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All I know about the issue of the laptop interfering with the compass is from
the IEE's Guide to EMC and Functional Safety, copied below:
*********
A routine flight over Dallas-Fort Worth was disrupted when one of the
compasses suddenly shifted 10 degrees to the right. The pilot asked if any
passenger was operating an electronic device, and finding that a laptop
computer had just been turned on requested that it be turned off, whereupon
the compass returned to normal. Following RTCA guidelines the pilot requested
that the laptop be turned on again 10 minutes later, when the compass error
returned.
Ref: Compliance Engineering (European edition) Nov/Dec 1996 p12
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I am led to believe that the incident was one that was officially
investigated, and not just someone's bad dream. Unfortunately CE Magazine's
on-line archives only go back to 1999, so I can't quickly find out more about
this incident.
But I am sure there is more to a compass in a modern airplane than simply a
bar magnet with a pointer attached. For example I have designed compass
systems for ships that used servomotors. So I assumed that the emissions from
the laptop were demodulated in some compass circuitry, probably after being
picked up by a cable from a remote compass sensor, causing the error (see
Edmund A Woodcox's earlier message 02/01/02 19:58:08 GMT).
A quick trawl through the 'Banana Skins' columns in the EMC + Compliance
Journal's archives (at www.compliance-club.com) reveals that most anecdotal
or official reports of interference problems are either caused by radio
transmitters (including ISM equipment used for the RF processing of materials
in industry or medicine) or are suffered by radio receivers.
However, there are some reports which don't fit into the above two
categories, where a device that one might expect to comply with EMC emissions
standards has caused interference with a device that is not a radio receiver.
These reports can be found in the above archives as follows:
No. 2, 4 Feb 98
No. 35 Dec 98
No. 49 Jun 99
No. 66 Dec 99
No. 96 Aug 00
No. 120 Apr 01
No. 129, 137 Jun 01
No. 157 Oct 01
But don't forget that publication in a magazine is not proof that the
incident occurred!
Regards, Keith Armstrong
In a message dated 02/01/02 16:48:43 GMT Standard Time,
ken.ja...@emccompliance.com writes:
> Subj:Re: EMC-related safety issues
> Date:02/01/02 16:48:43 GMT Standard Time
> From: ken.ja...@emccompliance.com (Ken Javor)
> To: cherryclo...@aol.com
>
> I forgot to mention the issue of the compass in my earlier reply. First a
> statement of fact. A compass is a magnet, and it can only respond to a dc
> or very slowly varying ac magnetic field. It is physically impossible for
> the compass movement itself to respond to rf. There is the concept of the
> "compass safe distance" and a requirement to measure it is included in
> RTCA/DO-160, "Environmental Conditions for Aircraft." But the effect is
> limited to a dc offset due to magnetic material. I cannot see a laptop
> causing such a problem unless it were in the immediate vicinity of the
> compass, which is unlikely in an aircraft. To illustrate how utterly
> insensitive a compass is to rf, consider the following true story. A
> flux-gate (a type of compass used on aircraft) was reported to be sensitive
> to rf energy when the HF transmitter on a particular aircraft was keyed.
> The frequency didn't matter, when the transmitter was keyed, the heading
> indication shifted. The flux-gate was located in the immediate vicinity of
> the HF antenna, which was the leading edge of the vertical stabilizer. It
> turned out that the 200 Watt rf transmission had nothing to do with the
> interference. When an HF transmission is keyed, an antenna tuner adjusts a
> tuning coil to match the antenna to the rf power source and that required
> 28 Vdc current to flow and it was that relatively low-level current, not
> the 200 Watts of radiated rf power that caused the offset.
>
> I wonder if the laptop disturbing a compass story is a distortion of the
> DC-10 event I related in the response to Mr. Woodgate's postings?
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