This graphic, released by the Australian Maritime Safety Authority, shows an area (bottom left) in the southern Indian Ocean that the AMSA is concentrating on during its search for the missing Malaysia Airlines plane. Picture: Australian Maritime Safety Authority

New York - One of the most commonly asked questions about Malaysia Airlines Flight 370 is: Why didn’t the passengers aboard use their cellphones to call for help? We all remember United Airliners Flight 93, whose passengers communicated with the ground by phone after the plane was hijacked during 9/11. Given how famous the incident became, it’s hard to see why none of the 227 passengers aboard Flight 370 tried something similar.

One part of the answer is that, at least at first, many or all of the passengers might have been oblivious that anything was awry. There is no evidence that the takeover of the plane was violent; the view has become increasingly widespread among investigators that the captain diverted the plane. By this time, it was past 1am, and many passengers were no doubt asleep.

Apart from the lights of the occasional ship, the sea below would have been dark. In such a situation, without visual reference or navigational instruments, it is almost impossible to determine one’s orientation. The only way a passenger could have detected a change in course is if he watched the plane’s course on the entertainment system – but the cockpit can shut that down. “There’s the circuit breakers for all that stuff,” says retired 777 captain Gerard Baer.

Some reports indicate that after the plane turned off its transponder and changed course to the west, it climbed sharply to 45 000 feet and then descended even more abruptly. If true, this could have alarmed the passengers. But it’s hard to know how much weight to give the altitude-deviation reports, since it seems unlikely that a fully loaded 777-200ER can reach such an altitude and impossible that it could have descended so quickly – the data might have been produced by faulty instrumentation.

At any rate, once an airliner is at 30 000 feet, cellphones no longer work. “At 3 000 feet, you can make a call, but go much higher and you can basically forget about that,” says Wouter Pelgrum, an assistant professor of electrical engineering at Ohio University. “You don’t have coverage.”

Part of the problem, he says, is that cell tower antennae are pointed down, towards the ground, not up into the sky. If you’re over a city, with its dense cluster of coverage, you will have a decent chance, but not in a rural area, and even less so over the ocean.

Malaysia Airlines offers an “air-to-ground phone” service in business class that also allows passengers to send e-mail, but the captain can shut this down. Only in the later stages of United Airlines Flight 93, when the plane was nearing Washington, DC and below 10 000 feet, did a few cellphone calls manage to go through.

Recent talk about allowing cellphones in flight depends on the airlines installing another technology, so-called “pico cells”, which are basically miniature cellphone towers located within the plane that relay the signal to the ground. Flight 370 appears not to have had this equipment, however, and even if it were, the pilot could have shut it down.

That’s not to say a cellphone would have been useless in this scenario. If it were to be taken out of “airplane” mode, a cellphone would begin sending out signals every 20 seconds or so, trying to locate the nearest cellphone tower. (This is why it’s a good idea to put your phone on airplane mode during a flight, even if you’re not worried about disrupting navigational equipment: the constant pinging will drain your battery.) This won’t result in your phone making a connection to a network, but it’s a source of electromagnetic radiation that eavesdroppers could pick up.

“If you have 200 cellphones all pinging repeatedly at 6/10ths of a watt, it would be a chorus,” says Paul Czarnecki, a pilot and cellphone network technician. “The US has listening stations all over the world to record and digitise every signal in the air. It blows my mind that we don’t know where it is.”

One reason for this lack of pinging might be that the passengers had their phones in airplane mode, and it never occurred to them that something was amiss. If the plane’s final destination was near Kyrgyzstan, it would have landed within an hour of dawn, giving passengers little time to notice that something was wrong with the landscape outside their window – and by the time they did, they might have been over a sparsely populated landscape with little cellphone service, and/or at low altitude, where electronic transmissions of any kind would have been difficult to detect.

And if the passengers did notice something was wrong earlier in the flight, and began to take action, the pilot could have taken swift action to silence them.

“The pilot can raise the cabin altitude (meaning, reduce the pressure to match that of a specific height above the ground) up to cruise altitude,” says 777 pilot Rick Solan. In essence, the pilot can depressurise the cabin, which at 30 000 feet will cause passengers to lose consciousness within seconds unless they grab their oxygen mask (which will automatically deploy if the pressure drops to such an extent). But sucking air from a tube will keep passengers immobilised in their seats, and at any rate the chemically generated oxygen will run out after 15 minutes. That’s a depressingly grim scenario, however, and there’s good reason to think that whoever took Flight 370 would be strongly motivated to avoid it.

If the most obvious motive for abducting a planeload of Chinese nationals – ransoming them for political leverage – is the correct one, then killing them would defeat the whole purpose of the mission.

Given the cunning strategy of the plan’s early phase, it’s more likely (assuming the flight didn’t end up ditching in the southern Indian Ocean) that he was able to keep the passengers quiet the way that airline pilots all over the world do every day: by exerting his authority as captain and offering a plausible-sounding explanation for the rerouting and delay. – Slate