How often are aircraft struck by lightning?
It is estimated that on average, each airplane in the U.S. commercial fleet is struck slightly more than once per year. In fact, aircraft often trigger lightning when flying through a heavily charged region of a cloud. In these instances, the lightning flash originates at the airplane and extends away in opposite directions. Although record keeping is poor, smaller business and private airplanes are thought to be struck less frequently because they usually do not adhere to rigid schedules.
Does lightning cause aircraft to crash?
The last confirmed civilian plane crash that was directly attributed to lightning in the U.S. was in 1967, when lightning caused a catastrophic fuel tank explosion. Since then, much has been learned about how lightning can affect airplanes, and protection techniques have improved. Airplanes receive a rigorous set of lightning certification tests to verify the safety of their designs.
What happens when an aircraft is struck by lightning?
Although passengers and crew may see a flash and hear a loud noise, nothing serious should happen because of the careful lightning protection engineered into the aircraft and its sensitive components. Initially, the lightning will attach to an extremity, such as the nose or wing tip. The airplane then flies through the lightning flash, which reattaches itself to the fuselage at other locations while the airplane is in the electric "circuit" between the regions of opposite polarity. The current will travel through the conductive exterior skin and structures of the aircraft and exit off some other extremity, such as the tail. Pilots occasionally report temporary flickering of lights or short-lived interference with instruments. We have heard reports of the activation of cabin oxygen masks; no doubt frightening for the passengers.
How is an aircraft protected from lightning?
Most aircraft skins are made primarily of aluminum, which is a very good conductor of electricity. By making sure that there are no gaps in this conductive path, the engineer can assure that most of the lightning current will remain on the exterior skin of the aircraft. Some modern aircraft are made of advanced composite materials, which by themselves are significantly less conductive than aluminum. In this case, the composites are made with an embedded layer of conductive fibers or screens designed to carry lightning currents. These designs are thoroughly tested before they are incorporated in an aircraft.
Modern passenger jets have miles of wires and dozens of computers and other instruments that control everything from the engines to the passengers' music headsets. These computers, like all computers, are sometimes susceptible to upset from power surges. So, in addition to the design of the exterior of the aircraft, the lightning protection engineer must assure that no damaging surges or transients can be induced into the sensitive equipment inside of the aircraft. Lightning traveling on the exterior skin of an aircraft has the potential to induce transients into wires or equipment beneath the skin. These transients are called lightning indirect effects. Problems caused by indirect effects in cables and equipment are averted by careful shielding, grounding and the application of surge suppression devices when necessary. Every circuit and piece of equipment that is critical or essential to the safe flight and landing of an aircraft must be verified by the manufacturers to be protected against lightning in accordance with regulations of the FAA or a similar authority in the country of the aircraft's origin.
The other main area of concern is the fuel system, where even a tiny spark could be disastrous. Therefore, extreme precautions are taken to assure that lightning currents cannot cause sparks in any portion of an aircraft's fuel system. The aircraft skin around the fuel tanks must be thick enough to withstand a burn through. All the structural joints and fasteners must be tightly designed to prevent sparks as lightning current passes from one section to another. Access doors, fuel filler caps and any vents must be designed and tested to withstand lightning. All the pipes and fuel lines that carry fuel to the engines, and the engines themselves, must be verified to be protected against lightning. In addition, new fuels that produce less explosive vapors are now widely used.
Radomes are the nose cones of aircraft that contain radar and other flight instruments. The radome is an area of special concern for lightning protection engineers. In order to function, radar cannot be contained within a conductive enclosure. Protection is afforded by the application of lightning diverter strips along the outer surface of the radome. These strips can be solid metal bars or a series of closely spaced buttons of conductive material affixed to a plastic strip that is bonded adhesively to the radome. These strips are sized and spaced carefully according to simulated lightning attachment tests, while at the same time not significantly interfering with the radar. In many ways, diverter strips function like a lightning rod on a building
Private general aviation planes should avoid flying through or near thunderstorms. The severe turbulence found in storm cells alone should make the pilot of a small plane very wary. The FAA has a separate set of regulations governing the lightning protection of private aircraft that do not transport passengers. A basic level of protection is provided for the airframe, fuel system and engines. Traditionally, most small commercially made aircraft have aluminum skins and do not contain computerized engine and flight controls, therefore they are inherently less susceptible to lightning. Numerous reports of non-catastrophic damage, such as to wing tips, propellers and navigation lights have been recorded.
A growing new class of kit-built composite aircraft also raises some concerns. Because owner-assembled, kit-built aircraft are considered by the FAA to be "experimental," they are not subject to lightning protection regulations. Many kit-built planes are made of fiberglass or graphite-reinforced composites. At Lightning Technologies' laboratory, we routinely test protected fiberglass and composite panels with simulated lightning currents. The results of these tests show that lightning can damage inadequately protected composites. Pilots of unprotected fiberglass or composite aircraft should not fly anywhere near a lightning storm or in other types of clouds, because non-thunderstorm clouds may contain sufficient electric charge to produce lightning. NASA has a Small Business Innovation Research project for the development of cost-effective lightning protection for kit-built aircraft. Conducted by Stoddard-Hamilton Aircraft, Inc. and Lightning Technologies, Inc., the program designed and tested lightning protection against severe in-flight strikes for Stoddard Hamilton's fiberglass composite Glasair III LP, a small high-performance, kit-built aircraft..
Hope this explains why in properly lightning proofed aircraft lightning should not be a problem.
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