Why Must All Electronic Devices Be Turned Off During Takeoff?

The question crosses our minds at the start of every flight, when we're told our cell phones and other electronic devices must be turned off: "What'll happen if I don't?"

Probably nothing. The Federal Aviation Administration (FAA) is overly conservative when it comes to their zero-tolerance electronics policy; devices like Kindles and Blackberrys in airplane mode almost certainly don't interfere with airplane controls.

But the FAA's caution is justified. According to Captain John Cox, a U.S. Airways pilot, there have been incidences in the past when electronic devices have indeed interfered with airplane navigation systems, which sometimes use frequencies near FM radio frequencies. Planes' Instrument Landing Systems, which control vertical distancing and alignment with the runway, are most sensitive of all. "I know of an example where a small radio caused electrical problems on a jet. The result was an airworthiness directive by the FAA to correct the problem," he told USA Today.

Cell phones also have the potential to interfere. "Another example was an older model analog cell phone that was found to cause problems with the navigation system on a European operator's jet," Cox said. Though that particular model of phone was not sold in the U.S., it still served as an example of the potential of phone emissions to cause problems.

Even transmitters in remote control toys have been known to interfere with navigation receivers.

Many modern jets now use GPS rather than radar positioning, reducing the risk of interference, but the FAA simply can't be sure which devices will and won't cause problems. "The growth of electronics has far outpaced the testing ability for aircraft," Cox said. "Maybe one day the manufacturers will be able to certify electronics to be 'airplane friendly,' but until then we should turn them off below 10,000 feet."

Fine, we'll do it.

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