Electronic use on planes: Can your cell phone bring down a plane?

Fess up.

You left your cell phone on during a flight, didn't you?

Maybe it was accidental, or maybe it was out of sheer contempt for a rule you believe is unnecessary.

Almost one-third of airplane passengers reported that they left on a portable electronic device during the previous year, according to a survey by the Consumer Electronics Association.

This month, a government industry group studying the safety of personal electronic devices on planes will deliver its findings and recommendations to the Federal Aviation Administration. The report may lead to looser rules on e-readers and similar devices on planes.

"The FAA recognizes consumers are intensely interested in the use of personal electronics aboard aircraft,' said an FAA spokesperson. "That is why we tasked a government-industry group to examine the safety issues and the feasibility of changing the current restrictions. The group is meeting again this week and is expected to complete a report to the FAA by the end of the month. We will wait for the group to finish its work before we determine next steps."

But it raises a big question: Can cell phones bring down planes? Are e-books really that dangerous below 10,000 feet? Can Bluetooth kill?

In a search for answers, we traveled from coast to coast, talking to experts and reviewing recent reports. Here's what we found.

The risk is low

We can find no instance in which electromagnetic interference from a portable electronic device brought down a commercial plane or was a contributing factor in an accident. And the National Transportation Safety Board says it has never issued a recommendation about such devices on planes.

But those who flatly say there's no evidence that electronic devices have caused interference on planes are wrong. (More on that later.)

The consequences are high

A cell phone doesn't have to jam the pilot's radio or distort a plane's glideslope to cause a catastrophe. That scenario is unlikely. But safety officials worry that portable electronic devices could contribute to an accident.

At Boeing's Electromagnetic Interference Lab in Seattle, engineer Kenny Kirchoff enters a room shielded from electrical energy. He turns on a laptop positioned near an antenna and then points to a monitor. The laptop's electrical emissions are superimposed over a line depicting airplane radio frequencies.

"You can see that some of the signals from the laptop actually jump over the limit," Kirchoff said. "So that means there's a potential that this piece of equipment could interfere with the VFR radios."

Will that bring down the plane?

"It's not necessarily that a phone can bring down an airplane," he said. "That's not really the issue. The issue is interfering with the airplane and causing more work for the pilots during critical phases of flight. So when they take off and when they land, those are phases of flight which require a high level of concentration by the pilots."

A problem caused by a portable electronic device could distract the pilots' attention, lowering the level of safety on the plane.

And making it a plane you wouldn't want to be on.

It has happened before

Pilots have reported cases of suspected interference.

From 2003 to 2009, there were 75 instances of suspected electronic device interference, including 29 involving mobile phones, according to a study by the International Air Transport Association.

That is one event for every 283,300 flights.

Though rare, the reports suggest that such interference can affect almost every aircraft system, from communication and navigation systems to flight controls (such as autopilot) and warning systems.

The International Air Transport Association could not independently validate that personal electronics caused the problems. Replicating the problems can be difficult or impossible.

But the pilots who reported the problems seemed convinced, saying that when passengers were instructed to turn off electronic devices, the problem went away.

Better safe than sorry

The FAA, saying that personal electronics "could be potentially hazardous to aircraft communications and navigation," has largely handed over responsibility to the airlines. The airlines -- not the FAA -- are responsible for determining which electronic devices may be used on planes safely.

Airlines also are responsible for determining when devices can be used. Most airlines follow FAA guidance that devices be allowed when the airplane is above 10,000 feet, which gives pilots time to troubleshoot problems that arise. Airlines that want to allow devices below 10,000 feet must certify that the specific devices are safe.

The cell phone conundrum

Cell phones and laptops with wireless network capabilities -- devices that transmit signals -- fall into a category of their own when in transmit mode. Those transmissions are banned by both the FAA, because of potential airplane interference, and the Federal Communications Commission, because of potential interference with wireless networks on

the ground.

And for that reason, the government industry committee reviewing portable electronic usage will not consider the airborne use of cell phones for voice communications during flight.

In the future, you're likely to see more and more cell phone and WiFi services offered on planes but only as part of airplane-based systems designed to handle the signals.

But when is a cell phone a cell phone? Many passengers just aren't sure.

"It remains a fact that passengers do not really know -- when turning their (smart phones) 'on' and 'off' onboard an aircraft -- if they activate WiFi only and/or cellular functions," Airbus wrote in its submission to the FAA.

And if passengers don't know how to operate the devices they own, can flight attendants be expected to understand the ones they don't?

The risk is growing, and shrinking

When the FAA first regulated electronic devices in 1966, regulators were concerned about FM radio interference. Today, travelers come equipped with smart phones and tablets, WiFi and e-readers, Bluetooth technology and noise-canceling headphones.

Meanwhile, planes have grown more sophisticated, too. In 1966, mechanical linkages connected the cockpit with the control surfaces; today, miles of wires do the trick. The potential for interference is growing.

But modern planes are also being built with portable electronics in mind. Newer aircraft are "hardened" against electromagnetic interference, "immunizing" them from problems.

Technology is creating a problem, and technology is solving the problem.

Other considerations

But the FAA must consider non-technical factors as well.

Will passenger use of electronic devices keep them from listening to safety briefings? Will laptop computers, if used during takeoffs and landings, become missiles if the plane encounters turbulence?

And what can airlines do to get compliance with new rules when the old rules are so routinely ignored?

During our return trip to Washington, we ask a fellow passenger whether she has turned off her phone. She has, she says, but only because of an unusually persistent crew, which made three warnings.

"Typically, (I will) always leave my phone on," she said.

Why?

"I don't think that having your cell phone on will actually interfere. And plus ... I do like to get my messages the second I land."

An unrelated traveler chimes in, saying she leaves her phone on airplane mode. "It's there, so I do," she explained.

"I think if people were honest, I think at least half of them have their phones on."

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