The Telegraph
Wednesday , March 19 , 2014
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Questions over absence of cellphone calls
Investigators mystified by Malaysia flight silence

Sepang (Malaysia), March 18: When hijackers took control of four airplanes on September 11, 2001, and sent them hurtling low across the countryside toward New York and Washington, frantic passengers and flight attendants turned on cellphones and air phones and began making calls to loved ones, airline managers and the authorities.

But when Malaysia Airlines Flight 370 did a wide U-turn in the middle of the night over the Gulf of Thailand and then spent nearly half an hour swooping over two large Malaysian cities and various towns and villages, there was apparently silence.

As far as investigators have been able to determine, there have been no phone calls, Twitter or Weibo postings, Instagram photos or any other communication from anyone aboard the aircraft since it was diverted.

There has been no evidence “of any number they’re trying to contact, but anyway they are still checking and there are millions of records for them to process,” said Ahmad Jauhari Yahya, the chief executive of Malaysia Airlines, said today.

The apparent absence of any word from the aircraft in an era of nearly ubiquitous mobile communications has prompted considerable debate among pilots, telecommunications specialists and others.

Most of the people aboard the plane were from Malaysia or China, two countries where mobile phone use is extremely prevalent, especially among affluent citizens who take international flights.

Some theorise the silence signifies that the plane was flying too high for personal electronic devices to be used. Others wonder whether people aboard the flight even tried to make calls or send messages.

According to military radar, the aircraft was flying extremely high shortly after its turn — as much as 45,000 feet, above the certified maximum altitude of 43,100 feet for the Boeing 777-200. It then descended as it crossed Peninsular Malaysia, flying as low as 23,000 feet before moving up to 29,500 feet and cruising there.

Vincent Lau, an electronics professor specialising in wireless communications at Hong Kong University of Science and Technology, said that the altitude might have prevented passengers’ cellphones from connecting to base stations on the ground even if the phones were turned on during the flight or had been left on since departure.

The hijacked planes on September 11 were flying very low toward urban targets when passengers and flight attendants made calls from those aircraft, he said.

Base station signals spread out considerably over distance. So cellphones in a plane a few miles up, like Flight 370, would receive little if any signal, he said.

Base station design has improved since the September 11 attacks to provide better, more focused coverage of specific areas on the ground. But that also means somewhat less signal intensity is wasted in directions where callers are unlikely to be located, such as directly overhead, Lau added.

Lam Wong-hing, a wireless communications specialist at the University of Hong Kong, said that cellphones transmit at one watt or less, while base stations typically transmit at 20 watts and sometimes much more. So even if a cellphone showed that it was receiving a signal while aloft, it might not be able to transmit a signal that was strong enough to make a connection, he said.

The metal in an aircraft reduces cellphone signals somewhat. If a passenger had pressed a cellphone against a plastic window with a line of sight to a cellphone tower then it is possible a connection might have been made even at a fairly high altitude, because plastic barely blocks a cellphone signal at all, Lam said.

Many aircraft carry air phones using radio or satellite technology, and the Malaysia Airlines jet was equipped with them in business class. The plane continued to send satellite pings for nearly seven hours after it was apparently diverted. But air phones these days tend to be part of an aircraft’s in-flight entertainment system.