Static Discharge: Windshield Light Show

On your next flight, while you sit back, relax and enjoy reading SkyMall, there is a good chance your flight crew is enjoying their own, shocking light show. Pilots see static discharges dancing on our windshields fairly often. As a passenger, you might be surprised or even concerned at the display. It can be very impressive, but it’s completely harmless. Take a closer look at one of the coolest light shows that pilots experience on the flight deck…

Too Much Static Electricity

As an aircraft flies through the sky, air and precipitation rub against the aircraft’s skin causing a buildup of static electricity. The same thing happens when you rub a balloon on your hair. When this electrical charge is strong enough, it can cause static on aircraft radios, interfering with communications. In typical conditions, the charge is continuously dissipated by small, pointed static discharge wicks mounted to the trailing edges of the wings and tail.

Most of the aircraft skin is bonded (connected) to the static discharge wicks with the exception of the windshields. When we fly through high altitude ice crystals (cirrus clouds), heavy rain, or snow, the static discharge wicks don’t dissipate the precipitation static that builds up on the windshields.  The precipitation static eventually discharges on its own, providing a fascinating web-like display a few inches in front of us.

You’ve seen or heard static discharges around your house. In the winter, when you take a blanket out of the dryer, you can hear the static electricity snap and crackle. If you turn off the lights and give the blanket a shake, harmless sparks can be seen as the excess static discharges. This is the same stuff we see on our windshields.

Though not nearly as impressive as seeing it in person, here are two video clips to give you an idea of what the discharges look like. The first is on a Boeing 737 windshield. The phenomenon begins at 2:20. It’s interesting to note that these videos are incorrectly titled as St. Elmo’s Fire (more on that later).

Another clip of precipitation static discharge:

Saint Elmo’s Fire

Most pilots incorrectly refer to windshield static discharges as St. Elmo’s Fire.

St. Elmo’s fire is a form of plasma that can occasionally be seen emanating from roof peaks, towers, spires, and other pointed objects in the vicinity of thunderstorms. The phenomenon is often seen prior to a lightning strike. If you see it, get indoors!

Documented sightings go back to ancient Greece. Early sailors saw St. Elmo’s fire glowing around the tips of their ship masts when near storms. They named the glow after St. Erasmus of Formia, the patron saint of sailors. The salty sailors, not known for their linguistic skills, mispronounced the Saint’s name as Elmo.

Here are two engravings from the 1800’s depicting St. Elmo’s visit to tall masted sailing ships. Click the pics for full size.

On aircraft, St. Elmo’s Fire appears as a bluish glow or flame emanating from an aircraft wingtip or nose. The phenomenon is rare on aircraft; most pilots have probably not seen true St. Elmo’s Fire. St. Elmo’s can appear in heavily charged air in the vicinity of a thunderstorm and is harmless to aircraft.

I’ve seen St. Elmo’s Fire only twice in over 30 years of airline flying. During the most impressive display, it appeared as a beam of bright blue light emanating from the nose of a DC-8. When I saw it, I first thought we had a landing light turned on.

Keep Your Eyes Open For St. Elmo’s Fire

Unfortunately, as a passenger, you probably won’t see static discharges. I’ve never seen or heard of them happening on side windows. If you are flying on a stormy night, however, you might catch a glimpse of St. Elmo’s Fire on a wingtip similar to the photo above. If you see it, be sure to take a photo. It’s a rare phenomenon and I’d love to see another picture of it!