In 1992 the FAA asked the RTCA to investigate potential interference effects of portable electronic devices (PEDs) upon aircraft electronic systems. The terms of reference included developing appropriate methods of testing and to make recommendations on acceptable operating restrictions to apply to the use of PEDs aboard aircraft. The RTCA set up Special Committee SC-177 which developed tests, carried out testing of various electronic devices and published its recommendations in 1996. The reference document is "Portable Electronic Devices Carried On Board Aircraft" Document No. RTCA/DO-233 dated 20 August 1996 - a hefty document consisting of 160 pages of discussion, scientific appendixes and tables.
The report recommended that the FAA should modify FAR 91.21, Portable Electronic Devices so that . .(a) The use of any PED is prohibited in aircraft during any critical phase of flight.. .(b) The use of any PED which has the capability to intentionally transmit electromagnetic energy is prohibited in aircraft at all times unless testing has been conducted to ascertain its safe use.
The FAA accepted the recommendations and revised FARs accordingly. The JAA also included similar but more stringent requirements into the appropriate section of JARs.
So, the answer to the question is that the ruling is based upon empirical data collected by scientific investigation of the interference radiated from various PEDs - including CD Players. A mobile phone obviously falls within the definition of being capable of intentionally emitting electromagnetic radiation and is banned at all times. Portable CD Players have been demonstrated to radiate unintentionally and are therefore prohibited during critical phases of flight.
Airlines interpret the ruling in different ways because there is no set definition of a "critical phase of flight" The best definition would seem to be that a critical phase of flight occurs whenever the commander of the aircraft considers that the aircraft is in a critical phase of flight. In practice, individual Flight Operations Departments set a policy to be follwed by all its crews which is usually transmitted to the passengers by a policy statement in the In Flight Magazine or other convenient document.
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