This struck me as a bit strange, 91.21 which covers electronic devices that can be used during IFR flight has some exceptions, one through three make logical sense (not really sure why portable voice recorders are in there though) but number 4, Electric Shavers, is specifically permitted by the regulation. Are that many people shaving under IFR conditions that this needed to be permitted? Is there some historical or practical reason why electric shavers are specifically named in this regulation?
(a) Except as provided in paragraph (b) of this section, no person may operate, nor may any operator or pilot in command of an aircraft allow the operation of, any portable electronic device on any of the following U.S.-registered civil aircraft:
(1)Aircraft operated by a holder of an air carrier operating certificate or an operating certificate; or
(2) Any other aircraft while it is operated under IFR.
(b)Paragraph (a) of this section does not apply to -
(1) Portable voice recorders;
(2) Hearing aids;
(3) Heart pacemakers;
(4) Electric shavers; or
(5) Any other portable electronic device that 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.