An extract from the JAA Guidance Leaflet No. 29 dealing with cellphones. TGL 29 has been published by the JAA on behalf of its 33 member European authorities and sets the policy for airlines operating under JAA regulations.
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Cellphones are both non-intentional and intentional transmitting PEDs, operating on spot channel frequencies in the bands of approximately 415 MHz, 900 MHz or 1800 MHz. (Some regions of the world use slightly different bands). Most use digital modulation but analogue types are still in use. Their maximum transmitted power is in the range of typically 1 to 5 watts. The actual power transmitted at a particular time is controlled by the cellular network and may vary from 20mW to maximum rated power of the cellphone depending on quality of the link between the cellphone and the network. Even in standby mode, a cellphone transmits periodically to register and re-register with the cellular network and to maintain contact with a base station.
The transmitted power and precise radio frequency of an operating cellphone is dependent on the traffic on the network, the distance of the cellphone from the nearest base station, and any obstacles or attenuation in the signal path. An aircraft on the ground at an airport is likely to be in close proximity to a base station resulting in a strong link between that station and an onboard cellphone. Under these circumstances, the cellphone would seek a free channel in the assigned communication band and its output power would be set by the network to a low level sufficient to maintain the link. Interference levels would, as a result, be low and probably harmless but this cannot be guaranteed. Closing of the aircraft doors increases attenuation in the signal path, and as the aircraft increases its distance from the base station, the output power setting of the cellphone is increased, eventually to its maximum rating. The risk of interference is then at its greatest. At altitude, the cellphone will transmit periodically attempting to register with the cellular network. The quality of the link is likely to be poor and the cellphone will radiate maximum power in these circumstances. Furthermore, since it is likely to be in line-of-sight range of multiple base stations, some degradation of the network operation may result and actual communication may not be possible.
The effect of this type of functioning is that, when the aircraft is on the ground near a base station, the interference risk can be low but not negligible, and it will increase as the aircraft taxies and then climbs away from the network base stations.
The simultaneous use in an aircraft of several cellphones will result in transmissions at different radio frequencies leading to a more complex interference environment.
Reports of interference are increasing but it has been difficult to positively confirm in all cases that a PED has been the actual cause of a problem. This is due to the difficulty in replicating the conditions that existed at the time of the occurrence due to the multiple factors involved (e.g. geographical location of the aircraft, system operating modes, interference frequency and intensity, source location in the aircraft, and path attenuation). Cellphones have been positively identified as the cause of degraded communications and of false baggage compartment smoke warnings. Cellphones have been strongly implicated in other spurious cockpit warnings, corrupted instrument displays, and pressurisation system malfunctions.
Although the total number of reports is relatively low considering the aircraft flight hours involved, the potential severity of the effects of interference means that the problem cannot be ignored.
JAA documents are available from Information Handling Services (IHS). Information on prices, where and how to order, is available on the JAA web site (http://www.jaa.nl) and on the IHS web sites
http://www.global.ihs.com and
http://www.avdataworks.com.
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