Ken, you may be right but it is like trying to convince the FAA that
there is no harm in using car gas in airplanes. There are just too
many ways for uncontrolled fuel to become contaminated from unknown
sources.
With respect to the EMC and immunity issues, it is not the technical
issues that are of concern, but rather the liability and publicity of
even being accused of causing interference, regardless of how it got
there. The potential for it exists, and lives are potentially at
stake. Who would argue with this?
I am also an aviation consultant as well as an engineering one. Most
devices are allowed to be used in flight. Cell Phones and 2-way pagers
cause too much GROUND interference when used from the air and THAT is
the primary reason they are not permitted. They use more resources
than intended when used from the air.
By the way, I was consulting to a company that did not even test ANY of
their equipment to FCC Part 15, and I discovered that they were out of
specification for Class A by several db. We had our attorney negotiate
with the FCC, who wanted to know if ANY of the over-limit frequencies
fell into the aviation bands. They did not, so we had to fix the
problems with no other action required other than submission of new
compliant verification reports. There is concern for this even
originating outside of the aircraft.
Warren Birmingham
Epsilon-Mu Consultants
(510) 793-4806
email: war...@epsilon-mu.com
website: http://www.epsilon-mu.com
On Monday, Sep 16, 2002, at 09:56 US/Pacific, Ken Javor wrote:
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: andrew.p.pr...@baesystems.com
To: emc-p...@majordomo.ieee.org
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: andrew.p.pr...@baesystems.com
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