flight to Latakia in western Syria.

Two minutes into the flight the plane disappeared from radar.

A recording of the final conversation between air traffic controllers and the pilot revealed no difficulties.

But Russian media have reported a cockpit conversation between the two pilots, apparently transcribed from one of the flight recorders, which indicates they were taken by surprise when an alarm began to sound.

Wreckage was found about 1.5km (about one mile) from the shore.

Fragments of the plane were found across a radius of about 500m (1,640ft), the defence ministry said, after a vast search operation involving a submarine, 45 other vessels, 12 planes, 10 helicopters, three drones and more than 3,500 personnel.

Who was on the plane?

image copyright Russian Ministry of Defence image caption "Dr Liza" was on the flight to deliver medication to a university hospital in Latakia

In addition to the eight crew members, there were:

Sixty-four men and women of the Alexandrov Ensemble, the official choir of the Russian armed forces

One of Russia's best-known humanitarian figures, Yelizaveta Glinka - known popularly as "Dr Liza" - who was due to deliver medicines to a Syrian hospital

Nine members of the Russian media including TV crews from Channel One, NTV and the military TV channel Zvezda

Eight military figures among whom is listed Lt-Gen Valeri Khalilov, the choir's conductor

Two civil servants

As of 29 December, at least 19 bodies had been recovered.

What do the 'black boxes' reveal?

Russian media say the plane was equipped with three flight recorders: a Flight Data Recorder (FDR), a Cockpit Voice Recorder (CVR) and a third "box" which duplicates recordings from both of the others (and is said to have been badly damaged).

A transcript of what is said to be the pilots' final conversation was leaked to Russian media:

"Speed 300 [inaudible]."

"[Inaudible]."

"I've pulled in the landing gear, commander."

"[Inaudible]."

"Oh bloody hell!"

Piercing alarm sounds

"The flaps, hell, what a…!"

"The altimeter [altitude meter]!"

"We're in… [inaudible]."

Alarm sounds about dangerous proximity to the ground

"[Inaudible]."

"Commander, we're falling!"

This suggests that the jet's wing flaps - panels which help lift an aircraft - were not moving together, causing the crew to lose control.

If the transcript proves to be genuine, it could indicate either a mechanical fault or a fault by the crew, such as mistakenly retracting the flaps instead of the undercarriage.

media caption Investigators examine one of the aircraft's 'black box' flight recorders

Russian newspaper Kommersant quotes a source as saying a flight recorder registered the failure of the plane's flap retraction system.

The crew's actions "only further worsened an emergency", it suggests.

Who was at the controls?

The crew was commanded by an experienced pilot, Maj Roman Volkov, who had more than 3,000 flight hours behind him.

He was flying with his regular crew, the Russian Armed Forces flight safety service says, including co-pilot Capt Alexander Rovensky, who had 10 years of aviation service.

image copyright Russian Ministry of Defence image caption Maj Roman Volkov, who commanded the flight, was an experienced pilot

Could it have been an attack?

The plane was flying to an air base in Syria, where Russia has been waging an air campaign on the side of President Bashar al-Assad for more than a year.

So-called Islamic State, one of Russia's deadliest enemies in Syria, claimed responsibility for the bombing of a Russian airliner returning from Egypt just over a year ago, when 224 people were killed.

A bomb has been ruled out this time because there is no evidence of an explosion on-board but Lt-Gen Sergei Bainetov, the Russian air force's head of flight safety, told reporters there were other forms of "terrorist attack".

"It could have been any type of mechanical impact, so we don't rule out a terrorist act," he told reporters

A missile attack downed Flight MH17 over neighbouring Ukraine in July 2014, with the loss of 283 lives.

However, other Russian officials have sought to downplay the possibility of an attack from the very start of the investigation.

image copyright AFP image caption A huge search of the Black Sea is taking place

What are the other theories?

Lt-Gen Sergei Bainetov said there were multiple theories which ranged from poor quality fuel causing engine failure to a bird getting caught up in the engines.

"There is no leading theory," he stressed.

The advanced age of the plane may have been a factor - it was 33 years old.

That said, few of the 39 fatal accidents involving this model of the Tupolev have been attributed to technical problems. Many were lost as a result of difficult weather conditions and poor air traffic control.

Another factor in past air crashes in Russia has been overloaded planes.