Following the recent discussion on the forum regarding whether the shielding
screens of the cables to the ignition switches for Rotax engines should be
grounded at one or both ends, I asked Nigel Beale of Skydrive (the UK Rotax
importer) to contact Rotax again and ask whether their advice had changed
since 1996 when we queried the double-ended grounding recommendation. We
also asked whether there was any difference between the 912 and the 914
ignition circuits in this respect.
You will see from Nigel's reply, reproduced below, that Rotax are still
recommending grounding at both ends. I cannot copy the enclosures to the
forum, but they show the same recommendation for both the 912 and the 914.
However, in answer to our query they do point out that the electrical
circuit's design for EMC and EMI does depend on each individual wiring
installation -- it is not therefore necessarily "wrong" to ground the screen
at one end only. Remember that the screening is there both to prevent
electrical noise from the ignition circuits causing interference to the rest
of the aircraft system (e.g. ignition noise on radio) and also to prevent
spurious signals (e.g. from radio transmissions or strobe circuits) from
causing false triggering of the ignition or damage to the ignition
electronics. Conventionally, as others have pointed out, the best shielding
effect is normally obtained with the screen grounded at one end only, to
avoid "ground loops". This is why in 1996 we asked Rotax to confirm their
recommendation before we repeated it in the Europa circuit diagram. Rotax
stated that their tests had shown they obtained better results from
grounding both ends.
I would not expect the difference between single ended and double ended
grounding to be very great. Unless there were a very powerful source of
pulsed radio transmissions or strobe energy with the antenna sited (or
cabling routed) very close to the shielded ignition cables, it is most
unlikely that you would cause spurious ignition triggering, still less
damage the ignition electronics. Unless anyone comes up with a good reason
to do otherwise, I would suggest that those who are concerned with the
possibility that double grounding provides a parallel earth return path
(which could be overloaded in the event of a fault with the main ground
return) could safely test their installation by running the engine with only
one shield end grounded. For preference, I would choose to ground at the
engine end. One circuit only could be modified in the first instance, and
comparisons made by switching off each ignition in turn. Following
satisfactory ground running, with no additional radio interference (and
certainly no spurious ignition triggering), an air test should be carried
out. If OK, the second circuit could be modified.
Before making any modifications to your electrical circuit, you may wish to
check with the person who inspected and passed the original electrical
installation. I must emphasise that, as Rotax say, it is the entire
installation that must be considered when deciding what is best for any
particular aircraft. I can only offer this as guidance as to what you may
wish to look for.
Fly safely
Mike
Europa Club Safety Officer
Dear Mike,
Following your recent email. I have today received the following reply from
Rotax. Hope it answers the question. I have also copied it to Andy Draper.
Best Regards
Nigel
>Dear Nigel,
Please be aware that the setup and installation of ignition switches and
shorting cables have to be done according to the current relevant
installation manual. See excerpts of the relevant pages enclosed.
Be aware that according to these instructions the shielding braid has to be
grounded on both ends. See relevant wording on page 47 in IM 912 F and page
80 in IM 914 F. The wiring diagrams show a general sketch. The details are
given in the text.
Further on in discussion with our R&D technicians a general rule on
installation in aircraft is not that easy to answer, because in regard to
the electromagnetic compatibility (EMC) and electromagnetic interference
(EMI) the wiring has big influence and has to be checked for each aircraft
installation. Depends e. g. on routing, components/material used, grounding
setup etc.
For reference it would be an idea to recheck the installation know-how and
their experience on EMC and EMI of the whole installation with an aircraft
manufacturer e. g. DIAMOND, who is operating certified aircrafts.
Best regards
Hannes
(See attached file: pagesofInstallationManual914F. pdf)(See attached file:
pagesofInstallationManual912F. pdf)
<< File: pagesofInstallationManual914F.pdf >> << File:
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