Lightnin’

Everybody seems to know that airplanes and storms are a bad mix. At least for the airplane anyway, the storm will be just fine either way.

A common concern is that the airplane might be “hit by lightning”.  This kind of makes sense because the airplane is flying and the lightning is flying all around. It just seems like the two could run into each other with terribly negative result. It just doesn’t happen that way at all.

There is a phenomenon we call a “lightning strike”  for lack of a better aviation terminology. 

An airplane can build up an incredible electrical charge when flying through dry snow, dust or near a thunderstorm. This is similar to walking across carpet on a dry day. The same if the carpet was at thirty nine thousand feet and you were walking at about five hundred miles per hour.

You may have noticed little pointy things hanging off of the back of the wings and tail of  an airplane. These are  “Static Discharge Wicks”, and are intended to dissipate the static potential in a controlled and non-impressive manner.  

Sometimes when the static potential is excessive an electrical field becomes evident by the discharge of  electrons. These little guys get kicked around and knock some photons loose.  This causes Saint Elmo’s fire, which at night looks like the blue sparky animation R2-D2 had all around him when the sand people zapped him. 

  Below is an unretouched picture I took of Saint Elmo in action. This is looking out the front window, you can see part of the wiper on the left.

  st-elmo5.jpg

 St. Elmo’s fire will dance around on the windshield and engine intakes. The stuff doesn’t really hurt anything but it does make radio communications difficult. Sometimes passengers can see it stream off of various parts of the wings, tail, and antennae.  Saint Elmo is the Patron Saint of sailors, the “fire” was first noticed at sea and must have scared the absolute beegeezez out of the guys in masted ships on stormy nights. That, of course, assumes they could be scared any worse that they already were in a storm at sea in a wooden boat.

If the static potential is kicked up a notch in intensity things start to get a bit interesting. Not dangerous, but weird. This is about the point where pilots stop playing with the windshield wipers to make Saint Elmo dance around, and turn the instrument lighting all the way up.  A cone shaped aura will project forward, ahead of the airplane.  

Usually the cone thing will go away quietly but once in a while you might notice a tiny little “stringer” of static discharge going off in one direction. This is a great time to squint your eyes since just about instantly a flash as bright as an arc welder is accompanied by a really loud clap of thunder. Hearing thunder in a jet going about 80 percent of the speed of sound is impressive.  Most passengers whoop or scream.

So, to me at least, it has always seemed that the airplane is providing the potential for a static discharge, creating an ionized path through the air. The “lightning” follows this ionized path right back to …. the airplane. 

Good news for us, “the occupants” is that an airplane provides a nice protective bubble in the dynamic of the electrical field. Airframe components are grounded together, sensitive equipment is shielded, and the aluminum skin provides very good conductivity and helps route the massive electrical discharge safely.

Airliner damage during one of these events is generally limited to little holes burned through the skin here and there, radio damage, and some odd system behaviours. A “lightning strike” event requires a maintenance inspection so a delay should be expected if you aren’t finished for the day.

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4 Responses to “Lightnin’”

  1. Me again. So just to clarify – cloud/ground lightning doesn’t hit planes then? They just make their own lightning?

  2. The airplane just gets it started. What happens next is a matter of scale. I don’t really know if valid research has been done to compare the size of “cloud to airplane” lightning with “cloud to ground” lightning but I imagine that it is much smaller.

    Electricity is tidy even in its weird field state. It just doesn’t much go much where it can’t leave from, airplanes don’t provide much of a ground path.

    The huge static potential airplanes can build up is why the fuelers always attach a ground cable to the plane before fueling. It’s also a great way to trip a pilot who isn’t watching their step.

    A good idea when fueling your car to be sure to touch the car with your hand before opening the fuel cap. A good zap of static elecricity from the car to the fuel nozzle could be a bad thing too.

  3. Very very cool. I’d love to see pictures of the aura, which I have just remembered is cone shaped.

  4. Fascinating, I had no idea! I’ll second the request for a pic of the aura, wow.

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