WHICH is worse: not being able to use a mobile phone while on board an aircraft—or being able to do so? Just about the last thing most people want is to be trapped next to someone nattering endlessly into a mobile phone, oblivious of everyone within forced earshot. Rudeness and lack of consideration know no bounds for some folk. And sad to report, as mobile phones have proliferated, such crass behaviour is no longer the isolated exception within an otherwise civil crowd. Were the Federal Communications Commission (FCC) to relax its rule banning the use of mobile phones once an aircraft leaves the ground, there would be fist-fights aplenty above the clouds.



Fortunately for those who value at least peace and quiet when crammed in an airline seat, the FCC is in no hurry to relax its ban. However, regulators are finally thinking of permitting the use, during take off and landing, of other sorts of electronic equipment currently forbidden, except when the plane is in level flight. Understanding why this change is possible illuminates a common misconception about what it is that means mobile phones and aircraft do not mix. For the ban exists not (as the public is often led to believe) because mobiles disrupt an aircraft’s sensitive avionics, but rather to stop them playing havoc with the phone companies' receiving equipment on the ground that is trying to handle their calls.



In theory, any piece of electical equipment might interfere with a plane's GPS navigation equipment, VHF omnidirectional range/locators and instrument-landing system. Even gizmos that are not supposed to, such as audio players and video-game machines, may emit spurious radio-frequency energy. The worst of these "unintentional" transmitters are probably the cables and power supplies that passengers bring on board to charge their laptops and tablets from in-seat power sockets provided nowadays by numerous airlines.



Intentional transmitters, like mobile phones, two-way pagers and walkie-talkies, present a different sort of problem. With these, engineers worry about the so-called "near-far” effect. Even if below permitted levels, any spurious emissions they might produce would occur close to an aircraft’s avionics, compared with the weak signal from a ground-based radio beacon hundreds of kilometres away, or the whispers from a GPS satellite thousands of kilometres up in space. The concern here is that weak, distant signals might be drowned out as a navigation receiver captures a spurious signal that may also be weak but is significantly closer.



There are dozens of anecdotal reports of instruments on the flight-deck being affected by passengers using portable electronic devices, or PEDs as they are known in aviation circles. Unfortunately, duplicating these under controlled conditions has proved nigh-on impossible. Both Airbus and Boeing have bombarded their aircraft with electromagnetic radiation at frequencies and power levels used by mobile phones, only to come away empty handed.

In practice, then, the chance of unintentional transmitters doing any harm is infinitesimal. Indeed, that is just as well, because flight crews have had permission from the FAA to use portable computers called “electronic flight bags” in the cockpit since the early 1990s. Today, they carry iPads and other tablets as replacements for the bulky aircraft operating manuals, flight-crew manuals and navigation charts. These portable electronic devices are in much closer proximity to the aircraft’s avionics than anything passengers are likely to bring aboard, and remain switched on throughout the flight.



On top of that many passengers leave their phones, laptops, tablets and other PEDs turned on during take off and landing, even though they are not actually using them. They may not realise it, but closing the lid of a laptop merely puts it into standby or hibernation mode. Meanwhile, the device’s clock circuit continues to hum away and the Wi-Fi chip carries on hunting for a connection. The same goes for mobile phones and tablets, which are often put into standby mode rather than being switched off properly. The latest tablets and smart phones have an “aeroplane mode”, which at least switches off the device’s various radios.



And that is not to consider the scoff-laws who deliberately leave their phones on after the aircraft doors have shut, so they can send text messages and make calls surreptitiously. A three-month study done in 2003 (published subsequently in the Institute of Electrical and Electronics Engineers’ IEEE Spectrum) found that between one and four covert calls were made on each of the 37 domestic flights the researchers monitored with a hidden spectrum analyser.

A decade later, with practically everyone on board now possessing at least a mobile phone, there are probably dozens of such illicit calls being made on every flight. Yet there has still been no airline incident, let alone an actual accident, caused by a personal gadget being used legally or illegally during a flight. Despite 20 years of testing, the authorities have been unable to demonstrate that mobile phones and other electronic devices can interfere with an aircraft’s navigation and communications gear.

The truth is that the FCC never was concerned about the possibility of electronic interference when, in 1991, it banned the use of mobile phones on board aircraft. All it was really worried about was their impact on cellular networks on the ground. These work on the principle that, at any given moment, a mobile phone is within range of only one or two nearby masts. Each mast uses a set of channels different from those allocated to the masts closest to it, but the same as others further away. In this way, each channel can be used, and reused, to carry calls from multiple users.



Unfortunately, a mobile phone operating in an aircraft flying overhead might be within reach of any number of masts using the same channels. This could not only cause calls to be dropped, but would also confuse the network's software—reducing the mobile system’s overall capacity by blocking the reuse of channels.



There is also the added problem of an airborne phone moving too fast across the sky for the ground-based network to respond. The highest speed a mobile network is expected to cope with is that of an express train—not a passenger jet travelling at just below the speed of sound. A mobile used on an aircraft could traverse a tower too quickly to register with the network. If that happened, it would then bombard multiple towers along its route with repeated attempts to register, causing yet further network confusion.



So there are sound technical reasons why the FCC prohibits the use of mobile phones in the air. But why are laptops, tablets, e-readers, audio players and other PEDs banned during take off and landing? Like aviation authorities elsewhere, the Federal Aviation Administration (FAA) says these cannot be used until the flight crew give permission and the aircraft has climbed to an altitude of at least 3,000 metres.



Why so arbitrary an altitude? The standard explanation is that, above 3,000 metres, there would be enough time and height for the flight crew to diagnose any problem potentially caused by an errant gizmo and make a correction. Close to the ground, during take off and landing, there would be a whole lot less.



That might sound reasonable, save for the fact that all those years of tests—and all those live gadgets in the cockpit—suggest the actual threat is approximately zero. Last March, following numerous demands for the ban to be lifted, the FAA unexpectedly told Nick Bilton of the New York Times that it was going to take a “fresh look” at the use of electronic gadgets on board commercial flights. Mr Bilton has been nagging the FAA to relax its regulations governing electronic devices that demonstrably cause no harm to an aircraft’s instruments. A bigger surprise is that he has now been joined by the FCC itself.



On December 6th the commission’s chairman, Julius Genachowski, wrote to the FAA’s acting administrator, Michael Huerta, urging the airline regulator “to enable greater use of tablets, e-readers and other portable electronic devices during [all phases of] flight.” The FCC agreed to work closely with both the FAA and industry to ensure a successful outcome.



It is not that the FAA has been dragging its feet out of bureaucratic bloody-mindedness. The problem is that the current guidelines require each airline to test every make and model of each gizmo it wants the FAA to approve for use on its flights—and then to do the same for every type of aircraft in its fleet. The airlines have baulked at such a monumental task because of the cost. The FAA is now looking for ways to bring airlines, aircraft manufacturers, electronics makers and other interested parties together to streamline the certification process at least for tablets, e-readers, game machines and a few other popular gadgets.



But do not expect such easing to extend to phones. One reason is that, with the enormous number of makes and models in existence, getting all mobiles approved for use on board aircraft would be prohibitively expensive. Another is that the ground-based interference problem has still to be resolved.



Perhaps it never will be; and maybe that will not matter. The FCC has allocated frequencies in the 450 megahertz and 800 megahertz bands for dedicated air-to-ground services that communicate with widely separated ground stations. Small cellular base-stations, known as pico-cells, are beginning to sprout in aircraft cabins, so passengers can make Skype calls to friends and family on the ground, and download content from the internet while airborne. The Wi-Fi connections to an airborne pico-cell are bounced up to a satellite before being beamed down to the ground using one of the special frequency bands allocated for the job.



The service is no bargain. Virgin Atlantic, which expects to have 20 aircraft offering internet access on its Atlantic routes by the end of this year, is charging $1.60 a minute for calls and 32 cents for text messages. Your correspondent is quietly relieved that the cost alone may deter many users—just as it did with those hard-wired, back-of-the-seat phones.