Plane crashes happen, but planes disappearing is extremely rare. How did one Malaysian Airlines plane and its 239 passengers just disappear?

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It left Kuala Lumpur for Beijing at 12:35am Malaysian time and was scheduled to arrive in China at 6.30am on Saturday 8 March. 40 minutes later, flight MH370 had dropped off the plane tracking services.

Two hours later, Malaysian Airlines declared the plane had been lost.

Somewhere over the South Chinese Sea, just after 1am in the morning, the plane and its 239 passengers vanished. Search parties from Malaysia, China and Vietnam have still not located the plane.

Safest plane in the world

It is a six-hour flight, several Malaysia planes flew the route every week, and both Malaysia Airlines and the plane manufacturer Boeing have impeccable safety records.

Boeing's 777 range are rated as the safest plane in the world. Boeing 777 planes have flown more than 5.5m flights, and up until a botched landing in San Francisco killed two people in 2013, there had never been any crash fatalities.

The carrier - Malaysia Airlines - has one of the best safety records in Asia. Though its fleet of Boeing 777s were slightly older than average, it's an airline with a near top safety rating.

There are no clear answers here.

Communication mystery

But it's the communication failure that is particularly mysterious. The aircraft was comfortably cruising – at a stage of the flight when the pilot would have had plenty of time to report any mechanical problems to Air Traffic Control. But no distress calls were received.

And then, it's not just the lack of active communication, the plane should automatically update Air Traffic via an inbuilt tracker known as a transponder device. The transponder continually responds to requests for location - but why it stopped working is unknown.

Since the start of the 1990s, there have been plane crashes but very few planes have "disappeared" - fewer than five are recorded here - and they tend to be small craft flying over particularly remote areas.

Transponders, built into the airplane's black box should continue working after crashes - and even underwater – though deep-water black box retrieval is hard and a race against the clock. They have a battery life of 30 days.

Black boxes on commercial aircraft also contain cockpit voice recorders which could provide some insight into what went wrong on that plane at 1am on Friday morning. Normally black boxes are fitted with underwater retrieval beacons – designed to make them easier to find in the event of a plane crashing into the sea.

Our Science Correspondent Tom Clarke writes:

With all the technology on board a modern passenger aircraft its seems impossible that one can just disappear without explanation.

However, in the case of MH370 that appears to be exactly what's happened.

The Malaysian Airlines plane was equipped with the industry standard ASD-B flight transponder. This device sends a package of GPS data back to air traffic controllers every second.

It gives the plane's altitude, speed and direction.Data from the transponder shows the plane taking off at 12:41 a.m from Kuala Lumpur.

Then at precisely 1.20 a.m. when the plane was cruising at an altitude of 35,000 feet the signal was lost.

The fact that the transponder data shows no change in altitude or course, and that there was no communication from the crew on board has led to speculation that a sudden and catastrophic event caused the plane to crash into the sea.

In emergency situations however, pilots are trained to "fly, navigate, communicate" in that order.

If there is an on-going emergency situation that requires the full effort of the pilot and crew it may be that a distress call is not made in time.

The aircraft was also equipped with an ELT - a floating GPS beacon - designed should transmit the aircraft's position if it lands in water.

There has been no report of a signal from this transmitter either. However its signal is only short range and won't work if the device sank along with any wreckage.

It's now 24 hours since the MH370 took off so it looks almost certain that it has crashed. But until physical evidence has been found authorities cannot confirm that the plane, with its 239 passengers and crew, has been lost or not.

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