Cell Phone Usage on Airplanes == Bad Idea

gclef writes "The New Scientist is reporting on a study done by the UK's Civil Aviation Authority that shows that older planes can't handle cell phone emissions. Hackernews has a little commentary on this as well. Good to hear that the newer planes can handle this, but why the heck were older planes *not* build with Faraday cages and shielded wires? Scary...." Look a ways down the page for the HNN piece - but at least now I know that this isn't simply one of the arbitrary rules that the airlines setup.

Comments

Radio emissions on modern aircraft -- true story.

[Bowie J. Poag]

I'm reluctant to admit to such a flagrant act of nerdiness, but, here goes: :)

My last airline flight happened about a month ago. I thought it would be neat to take my new portable hand-held shortwave radio with me on the plane to see if I could pick up the transponder on the aircraft's black box(es), or maybe some of the cockpit/tower discussion. The flight was pretty much empty (I always take the red-eye) so for about an hour, I sat there with my earphones on and my antenna pitched up and scanned the whole damn plane from 1 Mhz all the way up to about 400 Mhz or so in short, medium, longwave and FM.

I didn't find anything recognizable. I was seated far enough away from the engines to rule out any interference, and the whole spectrum was peppered with odd little noises generated by the aircraft (This was a Boeing 767-300 if I remember correctly) but nothing resembling any sort of communications. Considering the fact I was seated in a giant metal tube, I can also rule out ambient interference. There was a guy with a laptop about 10 rows up, and I could pick up his machine pretty easilly. Anyway, no luck.

Most analog cell phone transmissions occur between 800 and 950 MHz. Youre going to have a hard time finding a scanner than will allow you to listen to that range. If I remember correctly, there was a law passed in '93 or so which made it illegal to sell scanners with capability in that range, in order to protect the privacy of analog cell phone users.

You wouldn't believe the stuff I heard. People's conversations get sorta...weird after 9PM. It's a sick world. :)

Bowie

PS.. No "You didn't hear anything in the cockpit because the crew was asleep" jokes, please. :)


Bowie J. Poag

Re:An accident created the regulation

[phliar]

The local oscillator leakage caused a false indication on one of the ILS (Instrument Landing System) instruments (I think it was called the localizer).

Localizers use frequencies around 110 MHz. Consumer FM receivers use a first intermediate frequency (IF) of 10.7 MHz. What this means is that if you tune an FM radio to KOZY 101, the radio transmits noise at 101.1 + 10.7 = 111.8 - which is bang in the middle of the navigation band.

Fortunately, all commercial jetliners now use inertial navigation systems, so this particular failure mode is much less likely.

Inertial is not used for approaches, only for en-route navigation. The problem with inertial systems is that they drift, so the longer you've been airborne, the more inaccurate they are. An error of about a mile is no problem up in the stratosphere, but you can't be a mile off the runway when you touch down...

Right now ILS is the only precision approach available in the vast majority of cases. Once the FAA puts up WAAS and LAAS augmentation for GPS, aircraft can use GPS for precision approaches.

Re:It is, in fact ILLEGAL to use a cell phone...

[petros]

A cell phone on the ground has line of sight to maybe 2 cell towers at a time. Maybe 3 if you're downtown.

My phone is seeing quite clearly three tower where I am right now, and I'm in a small town. In high density urban areas, you quite often have more than that.

Just immagine the cell tower selection algorithim with an input size of 4000 possible towers.

You are correct that using a cellular phone in a plane causes all kinds of trouble, but this isn't the reason. Remember that cellular systems rely on channel reuse to achieve high capacity. In other words, if channel x is used on tower A, it can't be used on any of the neighboring towers, but it can be used on towers that are further away. The assumption is that with the right power tuning etc, the two towers won't interfere with each other since they are not next to each other (of course sometimes if the network is too dense there are trouble spots where this isn't the case, hence crosstalk). This assumption is based on the fact that users are close to the ground. As you said, when you are in an airplane your cellular phone can see many towers, some of them using the same channels. I'm not sure how a network would respond if one of its channels was used from a phone in a plane, but I imagine that after all the users of the channel suffered from crosstalk for a while, they would either lose their calls or handed off to another channel, rendering one channel useless in a large area. Just a few people using their phones in a plane would have a devastating effect on the capacity of the system underneath, especially if flying over a large city. Not to mention that if your provider has any fraud detection system in place you would probably trigger an alarm by appearing to be in more than one place at the same time.

Violation of FCC regs

[AJWM]

It's a violation of FCC -- not FAA -- regulations to use cell phones aboard aircraft because of the transmitting range at altitude. Your signal covers more ground, giving greater potential for interference and confusing the multiple cells you're overlapping. FCC couldn't give a rats behind if it happens to screw up navigation equipment too. (Well, except where that nav equipment is also based on radio signals.)

Now, the FAA and airlines may also have a legitimate beef, and maybe even some regulations, but it's the FCC who will slap you with a heavy fine if they catch you doing it.

Those Damn Airplaces

[Cannonball]

You know, I'd been using my cellphone in airplaces for quite sometime now, my provider hasn't said anything about it...and I pay them $30/mo for the service! Now I'm gonna have to find a fscking vacuum to talk in? Man, the nearest one is like 15 miles from here...but I hear it's expensive to get to, and you need a pretty important patron...but I hear the reception sucks :). Fix that headline Cmdr Taco.

This is contrary to other studies I've seen.

[ericlj]

At the link below (which goes to TELECOM Digest V19 #457) is information which directly contradicts this newer (and apparently much smaller) study.

I believe that the tower-switching issue is genuine, but I find it hard to believe that personal electronics actually have an effect on jets (mainly because I've been on plenty of flights surrounded by people ignoring those rules -- and I've yet to be involved in a crash).

http://hyperarchive.lcs.mit.edu/telecom-archives /archives/back.issues/1999.volume.19/V19_% 23457

Not just cell phones...

[wass]

Also understand that it's not just the cell phone transmitters that pose the threat of interference with the aircraft's electronics. There are all sorts of problems with radio receivers as well.

The simplest problem, and also probably least likely to affect the plane, is passive non-linear antenna radiation. Basically, an antenna connected to a non-linear passive device can re-transmit the incoming RF at sum/difference frequencies (IM distortion anyone?). Although these re-transmissions are far below the incoming RF signal strenth (and most likely the noise floor) and not likely to interfere with the aircraft.

The bigger problem comes from a powered heterodyne radio receiver. Ie, a receiver (like a standard FM radio) that down-converts the incoming RF to an IF. The mixer on board the receiver doesn't have perfect isolation, so some of the produced IF (which is heartily amplified) will leak back through to the antenna, which can re-transmit. (FYI, a mixer multiplies the incoming RF with a synthesized LO (local oscillator) to produce output at the sum/difference of those two frequencies. Work out the trigonometry if you're bored, it's pretty cool.) Once again, the re-transmitted IF power is pretty small, but it is produced, and may interfere with the aircraft's receivers. And seeing that most IF's are in the range of 10 MHz or so, there is much opportunity for interference, almost independent of device RF frequency. This is why many radios are not allowed during flights, even if they're receive only.

That's why the aircraft-certified electronics are so expensive. (example - compare prices of a marine GPS unit versus an aircraft GPS unit). The aircraft units have had many resources spent to properly shield them not only from incoming RF (other than the GPS signals, of course), but also for outgoing IF re-transmission.

This IF effect has it's beneficial uses, too. For instance, one of my coworkers lost his RC model airplane when some wind gusts picked up while he was flying it. One of his friends grabbed his multi-element yagi antenna, tuned his receiver (non-heterodyning) to the IF frequency, and by scanning around (and using variable attenuators) they were able to track down the plane. Even though it wasn't actively transmitting any RF signals!

Terrorism

[SheldonYoung]

Okay, now I'm a little concerned. Smuggling a bomb on an airplane is a very difficult thing to do. What ISN'T hard it shipping a crate of electronics equipment designed to give off a wide spectrum of high energy radio waves.

What's stopping a terrorist from cargo shipping an electronics system designed to take down one of the older planes? How would the airlines respond with a threat to turn on such a device? Would they even believe it?

There NEEDS to be a TECHNOLOGICAL solution...

[SvnLyrBrto]

... and *NOT* a legal one.

A number of people have pointed out that it's illegal to use cellphones on aircraft anyway. And then there're the similar restrictions about discmans, gameboys, laptops, etc.

"RF frequency can disrupt navigation / autoland / whatever, so let's ban electronics either completely or just during takeoff / landing." Yeah... Grrreat idea!!!

What people who say "it's illegal anyway" overlook, is the fact that there is just about always some yahoo who thinks that the rules don't apply to him.

They'll use those tiny headphones and keep the discman in their pocket. Or they'll use a headset with their cellphone (till the plane climbs out of cell tower range). Or they'll hide the game boy whenever a stewardess gets near. Or they'll say they're using a Palm III when it's really a Palm VII. Or mabye even, they don't mean to break the rules at all, but they just leave the cellphone ON during the flight (those suckers *DO* transmit even when you're not in a call, ya know).

You know it'll happen, no matter what laws or rules or regulations you impose, and whatever safety guidelines you publish, and no matter how many times you tell someone. It WILL happen.

And that's why the FAA needs to dump those "RF on an airplane" rules, and mandate a technological solution.

john

EMF on Aircraft

[PotatoMan]

The use of personal electronic devices on aircraft has been debated a lot in the literature. The issue is resonance. An aircraft cabin is basically a long metal tube. So if your laptop hard disk puts out the right frequency, the signal may be amplified and interfere with the electronics. One result I've seen (I think this was IEEE Spectrum) is that a laptop hard disk put out the same frequency used by navaids. There is no consensus on this, so the FAA takes a conservative stance.

As for Hemos' suggestion of Faraday cages, the issue is really cost. How much more are you willing to pay in ticket prices to haul around a cage so that a few passengers can play Doom? Also, remember that the flight instruments are housed in the same metal tube, so you'd have to put a second cage around that section of the aircraft. And I suspect it is far easier to say, "put a cage around it" than it is to actually do so. For one thing, people kind of like those pesky windows...

Weight is such a big factor that aircraft manufacturers went to Kapton wiring because the insulation weighed less. And yes, all the wiring is sheilded. But I can tell you from firsthand experience that even twisted, sheilded pairs driven by differential transceivers are affected by impressed noise.

Have you ever seen an RC plane take a 'radio hit' from some else's transmitter? Same deal with PED's; you'll never know what frequencies are being emitted, so why take chances?

A View From The Inside

[Marillion]

1. I work for an airline. I do not speak for them.
2. My airline does allow cell phone use on the aircraft as long as the main door is open. When the door is close an announcement is made to discontinue cell phone use.
3. It is a well established here that ground crew who use their two-way radios inside the cargo compartment often trigger the fire detectors. Say what you want about "That shouldn't happen," but it does. If the same thing happens in the air; first, the crew dumps halon into the cargo hold (fluffy the cat dies, too) and they land immediately. If the investigation shows a passenger cell phone caused it. That passenger will be fined lots of money.
4. Aircraft use a landing signal system called ILS or "Glide Slope" to guide the aircraft to the end of the runway. There are big radio transmitters at each end of the runway which emit multiple finely tuned radio signals that form an interference pattern that the onboard aircraft use to determine if it is properly aligned in the runway. This is a "Good Thing." It is very good if the pilot can't see the runway. Stray radio signals from almost anything can mess up the the interference pattern. This is why you are required to switch off your laptops, palm pilots, walkmans, etc ... During takeoff and landing. In case you were wondering why takeoffs too, is because every takeoff procedure assumes that there might be an immediate emergency landing.
5. Inside a every US flaged airplane and US airport, the FAA is god. FAA regulations are gospel. (Except for the US Secret Service, but that's another matter isn't it) If the FAA has a rule regarding radio gear, any FCC ruling doesn't mean jack. Many aviation agencies of other countries are set up the same way.