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I was reading about Dangerous Goods, and came across the fact that magnetized materials are classified as Dangerous Goods (item 12 in the linked page). We all see magnets every day, and they seem harmless.

Why are magnets dangerous to airplanes and require more careful handling?

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    $\begingroup$ Strong magnets can also cause problems to the person carrying/holding the package if the person has a pacemaker, as the magnets can interfere with the pacemaker. $\endgroup$
    – user7076
    Aug 17, 2015 at 14:54
  • $\begingroup$ @coderworks Any reference on permanent magnets causing interference to pacemakers? $\endgroup$
    – Wirewrap
    Mar 9, 2016 at 21:56
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    $\begingroup$ hi @Wirewrap, yes you can see it mentioned e.g on this page from American Heart Association: heart.org/HEARTORG/Conditions/Arrhythmia/… (under "Power-generating equipment, arc welding equipment and powerful magnets"), and on this page from MedScape: medscape.com/viewarticle/788958 $\endgroup$
    – user7076
    Mar 12, 2016 at 17:07

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98% percent of magnets - the everyday ones - are completely harmless. I believe the concern is with super-strong rare-earth magnets, where they can be problematic. Aside from Simon's valid point on interference to compasses:

  • Getting stuck to things, such as other bags or the infrastructure itself, in the baggage system.
  • Attract themselves to other packages and content, possibly damaging these. Think of hard drives and electronics.
  • Being a nuisance and safety concern should somebody decide to open it, such as customs or security. You can get your hand crushed in the process if you open the box and the thing springs out.

Be aware they do not appear to be banned - just regulated in that they have to be properly labelled and packaged for safe transport. More curiously in that list are 'First Aid Kits' and 'Seatbelt Pretensioners'.

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    $\begingroup$ Depending on how exactly you interpret "First Aid Kit", it may contain flammable liquids (alcohol), pressurized canisters (e.g. spray-on bandages, inhalers), sharp objects (needles, scissors). That may be the reason, although I don't necessarily see how that makes it more dangerous than someone carrying his shaving kit (blade, compressed shaving cream, maybe rubbing alcohol) as baggage. $\endgroup$
    – Jörg W Mittag
    Aug 16, 2015 at 8:09
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    $\begingroup$ Some pre tensioners contains explosive squibs and/or gas cartridges. $\endgroup$
    – Simon
    Aug 16, 2015 at 8:48
  • $\begingroup$ Total/half-joking speculation: they don't want bad actors to have the tools to stay safer when they do their badness. $\endgroup$
    – Cyphase
    Aug 16, 2015 at 23:36
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    $\begingroup$ @mins while there are strong magnets within the hard drive, strong magnets + hard drive = Bad Things™. $\endgroup$
    – FreeMan
    Mar 9, 2016 at 14:35
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    $\begingroup$ @FreeMan: That's correct, deleting my comment... I did some research and degaussing devices used to wipe HD out are in the range of 1,5 tesla, not so huge compared to a strong magnet. $\endgroup$
    – mins
    Mar 9, 2016 at 15:28
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I use neodymium magnets in my work as a research scientist and I regularly carry them aboard aircraft in packaging.

We are not talking fridge magnets here, these are serious high-field permanent magnets. If two of them get properly stuck together they cannot be separated again easily. (The fields are far too strong to be pulled apart by hand and you can't use any metal tools for obvious reasons. One trick is to use a heavy, non-ferrous plastic-coated mallet to smash one off the other at the edge of a table, but even then it's usually half the day wasted.)

The problem is that the strength of the force between two such magnets is not intuitively understood by people unfamiliar with them: for example, your typical customs or security inspector. There might be almost no noticeable force between two magnets when they are some distance apart in the packing box but once they get within 5 to 10 centimetres they will suddenly be attracted together and anything in the way (flaps of skin, fingers) will simply be crushed flat under the force.

Or if nothing slows the magnets down as they come together, the force of the impact can chip the surface of the metallic protective layer, spraying shrapnel and metal fragments everywhere which poses a risk in particular to eyes. (The alloys used in constructing these magnets are extremely brittle.)

If the magnets get stuck to other large metal objects it can be a pain to separate them. They also induce weak magnetism in metals you normally expect to be non-magnetic.

Finally if they get wiped across anything like a credit card or ID badge that's usually the end of it (not always actually, it's a bit random).

The magnets are not extremely dangerous, getting small bones broken or metal fragments in the eye is the worse case scenario on my risk assessment, but they are not toys and there is a risk to those untrained in handling them. So anything like this needs to be identified so that they can be treated with care if someone does want to open my luggage to fish around inside it.

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  • $\begingroup$ Could two of those magnets be safely sandwiched on opposite sides of, say, a thick slab of very dense wood? $\endgroup$
    – Vikki
    Nov 22, 2018 at 1:55
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    $\begingroup$ @Sean Depends what you mean by "safely". They are safe until they are disturbed and handled by someone who isn't trained to use them. They were usually wrapped in bubblewrap individually and packaged something like the way you suggest. (I no longer work in research, so my information here might be out of date.) $\endgroup$
    – Calchas
    Nov 22, 2018 at 17:53
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Because the aircraft has a standby compass in the cockpit which is a normal magnetic compass. This compass must be fitted and must be working in case of extreme instrument failure. There are three things which standby instruments must provide; height, speed and heading (the compass).

Magnetic cargo can interfere with this compass and therefore, is treated as dangerous cargo. Whilst the risk is low, the aviation industry leans on the side of safety, and we can all see the benefits of that.

This report will tell you what you want to know.

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    $\begingroup$ "Magnetic field decreases exponentially with an increase in distance as measured from the compass." (from the report linked above). Really? I thought for a typical permanent magnet it decreased with an inverse-cube law - faster than inverse-square, but not "exponential". Inverse cube would give a reduction of about x10 between 2.1m and 4.6m distance, which is roughly consistent with Appendix A of the report. $\endgroup$
    – alephzero
    Aug 16, 2015 at 12:51
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    $\begingroup$ @alephzero Never was a whiz at maths, but surely an inverse cube is exponential? Certainly when plotted, it looks exponential to me. Regardless, rather than say "no magnets, unless in a metal container at least 10m from the cockpit" take the safe route and classify magnetic material as dangerous. $\endgroup$
    – Simon
    Aug 16, 2015 at 13:41
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    $\begingroup$ FYI "The graph illustrates how exponential growth (green) surpasses both linear (red) and cubic (blue) growth." $\endgroup$
    – ChrisW
    Aug 16, 2015 at 14:41
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    $\begingroup$ Well, I would hope ICAO get the math and physics correct in their official documents. "Exponential" means the rate of decrease at any point is proportional to the strength of the field itself. That means it would decay faster than any "inverse power" relation. The field from any geometrical arrangement of permanent magnets can only decay as some inverse power. For special cases like a "quadrupole magnet" it could be faster than an inverse cube law, but for this risk analysis the slowest possible decay of the field strength is what matters. $\endgroup$
    – alephzero
    Aug 16, 2015 at 14:49
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    $\begingroup$ @Random832 It is possible to construct an array of magnets or shape magnets to have quite a complex field geometry locally. But once you are sufficiently far away the inverse cubic term dominates the decay rate. $\endgroup$
    – Calchas
    Aug 17, 2015 at 15:37
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Airplanes have many sensors which are sensitive to electrical noise. And moving magnets can cause an electrical flow, hence electrical noise. As you can not estimate who is carrying how strong magnets it is safer to forbid them in life-critical environments like airplanes.

And about compass, it is only needed for an initial heading estimate, and it replaces GPS heading after moving a few meters in aircraft. So the compass is not a big deal as long as GPS is available.

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  • $\begingroup$ Do you have a source for your last paragraph? I'm not sure why they would go to the trouble of calibrating the compass if it's "not a big deal as long as GPS is available." $\endgroup$
    – fooot
    Mar 10, 2016 at 23:30
  • $\begingroup$ Sorry for misunderstanding. I was trying to say commerical airplanes uses latest technologies and not solely relying on compass measurements in the air. But the risk is u can not know gps will be availabile all the time. Safety and redundency issues. Compass is used for IMU (Inertial Measurement Unit) initialization/alignment on the ground pre-takeoff. And it depends on the INS technology and how it uses compass measurements (How it contributes to the kalman filter). $\endgroup$
    – ugur
    Mar 12, 2016 at 15:17

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