Cell phones and other electronic devices can interfere with the pilot's comms and other crucial avionics, as well as cell towers on the ground, if the plane is low enough. Due to these issues, the FCC forbids using cell phones on planes, while the FAA allows airlines to choose when they may be used, once the plane is out of range of the ground cell towers.Source

The fact is, though, many people just don't put their phones on airplane mode. You can forbid it all you want, but without enforcing it (not feasible - too many ways to get around it), it doesn't do much.

Enter the Faraday cage.

Faraday shields are, in short, metallic surfaces that prevent the transfer of electromagnetic waves from one side to the other; Faraday cages are ones with holes in them, blocking waves of specific wavelengths while allowing all others through. You've surely seen Faraday cages before; they're in the doors of all microwaves.

This seems to be an easy (perhaps somewhat expensive) solution. Just put one up on every airplane, maybe with a wire going from one side to the other to allow the on-board Wi-Fi to still work. But it would block all outside communication, regardless of if you've got airplane mode turned on or not. If the cage is routed around/inside the sides of the planes and across the door between the cockpit and the cabin, the pilot would have no problem communicating with the outside world.

I feel like I'm overlooking something obvious, but I can't put my finger on it. Is there a hole (so to speak) in this theory?

  • 11
    Perhaps a better fit for avionics.stackexchange.com?
    – Itai
    Oct 2 '17 at 19:28
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    @Itai: There's an avionics stack now? Man, the stacks are really getting fragmented. Oct 2 '17 at 19:44
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    @MichaelSeifert - I thinks that's a typo. There is an Aviation SE.
    – brhans
    Oct 2 '17 at 20:12
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    I bet the average flight has >20 phones/devices that are never turned off. This is a non-issue.
    – Berwyn
    Oct 2 '17 at 20:15
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    My understanding is that most airplanes already are faraday cages, which is why it's so hard to get a GPS signal inside one unless you hold the device against the window, or at least close to it. They obviously don't block mobile telephony frequencies as effectively, though, since it's far easier to connect a phone to the network than it is to get a GPS fix.
    – phoog
    Oct 2 '17 at 20:33

Aviation electronics already are shielded to rigorous standards, such as RTCA DO-160 - the issue with cell phones and other transmitting devices is largely based on out dated scientific understanding and studies, and older requirements.

In the US, both the FCC (which is the body which actually bans cell phone use on aircraft due to issues with ground cell towers rather than interference with avionics - civil aircraft move quick enough that hand offs between cell towers can become too frequent and problematic) and the FAA (which leaves cell phone use up to airlines through exceptions to FAR 14 C.F.R § 91.21) have indicated they are willing to relax rules and change official stances, even on older aircraft.

The issue these days with cell phones on civil aircraft has rapidly shifted to social issues rather than technical issues - people don't want to be sat next to the "cell phone thug" who insists on shouting into his device for the entirety of a five hour flight.

  • Wouldn't most of a five-hour flight be at such an altitude that the phone would be unable to connect to the network?
    – phoog
    Oct 2 '17 at 21:28
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    @phoog 35,000ft (an average cruise altitude) is only 6.6 miles - that's nothing for a cell phone with decent line of sight.
    – Moo
    Oct 2 '17 at 21:30
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    Cell antennas are directional, and they don't point up. A quick perusal of the internet suggests that consistent service is unavailable above roughly 6000 or 8000 feet.
    – phoog
    Oct 2 '17 at 21:34
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    @phoog all those times I've got cell phone reception at cruise over Europe and the US must be a figment of my imagination. That, or the fact that while cell phone towers direct much of their signal in specific directions, they are not shielded for other directions - it doesn't have to be straight up, since a cell phone can talk to a tower as far as 40 miles away...
    – Moo
    Oct 2 '17 at 21:37
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    @Calchas the CAA changed its stance in 2008.
    – Moo
    Oct 2 '17 at 22:50

Airplane safety addresses the proverbial "one in a billion risk" (which is not an exaggeration as I explain below) that a problem can occur, it doesn't deal with the normal situations where typically all will be fine. So, the fact that you won't expect problems with cell phone use in planes as pointed out in Moo's answer is not all that relevant. What matters when considering safety is if theoretically there exists a loophole that could lead to problems. One then has to make sure that such loophole cannot actually occur due to further detailed studies or by directly exposing a test flight to the theoretical risk and proving that it doesn't pose a problem.

The loophole that makes it at the moment unsafe to let people use cell phones involves by many passengers in a plane using their phones at the same time when they all have bad reception. It is precisely the shielding due to the plane's airframe that causes poor reception. So, suppose everyone were to switch on their phones before landing (say the flight was delayed and everyone is calling home to tell that they'll arrive in half an hour). Then because the phones have very poor reception they'll crank up their output power to 1 to 2 Watt. You'll then have hundreds of phones transmitting a few hundred Watts in total inside the airframe. The electromagnetic field strength in the plane can then build up large values, each point in the plane will be exposed via direct line of sight transmissions or indirectly via reflections.

Each phone is itself blasted by the RF transmissions of all the other phones, which then causes intermodulation effects. The signal form other phones will enter your phone and that signal will be added to the signal produced by your own phone before the final amplification stage. This leads to a new signal at a frequency given by the sum or differences of the signals that are amplified. The most problematic of these are third order intermodulation signals that are the sum of two mines one other, as the frequency of that signal will be of the same order as the intended signals and these are then not blocked by the device itself.

So, the net effect is that all the phones will end up transmitting not just on their normal channels, but they'll also output strong signals at other frequencies, and these signals can then interfere with the aircraft's communication signals directly, or indirectly by causing intermodulation effects in the pilot's receiver. While there are many filters in receivers and transmitters that block unwanted signals, they have their limits, particularly for rejecting unwanted signals close to the frequency of the desired signal.

But suppose that the cockpit has a good RF shielding, would that not prevent problems? That's unlikely to work because of the many electric cables moving from the cockpit through the cabin. These will carry the electromagnetic fields inside the cockpit. We need to keep in mind that the plane's antenna is capturing a signal of the order of a microwatt of power and then the hundreds of watts of interference (that's not in the form of a clean signal at a totally different frequency) is going to pose a huge problem, even for the best electronics available today.

While this objection may seem far fetched, it cannot be ruled out at present that in exceptional cases you get an intermodulation signal at an ATC frequency or at a navigational beacon frequency causing a catastrophic outcome. We don't want an accident caused by cell phone induced RF interference effects every few years, but with more than 100,000 take-offs and landings per day, more than 100 million in a 3 year period, you're aiming to rule out one a billion freak incidents.

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