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

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!

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.