Monday, August 29, 2016

Photo of the Day

I'm not quite sure how to credit it this.  It's from a CNN Story, and appears to be in the twitter feed of someone called @GoldboxATL. 
Delta flight 67, ATL to Las Vegas (LAS), a Boeing 737-900 (that means it's a new model of the 737) getting hit by lightning while queued up in the traffic Atlanta Hartsfield-Jackson gets on days like that.  I've personally waited about an hour and a half on the ground in Atlanta for the roughly hour long ride to the Silicon Swamp.  In this case, flight 67 took off around two hours late, but arrived in Lost Wages only about 20 minutes late (I assume they were re-routed more directly).  I'll conclude nothing was damaged on the plane.

The fact that planes get hit by lightning generally tends to really upset passengers, but the designers expect every commercial airliner to get hit by lightning once a year or more, depending on where they fly.  Aircraft that stay in the tropics are expected to get hit more often than those that fly in more northern latitudes, but since you'll never know where a given aircraft is flying, they're all designed to take more strikes.  I have to say that I was surprised as a newbie when I was told that my radios were not only expected to survive a lightning strike to the antenna, they were expected to play through it and not experience a processor reset or anything that cause the crew inconvenience - although there isn't a whole lot that can be done about the static noise hit.  We can do that in the laboratory, where we can make standardized, repeatable lightning strikes, but real lightning doesn't come in standard voltages and currents ("I'll take 5000 Amps, please").  It's always nice to get feedback from the field that things really are working the way they're supposed to. 


  1. Was riding jumpseat from KC to Memphis one night on a 727 and got to see quite a display of St. Elmo's fire around the windshields. Pretty cool.
    Lightning usually leaves little pits in the skin. It will buff out. :)

    1. That must have been pretty.

      A guy I was talking with long ago ('76?) said the first time the plane he was driving was hit by lightning was at night. Said it was so bright, he couldn't see his instruments for 30 seconds and that was the only part that was scary. He figured that under power, he could lose a lot of altitude in 30 seconds. That stuck with me. (obviously - it's about 40 years since I heard that)

  2. Just curious- and guessing? Is that the ground discharge at rear landing gear?
    Would be very interesting to know how that marks the tires- or doesn't~
    I would think that must have been a really loud report inside- always startling LOL!
    Any input~ Thanks in advance:0

    1. I believe that's what it is. The way the fuselage is made conducts the lightning around the outside down toward the bottom. The landing gear bears the brunt of it.

      In flight, it tends to come out the trailing tips of the wings, although I'm not sure how the fancy winglets they're developing to reduce drag at the wingtips affect that.

    2. I remember reading that the winglets are no different to lightning than plain wings.

      Plain wings... when talking about plane wings... ouch I hurt my language co-processor.

  3. Most of the airliners also have little static "dissipators" that trail off the wings. They look like a piece of black plastic tube with some frayed "rope" hanging out the end.

    One of the more interesting places I've visited in my career was the Thomas A. Edison Laboratory near Milwaukee when I was working for the parent company, McGraw-Edison.

    They had a high-voltage lab that was really neat, but the short-circuit lab was really impressive. It's where they tested, to destruction, high-voltage switch gear used in the power distribution business.

    The test cell had wall that were 10' thick reinforced concrete, and they needed them! The generator had a 500 ton flywheel to keep it turning when they shorted it out with the equipment under test, and could produce something 100 million Amperes for several full line cycles.

    The inside walls of the test cell were deeply scarred by surviving bits of equipment that had exploded under test.

    And they had a faint copper sheen to them from all the vaporized metal that had been deposited on them over the years....

    1. That's the kind of test that really pushes the mechanics to its limits. When they tell you in basic electricity that a short circuit goes to zero voltage and infinite current, they really mean it. Testing things that have to pass a few million amps in service (transcontinental powerline components), but then open the breaker for infinite current is rough. Really only that one way to do it. When the breaker opens, they have to blow away the arc that starts, or the arc just replaces the contact.

      The "lightning box" we'd use is commercial gear, although we went through a period of using homebrew boxes. Large banks of low-ESR electrolytic capacitors. For the equipment inside the plane, it was found that the long cables or long twisted pairs stretch out the pulse in time; this has the effect of lowering the amplitude and frequency of the pulse. This is all in industry standard documents (DO-160), but there's a handful of waveforms and handful of levels we'd test at. The customer decides what level they want the box to survive, usually depending on how much exposure it has to the outside world, and what it's used for.

      Most of the time, it's strangely non-eventful to watch. No arcs, no sparks, no ozone smell just the test conductor pressing button. We're talking maybe 5000V at 5000A for 10 to 50 microseconds.

  4. Yeah, the whole reason for existence of that lab was to see how things failed when they were subjected to 1000's of times their ratings.

    Forgot to mention the high-speed film cameras. Since there wasn't much (if anything) left of the test articles, they filmed everything from multiple angles at 400, 1000, and 10,000 frames per second.

    Ever see a piece of 4/0 copper wire explode at 10,000 frames per second? Pretty interesting to watch how the magnetic field caused by that much current makes the individual strands of the cable repel each other!

    1. Ever see the quarter shrinking gallery?

      Using enormous pulses to generate magnetic fields that deform coins and all sorts of things. We're talking thousands of Joules and 10s of thousands of volts.

  5. Haven't seen that in years.

    Amazing what Eddy Currents and Induced Magnetic Fields can do!

    The first time I looked into one of our big control cabinets that had suffered some some of MAJOR fault, it impressed me that the LARGE cable ties we had used to secure the 4/0 480 Volt feeder cables had been ripped apart, and the cables were twisted every which way.

    Electromagnetism In Action!