# Why don't cell phones work on airplanes at 30,000 ft? **URL:** \ **Category:** Factual Questions\ **Created:** [April 13, 2009, 6:03pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960 "2009-04-13T18:03:22Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [April 13, 2009, 9:07pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/21 "2009-04-13T21:07:23Z")
> [@Shadowfyre](#): > > These types of antennas are designed to broadcast and receive over a large horizontal distance (in one direction), but a relatively small vertical distance (maybe a couple hundred feet up or down). But note that a couple hundred feet at short range implies thousands of feet at long range. I have used an ordinary cellphone in a sailplane at 7000’ above ground. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [April 13, 2009, 10:11pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/22 "2009-04-13T22:11:03Z")
As Shadowfyre and Pleonast said, cell phone tower antennas are directional. Once you get high enough the antennas start to get blind towards you, but it’s not an instant change. Cell phones will work fairly reliable up to about 5,000 feet. Between 5,000 and 10,000 feet the signal drops off fairly quickly, and it’s pure luck if you can manage to get a cell phone to connect above 10,000 feet. IIRC, all of the 9/11 calls were made below 5,000 feet. Cell phones connecting to multiple towers used to be a real problem in the early days. Cell phone users are stupid. Tell them that they can’t use a cell phone on a plane because the plane will crash (even though it isn’t quite true) and they still insist on using their cell phones. So, the airlines and cell phone companies have just had to cope. The major problem in the early days wasn’t so much that different towers would connect, but different systems would connect, and this wreaked major havoc since the systems weren’t designed to handle this. Because idiots insisted on using their cell phones, the systems were forced to cope with this just to keep running, and these days it isn’t a problem. The fact that planes move a lot faster than cars causing you to travel fairly quickly from one tower to another isn’t a problem any more either. Cell phones can’t interfere with the control systems on most planes, because the control systems are mechanical and not electrical. Modern planes are tested very thoroughly for cell phone interference. Older planes will be much more vulnerable. Communication and navigation equipment can be affected. From pilots that I have talked to about it, navigation equipment problems are rare. The most common problem by far is that cell phones cause noise in the pilot’s headsets, making communication to the tower much more difficult. > [@Projammer](#): > > So why does the big aluminum can you’re in not act as a faraday cage and quash the signal? The fact that it’s not grounded directly to ground? Or is it more about the frequencies in use? It’s mostly the windows in the plane that allow the signal to get in and out. > [@Kimmy\_Gibbler](#): > > There doesn’t seem to be [any difficulty at all.](http://www.informationweek.com/news/telecom/voice/showArticle.jhtml?articleID=216403167) From the article, it appears airlines in Europe have already started allowing passengers to use phones, and unless Sen. DeFazio gets his way, this looks like it may soon be an amenity that American, United, Delta, et al., will be glad to afford you for only a nominal surcharge. The FCC recently did a long study on cell phone use in planes, and at the end of it they decided to leave the ban in place. The problems have mostly been solved, so calls aren’t going to be dropped and systems don’t crash any more, but there is still the issue that an airborne cell phone hogs channels on multiple towers, which reduces the overall bandwidth available for the system. The FAA also bans cell phones, citing the FCC ban and also citing safety concerns. I am personally aware of several incidents where cell phones have caused communication problems between the tower and the pilot, and I don’t even work in that industry. Those who believe that cell phones don’t cause any problems at all are just plain wrong. ---
**Author:** ![vivalostwages](https://avatars.discourse-cdn.com/v4/letter/v/d07c76/32.png) [@vivalostwages](https://boards.straightdope.com/u/vivalostwages)\ **Post date:** [April 14, 2009, 2:24am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/23 "2009-04-14T02:24:20Z")
Here’s another tidbit o’ trivia: Cell phones don’t work in Death Valley. There are no towers allowed in it since it’s a national monument, and the topography makes it hard to get a signal anyway. If you get closer to Beatty, in Nevada, you can get a signal. I assume it’s coming from Vegas, which is about 80 miles south. Just so you know if you ever travel there. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [April 14, 2009, 3:00am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/24 "2009-04-14T03:00:25Z")
> [@Spectre\_of\_Pithecanthropus](#): > > I’m still wondering why, when I live in a densely populated, very urban area in which cell phones have been standard equipment for just about everybody above the age of twelve, my cell phone doesn’t work in the bathroom, kitchen, or dining area–in fact, all along the southern wall of the apartment. At least the carrier did let me cancel our air cards and waive the early termination fee, because it’s a known issue. The building (or other nearby buildings) can block a lot of the signal. Different building materials will be more or less efficient at blocking cell phones – IIRC, cinder block and concrete are particularly bad. 30.000 feet of air is nothing compared to a few feet of brick or concrete or steel walls. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [April 14, 2009, 7:18am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/25 "2009-04-14T07:18:12Z")
I’ve used my cell at a bit over 10,000 feet with no problems, I haven’t had any luck much higher than that though. The other issue is that at 10,000 feet we’re normally about 30 miles away from the tower (flying into a small remote town with a single cell tower) so the actual distance is significant. But yeah, the noise cell phones make over the intercom can be very distracting, that is the most obvious problem I have with them as flight crew. > [@engineer\_comp\_geek](#): > > Cell phones can’t interfere with the control systems on most planes, because the control systems are mechanical and not electrical. Modern planes are tested very thoroughly for cell phone interference. Older planes will be much more vulnerable. Communication and navigation equipment can be affected. From pilots that I have talked to about it, navigation equipment problems are rare. The most common problem by far is that cell phones cause noise in the pilot’s headsets, making communication to the tower much more difficult. It depends on your definition of “most.” All Airbus aircraft have electric controls (fly by wire), and I think some of the later Boeing models, such as the B777, do as well. I haven’t actually heard people claiming that the phones interfere directly with the controls, it’s more that they could interfere with the nav equipment and the aircraft are often steered directly by the nav signal via the autopilot. ---
**Author:** ![elmwood](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/elmwood/32/3060_2.png) [@elmwood](https://boards.straightdope.com/u/elmwood)\ **Post date:** [April 16, 2009, 5:13pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/26 "2009-04-16T17:13:00Z")
> [@Xema](#): > > I hear varying reports on this: some say these were cellphone, others that the phones were the “airline seatback” types, specifically designed to work in flight. Anyone have the straight dope here? A slight hijack: whatever happened to seatback phones? A few years ago, they were everywhere. Today, they’re nowhere to be seen. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [April 16, 2009, 7:25pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/27 "2009-04-16T19:25:06Z")
Old analog systems would be more likely to work. Those were just phased out. Hitting too many cell sites, as mentioned above, could be a problem. Newer digital systems have a hard limit of ~15 miles between the tower and the handset, due to the required timing. Having a ground speed of 600mph might mess with the timing as well…probably not though. The biggest issue is probably just range. I’ve had people tell me that there is digital cell coverage “virtually everywhere”. IME this is true only if you keep to populated areas or the interstate highway system. Until it was phased out, analog ruled in remote areas, range being limited only by signal strength. ---
**Author:** ![manila](https://avatars.discourse-cdn.com/v4/letter/m/e5b9ba/32.png) [@manila](https://boards.straightdope.com/u/manila)\ **Post date:** [April 16, 2009, 8:41pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/28 "2009-04-16T20:41:28Z")
My cell has its sim set to roaming. ( Smart Philippines) A few times I have thrown my jacket including the cellphone absentmindedly still switched on in the overhead locker. When I landed at Dubai I read several “welcome” text messages from various countries that we had flown over. So obviously the phone had picked up some signals, despite flying at jet airliner level which could have been upto 45k ft I also understand that now you can legitimately use your cell on some flights? Does anyone know more? ---
**Author:** ![jackdavinci](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackdavinci/32/3259_2.png) [@jackdavinci](https://boards.straightdope.com/u/jackdavinci)\ **Post date:** [April 17, 2009, 7:23am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/29 "2009-04-17T07:23:15Z")
> [@Whack-a-Mole](#): > > As I understand it the main problem with using a cell phone on a plane is you pass from tower to tower too quickly. Cell towers work by passing you from one tower to the next as you move. In a plane you are zooming from tower to tower so fast the system has trouble handing you off one to the next. Assuming you were driving in a ground vehicle that wasn’t limited in speed, but would be the maximum speed the towers could handle? Or many a more appropriate question: what rate of tower switching can the phone system handle? Is there some speed at which some kind of doppler effect affects things? ---
**Author:** ![Shadowfyre](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/shadowfyre/32/9972_2.png) [@Shadowfyre](https://boards.straightdope.com/u/Shadowfyre)\ **Post date:** [April 17, 2009, 3:03pm UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/30 "2009-04-17T15:03:18Z")
The timing issue is only an issue for GSM equipment where the tower broadcasts a number of calls on a single frequency but transmits each call in it’s own time window. For instance Call A is transmitted for X microseconds, then Call B is transmitted for X microseconds, then Call C, then back to Call A to start the cycle over again. The cell phone needs to shift its receiving time window to match the tower’s as distance changes cause the tower’s signal to take more or less time, depending on if the cell phone is moving away from the tower or towards the tower respectively. As the cell phone moves away from the tower, it becomes more and more difficult to keep the time window on the cell phone in sync with the time window on the tower. The system will probably try to hand the cell phone off to a closer tower if one is available at this point. That 30,000 foot distance may be too much of a time window discrepancy for the system to handle. In the U.S., Sprint and Verizon use CDMA (no sim cards) instead of GSM. In CDMA, all the calls are on the same frequency but use codes to differentiate cell phones. But as the number of cell phones calls increase on a single tower, the noise floor gets higher and weak calls can be lost in that noise. So in response to that, the cell phone has to crank up its power to be heard over the noise. The tower can also adjust its power output, resulting in a varying geographical cell size. Towers try to keep their power as low as possible to keep the noise floor as low as possible and it’s not uncommon for a cell phone on one of these networks to be communicating with two or more cell towers at one time, so the system tries to hand off as much as possible to keep the noise floor low. The system is also programmed to drop calls if the tower’s power gets too high and it can’t hand the call off to another tower. Which is very likely to be the situation of a phone on a plane. Also, uban and suburban areas have a lot of towers with small geographic cells. This increases system capacity for both types of systems. With the towers using directional antennas and broadcasting at a lower power to keep the cell sites small, you might not get any signal in a plane. ---
**Author:** ![Cfgray](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@Cfgray](https://boards.straightdope.com/u/Cfgray)\ **Post date:** [September 12, 2011, 12:54am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/31 "2011-09-12T00:54:37Z")
> [@dolphinboy](#): > > Let’s wait for an expert to show up… but I vote for A, since we learned from 9/11 that apparently you can use a cell phone from say 10,000 feet. The cellular signal cant keep up at high speeds. Traviling in a car at anything over 180Km per hour you will find great difficulty making a call. So the truth is those so called 911 cell phone calls did not happen. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [September 12, 2011, 1:18am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/32 "2011-09-12T01:18:25Z")
> [@Cfgray](#): > > The cellular signal cant keep up at high speeds. Traviling in a car at anything over 180Km per hour you will find great difficulty making a call. > So the truth is those so called 911 cell phone calls did not happen. **Cfgray,** this is GQ (General Questions), and if you make a statement like that in this forum, you need a damn good cite to back it up. Your word alone is worthless unless you can also give us your credentials and they are appropriate. So, cite? ---
**Author:** ![cochrane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cochrane/32/10441_2.png) [@cochrane](https://boards.straightdope.com/u/cochrane)\ **Post date:** [September 12, 2011, 1:29am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/33 "2011-09-12T01:29:42Z")
> [@Cfgray](#): > > The cellular signal cant keep up at high speeds. Traviling in a car at anything over 180Km per hour you will find great difficulty making a call. > So the truth is those so called 911 cell phone calls did not happen. Cite? According to 9/11 Commission report (cited by **HorseloverFat** in [Post #15](http://boards.straightdope.com/sdmb/showpost.php?p=11034120&postcount=15)), there were two confirmed cell phone calls made from United Flight #93 on 9/11/2001. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [September 12, 2011, 2:30am UTC](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960/34 "2011-09-12T02:30:31Z")
> [@Cfgray](#): > > The cellular signal cant keep up at high speeds. Traviling in a car at anything over 180Km per hour you will find great difficulty making a call. > So the truth is those so called 911 cell phone calls did not happen. No offense, but including that last sentence makes your post sound a bit like conspiracy whack job nonsense. Your understanding of cell phone systems is a bit flawed. The speed of travel has never been much of an issue with cell phones. The problem with airborne cell phones in the early days was that they would contact multiple towers simultaneously, and this wreaked havoc upon the ground systems since they didn’t handle the handoff from one system to another all that gracefully when multiple towers from multiple systems were involved. Modern systems are more robust, and this is no longer a problem. Cell phones are banned on airplanes because of potential interference with aircraft systems, and also because they tie up multiple communication channels on multiple towers, which the FCC considers to be an unacceptable waste of available bandwidth. If you are willing to violate the law, however, you will find that cell phones will usually connect just fine up to about 3,000 to 5,000 feet or so, even at speeds well in excess of 180 kph. After that, the higher you go the less likely it becomes that you will be able to connect, and once you get above 10,000 feet your chance of a successful connection is pretty close to zero. Note that in some European countries, their equivalents of the FCC and FAA do not have the same restrictions as ours, and they do allow airborne cell phones to be used. [Previous page](https://boards.straightdope.com/t/why-dont-cell-phones-work-on-airplanes-at-30-000-ft/492960.md?page=1)