Q17:  Can I use a radio, either a broadcast or aviation receiver, in an
      aircraft?

A:   FAR 91.21 governs portable electronic devices.  Use of a receiver is
     prohibited except for units which "the operator of the aircraft has
     determined will not cause interference with the navigation or
     communication system of the aircraft on which it is to be used."
     "(c) In the case of an aircraft operated by a holder of an air carrier
     operating certificate ... the determination ...  shall be made by that
     operator of the aircraft on which the particular device is to be used.
     In the case of other aircraft, the determination may be made by the
     pilot in command or other operator of the aircraft."

     In plain English, this means that on an airliner, the _airline_ must
     allow you to operate the radio -- the captain does not have the sole
     authority to authorize its use.  On a private aircraft, the captain does
     have that authority.  Note that amateur (ham) radio operators are
     forbidden by FCC regulations from transmitting on _any_ IFR flight.