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Electric arcing creates a huge temperature in the air around it. When lightning strikes planes, why doesn’t this high temperature air melt/deform the aircraft skin? Also can these high temperatures ignite fuel in the wings?

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Potentially, yes, but this would be a very rare event. The principal reason that lightning does not create damage to an aircraft is that the airframe, particularly aluminum airframes, are electrically conductive, and carry the charge easily around them with very little resistance, which would otherwise create heat. Essentially, the airframe behaves like a Faraday cage and protects the contents within from damage. The airframe is also designed to dissipate static electric charges through static wicks, lightning runners and lightning spikes on the exterior of the skin. Fiberglass parts such as radomes are protected by lightning runners which carry the charge and protect electronic components underneath. Composite aircraft made from e-glass are constructed with a stainless steel mesh ply imbedded in the skin which channels and carries electric charges.

Lightning strike protection is a critical part of aircraft certification under parts 23, 25, 27 and 29. Very rarely has lightning caused any kind of an accident, and commercial aircraft are frequently struck by lightning every year and suffer almost no ill effects from it.

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    $\begingroup$ The question is about "high temperature air" so I do not see how the conductivity of the airframe answers that part of the question. $\endgroup$
    – Anonymous Physicist
    Apr 23 at 18:00
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    $\begingroup$ Those kinds of air temperatures last in the timeframe of milliseconds. Not exactly enough time to heat up and melt and/or deform the skin in. $\endgroup$
    – Carlo Felicione
    Apr 23 at 18:21
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    $\begingroup$ Just because the temperature is high, doesn't mean that it is magic and blows up like a bomb. The spark from a grinder is melted metal, but the total heat energy is so low that the workers usually don't need any extra protection around it, even if it is sprayed all over their body. $\endgroup$
    – Nelson
    Apr 24 at 2:08
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    $\begingroup$ @AndyDent sure, but that it took minutes to burn that channel is supporting evidence for the claim that microsecond exposures aren't going to matter. $\endgroup$
    – fectin
    Apr 24 at 17:04
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    $\begingroup$ Side note: commercial aircraft are frequently struck by lightning every year and suffer almost no ill effects from it. Granted, but what about the passengers? See: Frequent Flyers Could Take a Hit of Radiation from Lightning and 'Dark lightning': The mysterious radiation that might be zapping air passengers mid-flight An issue [or not]? $\endgroup$
    – Craig Estey
    Apr 25 at 2:17
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The other answers mention aircraft have a high electrical conductivity. However, air has a much lower electrical conductivity and does get hot, as you mentioned.

The hot air causes little damage because:

  • The electrical current only heats a small area of the air for a brief period.
  • The air expands quickly, causing it to cool quickly.
  • The aircraft moves quickly, so it is only exposed to the heat briefly. After that, it is exposed to very cold air that can remove the heat.
  • Since the aircraft skin is electrically conductive, it also conducts heat away from the hot area.
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    $\begingroup$ Also the metal has much higher heat capacity, so the same amount of energy heats it less then it heats the air, and good thermal conductivity, so the heat spreads quickly further reducing the thermal stress on the point of contact. $\endgroup$
    – Jan Hudec
    Apr 23 at 18:54
  • $\begingroup$ Bullet 1: Actually, part of the point is that a lightning strike is much LESS concentrated, and much shorter than pretty much any other arc, whether that be an accidental wiring issue, or from some sort of welding. Bullet 4: Electrical conductivity and temperature conductivity are not the same thing. $\endgroup$
    – MikeB
    Apr 24 at 7:33
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    $\begingroup$ @MikeB while not being the same thing, good electric conductors are usually also good thermal conductors (see an answer at physlink.com, or the sortable list on Wikipedia) it's not working the other way. $\endgroup$
    – Arsenal
    Apr 24 at 12:19
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Why doesn't it punch holes in the fuselage? It can.

Can it ignite fuel in wings? Yes, lightning creates sparks that can potentially ignite fuel vapors. The liquid fuel itself does not ignite, but the vapors can.

Both of these things are extraordinarily rare, however. Planes are specially designed to be resistant to lighting strikes, and most lightning strikes result in little to no damage.

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    $\begingroup$ All true. But please speak to how rare this is. I have to fly with people who read this. $\endgroup$
    – candied_orange
    Apr 23 at 16:44
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    $\begingroup$ Each airplane is struck, on average, 1-2 times per year: weather.gov/safety/lightning-planes flightsafety.org/wp-content/uploads/2016/11/… $\endgroup$
    – Adam
    Apr 23 at 17:22
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    $\begingroup$ It should be noted that damage like shown in the linked picture is, at the moment it happens, completely inconsequential. The structural elements around it are strong enough to prevent the hole from growing bigger and the outflow valves are bigger than that so they'll just close a bit and the pressurization can keep up just fine. Only long term the damage would spread mostly due to metal fatigue, so it has to be repaired after landing. But it wouldn't cause any immediate danger. $\endgroup$
    – Jan Hudec
    Apr 23 at 19:01
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    $\begingroup$ @Michael The cabin is not sealed to keep air inside, but the engines or compressors are continuously pushing quite a large volume of air into the cabin while it leaks through all the imperfect seals and the outflow valves. Quick search says the outflow valves on a 777 have (the two of them together) area about 90 in²/580 cm². You can definitely put your hand through that. $\endgroup$
    – Jan Hudec
    Apr 25 at 22:05
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    $\begingroup$ @Michael The hole depicted is in the skin of the wing, not through the pressure hull. The insides of the wing are not pressurized. $\endgroup$
    – Chris
    Apr 25 at 22:25
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When electric current flows through a medium, the amount of power dissipated is inversely proportional to the resistance of the medium. This is why the heating element in a space heater heats up, but the power cord leading to it doesn't (much): the element has a much higher resistance than the cord. This is also why using too small of an extension cord for the load can cause a fire: thinner conductors have a higher internal resistance.

Air has a much, much higher resistance than the aluminum, composites, etc. that they use to build planes, and hence gets much hotter during a lightning strike. This is easily seen in videos of planes being hit by lightning, where the air glows from the heat and the plane doesn't. Furthermore, heat can't transfer from the air to the plane because it's almost immediately blown away by the slipstream.

Of course, that's not to say that it's impossible for lightning to cause damage, because it absolutely can. If the current finds a spot of higher-than-average resistance, or gets concentrated to a point, it can (and has) caused significant damage.

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  • $\begingroup$ Wait, composite materials like carbon fiber have low electrical resistance? Apparently that's true: carbonfibergear.com/blogs/carbonfiber/… - neat. Unlike fiberglass, which being made of glass is an insulator (one which doesn't ionize nearly as easily as air). $\endgroup$
    – Peter Cordes
    Apr 23 at 22:31
  • $\begingroup$ Fun fact: the amount of current that the power cord of a space heater carries is equivalent to about one typical lightning bolt every second. Passing that amount of current through air produces tremendous amounts of light, heat, and sound; passing that amount of current through a copper wire does practically nothing. $\endgroup$
    – Tanner Swett
    Apr 24 at 2:25
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    $\begingroup$ @TannerSwett I don't know where you picked up that little factoid, but it's not exactly accurate. A typical lightning bolt is well over 30,000 amps, while a typical household electrical socket is limited to 15 amps. $\endgroup$
    – HiddenWindshield
    Apr 24 at 3:07
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    $\begingroup$ @HiddenWindshield Yes, now that I think about it, my comment is really not accurate at all. What is true is that the amount of charge carried by the power cord in a minute (about 700 coulombs) is a reasonable amount of charge for, say, 40 lightning bolts. When I wrote that comment, I was thinking that the actual instantaneous current doesn't matter—in other words, I was thinking that 30,000 amps for half a millisecond would be practically the same as 15 amps for one second—but now that I think about it some more, that's not true at all. Sorry for the mistake! $\endgroup$
    – Tanner Swett
    Apr 24 at 3:52
  • $\begingroup$ Wire heating from intermittent loads will be roughly proportional to duty cycle, but the square of the current, so reducing the duty cycle by a factor of 100 would only allow a 10-fond increase in current (rather than 100-fold) to keep the same level of heating. $\endgroup$
    – supercat
    Apr 24 at 19:43

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