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Lightning protection important for composite aircraft

 

This year we’ve had a bumper crop of nasty thunderstorms, especially in the lower 48 states. Storm counts are up, lightning strikes are up, and tornado counts are up. While I won’t debate the certain hazards of flying in or near a thunderstorm, I will speak to the hazards of one of the thunderstorm’s most dangerous hazards; lightning.

Average statistics indicate that each commercial aircraft in the U.S. fleet is struck by lightning at least once per year. Smaller business type aircraft are struck less often and small aircraft rarely. However, as technology improves, and becomes more accessible to the small aircraft pilot, these aircraft are more likely to find themselves (with their pilots!) in or near hazardous weather. Counter to intuition, radar, strike finders, and other high-tech weather avoidance gadgetry, although when used correctly can help the pilot avoid lightning, can also lead a pilot to fly in weather conditions that should otherwise have been “waited out.” And as technology continues to advance, we see more composite aircraft on the light-aircraft scene. This is a good thing for the light-aircraft industry, but also presents some new issues regarding lightning protection.

Conventional aircraft structures (aluminum) tend to be good at conducting electricity, and thereby the electricity of a lightning strike generally flows through the fuselage, to exit at some extremity such as a wing or tail. The advent of composite aircraft has complicated the problem of absorbing an electrical charge, and subsequently discharging it back into the environment.

If you’ve seen what happens to nonmetallic objects when struck by lightning, you have a little bit of an understanding of the problem already. If the electricity can’t flow, it goes “bang,” and off comes a wing-tip or other nonconducting part. And how much electricity are we talking about? Try anywhere from 10,000- 200,000 amps at 10’s of kilovolts! That’s more than enough for a wiener roast!

NASA says that, although the nature of hazard has not changed, composite “aircraft are far more vulnerable to lightning strikes.” The problem they cite is academic. Composite fibre construction, which amounts to insulated carbon fibre or epoxy type structures, is easily damaged, particularly at entrance and exit points. An even bigger problem is that composite aircraft have much less stringent lightning protection requirements than commercial aircraft, and experimental aircraft have no such requirements. This isn’t to say that all small composite aircraft are unprotected, just that they don’t have to be protected to the same standards,and some arenot built with protection of any kind!

While it is fairly clear that lightning rarely brings down an aircraft, the subsequent damage can be anywhere from a minor annoyance to extreme. Radomes can be blown off, wing tips shattered, holes in the skin at entrance and exit, crankshafts magnetized (that’s bad beyond a certain level; ask an engine repair shop why!), radios damaged, interruptions to communications and other electrical systems, etc. Unless you have a money tree growing in the backyard, best to avoid lightning!

So what do manufacturers do to protect composite structures from lightning? Well, in general, the idea is to make sure there are no conductive gaps in the structure of the aircraft; think “everything bonded to everything else.” And where you wouldn’t want to conduct such large jolts of electricity, you shield or protect with thicker structures so that the lightning can’t burn a hole through; e.g. fuel tanks.

Since most composite materials are significantly less conductive than conventional aluminum structures, the “gap” must be addressed in a novel way. To this end, conductive fibres or mesh screens can be embedded within the composite material. This aids in carrying the current introduced by a lightning strike.

Without this embedded conductive material, composites are at risk, some more than others. Fiberglass for example is not conductive at all, so if you fly a non-protected fiberglass kitplane, watch out!

Carbon fibre aircraft are conductive but significantly less than their aluminum brothers and sisters. Direct effects of low conductivity, with electricity seeking the path of least resistance, means that other critical conductive materials may be impacted. Control cables, control hinge points, and fasteners are then vulnerable. The more indirect effects include any unshielded or improperly shielded radios or other electronic equipment.

Protection is in the manufacturing. If the manufacturer of a composite aircraft has protected their product from potential lightning damage they’ll tell you so; and they’ll be able to provide details of how they do it. And considering how they do it, be aware that there are lots of ways to do it, and little actual data to support which way is best.

It could very well be that everyone is doing it to a level that is sufficient, even though their individual methods of improving composite conductivity are different.

In one article I read there were at least a dozen different techniques and material applications promoting lightning strike protection. All perform with various levels of success in the laboratory, but I’ve never seen evidence that anyone has created a real lightning bolt in a lab, so we’ll have to rely on actual lightning strike data for our analysis. But those haven’t occurred with enough frequency from which to draw any reasonable conclusions, so perhaps staying away from lightning is the best protection after all.

How do you keep from getting struck? Simple, avoid thunderstorms and it won’t happen in the first place. In fact, give thunderstorms (and that means the cloud, whether it’s producing rain or not) a berth of about 20 nm. If you’re not sure if it’s a thunderstorm, then avoid it anyway, just to be safe.

Lightning strikes the earth surface around 100 times per second so there’s plenty of thunderstorms out there to avoid. Using more than just your eyes to spot one is prudent. There are plenty of decent online radar resources these days that serve as a last minute check of the weather, but know what to look for.

A thunderstorm cloud that is not producing rain will not show up on radar, or if it does, may only depict higher altitude precip that is not yet reaching the surface. Under the right conditions know that a cumulus cloud can develop into a thunderstorm in about 30 minutes!

If avoidance is not possible, and we won’t debate why the flight went so far as to not be possible, then a few pointers are worth remembering: 1) Avoid areas of heaviest precipitation if at all possible. 2) Reduce airspeed to minimize static electricity buildup. Static electricity buildup can attract lightning leaders. 3) Find an altitude of least turbulence. Turbulence in or nears storms seems to be associated with lightning strikes. 4) Find an altitude that puts you farthest away from the freezing level. Again freezing levels seems to be associated with a higher incidence of strikes, and 5) If flying at night, turn up the lights to negate the effects of the blinding light of a strike.

Lightning is a dangerous problem for aircraft of all kinds, and especially for some composite aircraft. Know how your aircraft is protected and think about what might be affected by a lightning strike, and what you would do if that happened.

Best yet, avoid, avoid, avoid. Don’t get yourself into the position of finding out what a bolt of lightning can do!

This month’s Pilot’s Primer is written by Donald Anders Talleur, an Assistant Chief Flight Instructor and Researcher at the University of Illinois, Institute of Aviation. He has been flying since 1984 and in addition to flight instructing since 1990, has worked on numerous research contracts for the FAA, Air Force, Navy, NASA, and Army. He has authored or co-authored over 200 aviation related papers and articles and has an M.S. degree in Engineering Psychology, specializing in Aviation Human Factors.