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" Maybe you were fiddling with the air-conditioning nozzle, but you ignored the flight attendant&amp;#039;s annoucement to turn off your phone or iPad. Later, when the plane lands and you disembark at your destination, you look at your gadgets only to realize they were powered on (and not in airplane mode) during the entire flight.  The fact that you landed safely might make you join with myriad other air travelers who scoff at the rules about phones during flights and wonder whether it&amp;#039;s really necessary to turn them off (we&amp;#039;re looking at you, Alec Baldwin). But to Jay Apt, a professor of engineering and public policy at Carnegie Mellon University, that type of logic is dangerous. Despite the fact that more wireless signals are bouncing around than ever, Apt says there&amp;#039;s still a good reason why the flight attendants are sticklers about gadgets.  Back in 2003, Apt and his colleagues closely examined the radio-frequency (RF) spectrum for three months&amp;#039; worth of flights. Though they could detect a strong signal radiating from active cellphones, their data did not explicitly link it to a specific airplane malfunction during flight.  There may not be a smoking gun to connect cellphone use and equipment problems, but Apt stresses that the consequences of a malfunctioning plane are plenty high already. &amp;quot;Don&amp;#039;t come away with the idea that cellphone activity will take down a plane,&amp;quot; he says. &amp;quot;The probability of a crash occurring due to cellphone interference is low, but there are lots of flights each day. If a flight with 150 passengers crashes only once per decade, that is still too much.&amp;quot;  Why Do They Have Those Rules Anyway? The risk is a result of the basic way cellphones work\u2014they behave like powerful radios. Apt says that when your phone is first turned on, it broadcasts high-power radio waves while it&amp;#039;s looking for a cell tower to connect with. Essentially the phone yells out, &amp;quot;Can you hear me now?&amp;quot; and a cell tower replies, &amp;quot;Loud and clear!&amp;quot;  Once the cellphone establishes a connection, it reduces the strength of its radio waves. The back-and-forth between tower and phone repeats, with each cycle decreasing the cellphone&amp;#039;s broadcasting power. When the tower gets an unclear signal, though, it flags the phone to let it know that it needs to speak a little louder. Now the cellphone knows to send out a slightly stronger signal. And every so often, even when it sits there idly without actively making calls or using the 4G connection, the phone periodically sends messages to the cell tower just to confirm the connection. But if you&amp;#039;re at a cruising altitude of 35,000 feet, there&amp;#039;s no cell tower to talk to. The phone doesn&amp;#039;t know that, though, so it will continue sending out energetic signals to see if there&amp;#039;s anybody out there. The worry with planes is these loud and powerful signals could cause radio interference with GPS signals beamed down from satellites, resulting in a lost connection or even incorrect navigation information. But What About Wireless? If radio waves are causing interference, why are more and more airlines offering onboard wireless? It&amp;#039;s because laptops send and receive signals from an onboard server rather than a faraway antenna. Because the connection is only meters away instead of miles, your machine doesn&amp;#039;t need to transmit radio waves at high power. (The iPads now used by pilots are meant to replace binders full of maps and other reference materials; their Bluetooth, 3G, Wi-Fi, and other wireless features are kept off.) But you can take advantage of the wireless only when the plane is at cruising altitude. Takeoff and landing are different matters because, as Apt says, you&amp;#039;re that much closer to the ground without much time to correct for any mistakes. The majority of accidents happen during these stages. That&amp;#039;s why flight attendants disrupt your slumber and ask you to turn off your music so the plane can land. &amp;quot;While cellphones produce electromagnetic interference, all electronic devices [including CD players and game devices] can too,&amp;quot; Apt says.  Jerrold Sandors, the manager of the Spectrum Assignment &amp;amp;amp; Engineering Team at the Federal Aviation Administration (FAA), says that planes fly through a host of radio activity. &amp;quot;Anything that transmits a signal can get through.&amp;quot; If RF emissions are present when close to the towers and the antennas on the ground, what&amp;#039;s keeping the plane&amp;#039;s equipment from malfunctioning as it lands?  Apt doesn&amp;#039;t deny that RF activity on the ground can breach the cockpit. However, he says, the key factor is that the external interference is well-documented. Planes are regularly scheduled to perform &amp;quot;instrument approaches&amp;quot;\u2014test flights that are conducted without any passengers on board. During the flight, the pilot makes sure that the plane receives signals from a variety of navigation beacons and GPS satellites. If the pilot experiences any difficulty receiving those signals due to ground RF activity, then subsequent flights at those particular destinations are suspended until the issue is resolved. Unlike cellphone interference, ground transmission is regulated by the FCC. One way to try and minimize the interference is to install electromagnetic shielding and block off any unwanted interference. Gregory Huff, an electrical engineering professor at Texas A&amp;amp;amp;M University, says that newer planes made in the past five years or so have cockpits with better RF shielding. However, it&amp;#039;s difficult to retrofit planes that were built more than 20 years ago. &amp;quot;You can&amp;#039;t just shield the cockpit and expect to keep all the radio energy out. Any small crack or crevice is a place that RF energy can escape.&amp;quot; Huff says that while the technology will advance, he doesn&amp;#039;t anticipate that we&amp;#039;ll be able to make cellphone calls while aboard a plane. Instead, he sees a smarter generation of smartphones\u2014phones that could automatically switch to airplane mode or power down when they detect that they&amp;#039;re in an aircraft.  In any case, Huff says, he thinks airlines would be better served by improving onboard wireless rather than cell-tower communication. In the air or on the ground, people are using more and more data. &amp;quot;Data access is going to be critical, and for many people that will make or break the airline you choose,&amp;quot; he says. &amp;quot;It&amp;#039;s just a factor in planning your trip, no different than choosing one hotel over another for the better Wi-Fi.&amp;quot; "^^<http://www.w3.org/2001/XMLSchema#string>
<http://www.popularmechanics.com/flight/a7821/how-do-we-know-smartphones-endanger-planes-10065090/>