Most of what you say below meshes with my experience and does not contradict my basic premise, that PEDs can only interfere through aircraft antennas. I am curious what the resolution of the Boeing installation was. Equipment undergoing EMI qualification must be tested with a representative flight harness. Did your company test with screened cables and then try to force Boeing to use the same? Bad form. Did Boeing try to buy an off-the-shelf system qualified with screened cables and then install the system using unscreened cables? Equally bad form. This must be worked out before design and testing for procured equipment, and if the equipment is off-the-shelf, then the qualification configuration harness must be installed, or the equipment must be requalified using the planned/existing configuration wiring. Another question of interest: Was the system you provided Boeing flight critical?
There is one place that what you say could be interpreted to imply that PED emissions get into aircraft wiring: "It has been found through surveys carried out on aircraft by ERA and QinetiQ that the interference appears to get into these systems from certain locations within the aircraft where cable run reside under certain seats." Consider the physical parameters. The PED is small and low power and while it may not meet RTCA/DO-160, it does not blanket the entire aircraft with emissions. The intensity falls off rapidly with separation from the source. This is clearly a case where the emitting device is electrically small over almost the entire communications band. Assume the emitted radiation intensity were 100 mV/m, 5-6 orders of magnitude above CISPR limits. The transfer function of coupled current to a cable above ground is 1.5 mA per Volt/meter. I can supply that derivation if you like, but it is inherent in both RTCA/DO-160D and MIL-STD-461D/E. It is different in IEC 61000-4-6, but that is because the ground plane is far away or nonexistent in buildings and the cable-under-test is a more efficient pick-up device in that environment. Anyway, the coupled current would be 150 uA, and that assumes at least one half wavelength of the cable was immersed in a plane wave with precisely the right orientation relative to the wire in order to get that. If the "victim" circuit contains information represented by low potentials, such as below 0.1 Volt, then I would expect the cable carrying that signal to be shielded, as in a twisted shielded pair. 150 uA riding on a shield should not cause any problems to any flight critical signal, even with a pigtailed shield termination. For instance, if the pigtail termination yielded a transfer impedance as high as 50 Ohms at some frequency, the resultant common mode coupling to the interior pair would still only be 7.5 mV. Again I contend that Boeing and Airbus would not route a flight critical signal with a threshold of susceptibility that low. And if the circuit is totally unshielded, that implies it is a discrete or other relatively high level signal, where information is carried in such a away that it takes Volts of induced potential to cause an upset. Coupling to an unshielded wire above ground occurs at a transfer function of 75 mV per Volt/meter. [ Cf. IEC 61000-4-6, coupling efficiency of 1 Volt per Volt/meter, open circuit from a 150 Ohm source impedance.] Given the original 100 mV/m assumption, that translates into a coupled common mode potential of 7.5 mV and the conclusion still stands: no possibility of interference. ---------- >From: [email protected] >To: [email protected] >Subject: Re: New EU regulations - civil aviation >Date: Mon, Sep 16, 2002, 8:55 AM > > > Ken > > During the mid 90s we manufactured equipment that was installed on 747s > which was tested to RTCA/DO-160C and all the cables on that aircraft for > that system were unscreened. Boeing informed us that they would not permit > screened cables due to the increase in weight that would then affect the > passenger cargo carrying capability of the aircraft. > > I know that rf signals are coax and that certain control signals are > screened for flight critical systems. > > It isn't so bad for newer aircraft but some of the older ones that use Omega > and the earlier flight nav systems have reported interfernce with these > systems and the autopilot. When the passengers have been requested to switch > off their equipments the interference has dissappeared. > > It has been found through surveys carried out on aircraft by ERA and QinetiQ > that the interfernce appears to get into these systems from certain > locations within the aircraft where cable run reside under certain seats. > > Incidents of interfernce breakthough on coms have been more difficult to > identify. Investigations are still being carried out. > > If you want more data suggest you get in touch wuth Dr Nigel Carter @ > QinetiQ or Eric Stevens @ ERA. > > Regards > Andy > > Andrew Price > Principal Development Engineer (EMC Specialist) > BAE SYSTEMS Avionics > A125 > Christopher Martin Road > Basildon, Essex > SS14 3EL > > tel: +44 (0) 1268 883308 > email: [email protected] > > > > *************************************************************************** > > This email and any attachments are confidential to the intended > recipient and may also be privileged. If you are not the intended > recipient please delete it from your system and notify the sender. > You should not copy it or use it for any purpose nor disclose or > distribute its contents to any other person. > > *************************************************************************** > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: [email protected] > Dave Heald: [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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