Mobile phones on aircraft
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The use of mobile telephones is generally forbidden aboard commercial aircraft during flight.
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[edit] Technical issues
[edit] Channel reuse
The U.S. Federal Communications Commission prohibits the use of mobile telephones aboard any aircraft in flight. The reason given is that mobile phone systems depend on channel reuse, and operating a phone at altitude may violate the fundamental assumptions that allow channel reuse to work.
Mobile telephones are intentionally designed with low power output. A tower is the center of a "cell" and due to attenuation with distance (inverse square law) a phone can usually be received only weakly by towers in adjacent cells, and not at all in cells farther away (non-adjacent cells). This allows the channel used by any given phone to be reused by other phones in non-adjacent cells. This principle allows tens or hundreds of thousands of people to use their phones at the same time in a given metropolitan area while using only a limited number of channels.
Channel reuse works because from a mobile phone on the ground, there will only be one "closest" tower that can possibly use a particular group of frequencies, CDMA codes, or time slots. The software that manages the system assumes that the signal from a phone on a particular tower can, on other towers, only be "heard" at greatly reduced signal strength. The frequency, code, or time slot used by the phone can therefore be reused by other phones on other towers.
In the old analog cell system a channel was simply a frequency pair; there were seven groups of 35 channels each, and no two adjacent cells used the same channel groups. Modern CDMA and TDMA systems are more complex: A channel in TDMA is a frequency pair and a time slot, and a channel in CDMA is a spread spectrum key, but the principle of channel reuse still applies.
If a mobile phone is operated from an aircraft in flight above a city, this assumption is no longer valid, because the towers of many different cells may be about equidistant from the phone. Multiple towers might assume that the phone is under their control. The phone could be assigned a free channel by one tower, but could be heard on other towers using the same channel group, and the channel might already be in use on those towers. This could cause interference with existing calls. It is possible that the software controlling the towers could crash. Even if the software can cope with hearing the same phone on multiple, non-adjacent towers, the result at best is an overall decrease in the system's capacity.
An additional concern is the output power of the mobile handset. Because the towers might be many miles below the aircraft, the phone might have to transmit at its maximum power to be received. This will increase the risk of interference with electronic equipment on the aircraft.
[edit] Electromagnetic interference
Some level of electromagnetic interference to aircraft systems is theoretically possible not only from active radio transmitters (such as mobile phones, small walkie-talkies, or radio remote-controlled toys), but also from unintentional emitters (such as ordinary radio receivers, computers, and in fact virtually any digital electronic device).
There are some reports that seem to support this:
- A NASA report from 2001[1] summarizes "14 years of incidents reported by pilots to the Aviation Safety Reporting System" of interference caused, or suspected to be caused, by passenger electronic devices. Mobile phones were the most frequently identified source of interference, with laptop computers a close second. One must remember, however, that the Aviation Safety Reporting System is a completely unverified collection of anonymous, anecdotal reports. The ASRS was set up by NASA to encourage reporting of safety related incidents by pilots, crew and mechanics anonymously, without fear of repercussions. None of these incidents have been verified, nor is the identity of the reporter known.
- Another NASA publication[2] details the fifty most recent reports regarding "avionics problems that may result from the influence of passenger electronic devices." Although some of the reports merely have to do with passenger interactions with flight crews, a number of them document cases of anomalous instrument readings and other problems, apparently associated with passengers' use of mobile phones as well as other devices. Of course, this publication, based as it is on the unverified reports in the Aviation Safety Reporting System, cannot be considered conclusive by any means. Pilots and flight crew have neither the expertise in radio frequency technology nor the time to effectively determine whether a given anomaly experienced during a flight is actually attributable to a passenger using an electronic device. ASRS reports amount to crews reporting experiencing an anomaly at the same time a passenger was witnessed using a mobile phone. At best, this shows a weak correlative relationship, not causality.
- A 2003 study [3] found that mobile phones and many other passenger electronic devices can and do interfere with aircraft systems. This is true even for some unintentional radiators, even though the devices meet FCC type approval requirements for unintended, unlicensed emissions. The report is mostly speculative, however, and admits that "There is no smoking gun to this story: there is no definitive instance of an air accident known to have been caused by a passenger's use of an electronic device."
According to the BBC "most of the evidence is circumstantial and anecdotal. There is no absolute proof mobile phones are hazardous." [2] This opinion is consistent with the unverified nature of the studies above. To date, no study has ever shown an avionics malfunction caused by an actual mobile telephone in a real-life situation, despite the fact that this type of study would be very easy to set up. All studies have drawn speculative conclusions based on observations of signal strength in worst-case scenarios and the design tolerance of the avionics.
Whether interference from small battery-powered devices should have any influence on electronic systems that should be designed to fly through lightning storms without failing is often disputed by critics of the ban. An article[4] by Tekla S. Perry and Linda Geppert, then editors of IEEE Spectrum, offers an explanation: While a brand new aircraft may indeed be completely immune from such interference, shielding and other mechanisms that normally protect the avionics do degrade over time, after thousands of takeoffs, landings, and pressurization cycles and various maintentance procedures. Similarly, the shielding in passengers' devices also degrades, due to the simple passage of time and, in some cases, repair procedures.
The 2003 study also found that on a typical flight at least one mobile phone is likely to be left on throughout a typical flight. Some have suggested that interference from mobile phones must therefore not be a significant problem, since no actual accidents have ever been traced to this cause.
The same study, however, also showed clearly that a mobile phone in use produces a far stronger signal than one that is simply left switched on; therefore the risk of mobile phone calls cannot be dismissed. This argument also ignores the fact that should the ban be lifted, a great many mobile phones would be switched on and in use at the same time; the risk of this cannot be dismissed on the basis of experience with the present very small numbers.
Since the number of commercial aviation accidents from all causes each year is extremely low compared to the total number of flights, and mobile phones are indeed used in spite of the ban, it is clear that the risk of an accident stemming from present (illegal) use of mobile phones must be small. Weighing against this is that the cost of an accident, should one occur, could be extremely high in terms of human life, and that the risk is completely avoidable, in that no one absolutely needs to use their mobile phone in flight. The regulatory agencies and aviation industry understandably take the position that any increased risk is unacceptable if it is avoidable.
Some mobile phone systems, such as GSM may cause an irritating buzz (explained in the TDMA article), which could disrupt communications from the pilot to ground.[citation needed]. The high speed of air travel may make interference more likely than it would otherwise be. The maximum speed of travel in a mobile phone system is limited by several factors: frequency changes, rate of change of timing offset, etc. The speed of an airplane often exceeds these (typically phones are designed for use in a fast car) which means the phone will fail to register to the network and retry registration repeatedly.
While certainly not a rigorous scientific study, the MythBusters conducted several tests and determined that the myth that cell phones are dangerous to aircraft is "busted".[5]
Despite the FCC ban on the use of mobile telephones on aircraft, virtually every pilot headset sold on the market today comes with a cell phone adapter so that the pilot can use his cell phone through the headset. The manufacturers avoid legal responsibility by claiming the adapters are for use on the ground, but many general aviation pilots have used their cell phones in flight at one point or another.
[edit] Other factors
[edit] Social resistance to mobile phone use on flights
Many people prefer the ban on mobile phone use in flight, fearing undue amounts of noise from cellphone chatter. People tend to talk more loudly into mobile phones than they do in person.
There are several reasons for this. One reason is people subconsciously rely on hearing their own voice back to modulate their own speech volume. Mobile phones don't send "backtalk" (a reduced volume "copy" of the person's speech) back into the earphone. Land-based phones have done this for a long time (this is also the cause of the "echo" effect you can hear on many cordless phones). Another reason is the conversation in the phone may demand more mental processing power. This is because mobile phones sound different from normal speech due to audio data compression or analog background noise. Whatever the reason, the net effect is that the person is less aware of the people around him/her.
Also, one must not forget the very high level of background noise on an airplane which would invariably force people to talk even louder into their cell phones.
Cingular has suggested that in-flight mobile phone restrictions should remain in place, in the interest of reducing the nuisance to other passengers caused by someone talking loudly on a phone next to them. [3]
[edit] Competition for airlines' in-flight phone service
Some believe that the airlines support the ban because they don't want passengers to have an alternative to the in-flight phone service (such as GTE's Airphone). These services are much more expensive than mobile phone service. They also provide extremely slow data services at a similarly high price. In general, the airlines have had little success in selling these services, and the in-flight phone equipment has disappeared from most U.S. domestic flights.
Nevertheless it is apparently easy to believe that the airlines support a continued ban on mobile phone use so as to force customers to use the in-flight phone service. [4]
This idea is not supportable in light of the following:
- Use of all passenger electronic devices is banned by nearly all airlines during take-off, climbout, approach, and landing phases of flight (essentially whenver the aircraft is below 10,000 feet); and
- Mobile phones are not at all reliably usable at cruise speed and altitude.
The radiation pattern of the antennas on mobile phone towers intentionally causes strong attenuation at angles much above horizontal, and Doppler shift causes non-recoverable phase shift errors at high relative speeds between phone and tower.
Mobile phones therefore offer no real competition to the in-flight phone service.
[edit] Current regulations
[edit] United States
The use of cell phones aboard airborne planes is banned by the FCC in 47 C.F.R. § 22.925: "The use of cellular telephones while this aircraft is airborne is prohibited by FCC rules.... The use of cellular telephones while this aircraft is on the ground is subject to FAA regulations." This ban applies to phones that use the 800 MHz spectrum, but technically does not apply to phones that use the 1.9 GHz spectrum [5]; nevertheless it is usually interpreted as applying to both.
The FAA, in 14 C.F.R § 91.21 bans the use of all portable electronic devices (with a few odd exceptions) for all flights operated by an airline or under IFR, but allows that the pilot (or airline) can make any exception to this rule if the operator deems that devices safe. This effectively gives the operator (airline or pilot) the final word as to what devices may be used aboard an aircraft as far as the FAA is concerned, although the FCC restriction still applies.
No U.S. airlines have approved the use of mobile phones while in flight.
Note that for aircraft operated by an airline, the pilot is not considered the "operator" and cannot legally allow exceptions to the airline's restrictions, but the pilot may dictate additional restrictions.
The FAA, in Advisory Circular 91.21-1A recommends that aircraft operators blanket ban all intentional transmitters, mentioning specifically: CB radios, remote control devices, and cellular phones. While Advisory Circulars are not legally binding, air carriers rarely ignore official written advice from the FAA.
However, this Advisory Circular has been superseded by AC 91.21.1B.
FAR 91.21 elude that the Pilot In Command of an aircraft that is NOT IFR, and NOT Part 121 (Commercial Air Carriers), can allow usage of "Portable Electronic Devices". However to take the attitude that "The FAA doesn't say I can't do it" is incorrect, particularly in the category of Radio-Telephone communications, governed by the FCC. In the FCC regulations, specifically Title 47 Part 22.925, (Oct 1, 2006 revision) states "Cellular telephones installed in or carried aboard airplanes, balloons or any other type of aircraft must not be operated while such aircraft are airborne (not touching the ground). When any aircraft leaves the ground, all cellular telephones on board that aircraft must be turned off.".
[edit] Possible future changes
A few U.S. airlines have announced plans ([6] and [7]) to allow mobile phones to be used on aircraft, pending approval by the FCC and the FAA. The idea is similar to that used in some cars on the German ICE train: the aircraft will contain a mobile signal repeater that will then transmit the signals to a terrestrial-based system on separate frequencies that do not interfere with the cellular system. Since the repeater would be very close to the passengers and inside the aircraft's metal shell the phones' output power could be reduced to the lowest level possible, reducing interference both with cells on the ground and with the flight-critical avionics. Such systems have been tested on a few flights within the United States, under waiver from the FCC.
ARINC and Telenor have formed a joint venture [8] company to offer such a service on board commercial aircraft. The mobile phone calls are routed via satellite to the ground network and an on-board EMI screening system stops the cellphones contacting the ground network.
These systems are comparatively easy to implement for customers in most of the world, where GSM phones operating on either of just two bands are the norm. The multitude of incompatible mobile phone systems in the United States makes the situation more difficult. It is not clear that the onboard repeaters will be compatible with all of the different cell-phone protocols (TDMA, GSM, CDMA, iDen) and their respective providers. If a U.S. airline choose to support one mobile carrier and not another, they would be in the unenviable position of having to tell passengers that some of them could use their mobile phones while others could not.
On 30 August 2006, the Irish low-cost airline Ryanair announced it is to introduce a facility to allow passengers to use their mobile phones in-flight.[citation needed]
[edit] See also
- Crossair Flight 498 - an alternate theory of the 2000 crash of this flight was based on the use of passenger cell phones, which resulted in a number of countries outlawing use of cell phones on flights.
[edit] References
- ^ NASA CR-2001-210866 Personal Electronic Devices and Their Interference with Aircraft Systems
- ^ NASA ASRS Database Report Set Passenger Electronic Devices
- ^ IEEE Spectrum. (March, 2006). Unsafe At Any Airspeed?
- ^ Airline Pilot. (Aug., 1997). [1]
- ^ MythBusters "bust" cell phones on airplanes myth. MythBusters Website
