Page 22 - Volume 18 Number 3

From The Flight Deck by Kevin R. DingmanShockingthis hasn’t happened to an airliner since 1963. Each time I’ve been hit, it was a non-event. I didn’t think too much about a lightning strike having a result somewhere in the middle, not a non-event, but not an explosion either..... until recently.Mary Ann.... or Ginger?We were one of the last flights from DFW to ORD, the end of a four- day sequence and a long day – or so we thought. One hundred and forty passengers set sail that night for a two-hour flight..... a two-hour flight. The weather started getting rough; the tiny ship was tossed. If not for the courage of the fearless crew, the Minnow – no wait, that’s a different story; well, part of it’s different. The night was as dark as they come. As we climbed through 23k, there was not a glimmer of light but for the flash of strobes in the clouds. We were in thick soup with every bit of the anti-ice system engaged and the radar sweeping to 80 nm. There was no convective activity and, despite the clouds and ice, the ride was as smooth as a baby’s bottom.There was no VHF static or St. Elmo’s fire alerting us to the charged air. A flash of light from the right side of the plane was brilliant and blinding, amplified by the IMC and pitch-black darkness. Another reason for the level of brightness was revealed a millisecond later as a concussive shock-wave enveloped the jet with a sharp, attention-getting crack. Along with counting seconds between the flash and clap, the type of sound generated by a lightning bolt can help determine its range. As a shock-wave travels, heat energy is transferred to the air and the sound progresses from a sharp crack or snap to a bang or boom, then a thud and, eventually, a rumble. The brightness of the flash and the sharpness of the crack indicated that this one was close – very close.Confirmation of its proximity was MARCH 2014Our group was barely a thousand yards from the impact zone. A single B-52, flying at 500 feet and 400 knots, was dropping a string of twenty 500-pound high-explosive bombs. We had escorted several dozen politicians and VIP’s to a demonstration at a bombing range near Vegas. Watching from bleachers, scenes of Vietnam carpet-bombing played through my mind.The shock-waves from the explosions were unforgettable as they raced through the crowd at the speed of sound, reverberating in our lungs two seconds after each flash of light. The sensation was produced by rapidly-expanding gasses from the explosions, the primary mechanism of destruction from high-explosives. First comes the blast-wave, then shock-wave and finally the stress-wave. If near the blast, these are rapidly followed by shrapnel, fast-moving debris and fire – the secondary destructive forces. Anything that causes a fluid to be displaced at the speed of sound will generate a shock-wave, including the20 • TWIN & TURBINEone inch-diameter column of ionized gas from the average lightning bolt.Fuel Tank ExplosionAirplanes are routinely struck by lightning in flight. I’ve been hit a few times in airliners but never in a GA-size airplane – plenty of St. Elmo’s fire and VHF static, but never a strike. The location of a lightning strike on a metal airframe can be recognized as an arc spot the size of a dime, often with a trail of arc-like discoloration. Sometimes, on a non- metallic surface such as a radome, you may find a pencil-size hole or larger, accompanied by substantial delamination and discoloration. A Boeing study concluded that airliners are struck by lightning an average of twice each year and, in fact, the airplane itself is often the trigger. Airplanes usually handle a strike quite well. The times when a lightning strike goes badly, it usually goes very badly, the most lethal result being a spark-induced fuel tank explosion.In fuel tanks, even a small spark can be disastrous. Extra precautions insure thick fuel tank skins, so as to prevent burn-through, and joints and fasteners designed to prevent arcing. Additionally, fuel system components such as access doors, vents and fuel caps are protected. A few years ago, an AD was issued for the Duke, and likely all aircraft with a similar fuel cap configuration. The Duke had metal fuel-cap retainer chains, similar to the chain on a ceiling fan, which dangled into the top of the tank – ideal for spark generation. The AD specified plastic lanyards, like on an automotive fuel cap. Statistics show that a few lightning strikes have occurred in military fighters causing fuel tank issues, but, as far as they can figure,