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Although there are no documented cases of civilian airline crashes caused by cellular telephone or other interference, electronic devices may pose problems to airplane control systems. (see footnote 16) Of the approximately 100 reported cases of alleged interference, about one-third appear to have some validity, according to technical experts. (see footnote 17) FAA regulations hold the airline companies responsible for setting policies on the use of portable electronic devices; given the difficulty in assuring safe operation under all operating conditions, all airlines have decided to prohibit the use of any electronic devices during take-offs and landings. (see footnote 18)
Inside an aircraft, radio transmitters, such as cellular telephones, can induce transient currents in wires and even be amplified in the aluminum airframe, because any unshielded metal can act as an antenna. CTIA is currently testing cellular telephones in planes to certify their safe use on the ground. (In addition, cellular telephone use on commercial aircraft in flight is not allowed because a single cellular telephone at even moderate altitudes would tie up many terrestrial cellular base stations simultaneously, since many base stations could be "seen" simultaneously by an airborne cellular telephone.)
A potential problem with American Mobile Satellite Corp.'s (AMSC's) transportable telephone is that it will operate at a frequency adjacent to that used by the Global Positioning Satellite (GPS) system, which will serve as the basis of the new generation of air traffic control systems in the United States. Operating such a telephone in an airplane may jam the GPS navigation system. The FCC, the National Telecommunications and Information Administration (NTIA), and the Federal Aviation Administration (FAA) have established a procedure with AMSC and other interested parties to address this problem; a memorandum of understanding that was concluded in November 1994 provides the means to prevent interference and allows AMSC to proceed with its system's deployment. (see footnote 19)
Interference could be more serious between portable electronic devices and digital flight equipment, including navigation systems. These systems work with digital bit streams, which can be thought of as strings of ones and zeros. Interference might occur by inducing spurious currents and thus introducing new data to the normal data stream. Such data would probably be rejected by error- correcting routines in current avionics, resulting in an interruption rather than a deviation of normal aircraft control systems, but it is difficult to know with certainty that this would always occur. Even devices that are not designed as radio transmitters emit electromagnetic radiation. This has led to concern that uncontrolled use of any electronic device might cause interference. One recently publicized case involved a pilot who believed that a CD player in use in the first-class compartment interfered with the normal operation of the aircraft during landing. (see footnote 20)
Because analog avionics systems are not dependent on data streams, they are not susceptible to such interference. Thus, where a digital cellular telephone may affect new Airbuses or Boeing planes, it is unlikely to affect an older Boeing 727. On the other hand, newer aircraft use fiber optic cabling for control systems and more fault-tolerant architectures, making them less susceptible to radio interference.
It is extremely difficult and costly to model these internal interference problems. Because there are so many variables-- the type of emitting device; its power, frequency, and modulation schemes; the effectiveness of its filters; its place in the aircraft, the location of sensitive instruments, the location of wire or airframe with respect to the emitting device, and the activity the aircraft is performing (e.g. landing or cruising at altitude)-- determining all the conditions for trouble-free operation of portable devices is nearly impossible.