Image: Bill Dedman
By Bill Dedman Investigative reporter
msnbc.com
updated 6/1/2009 12:20:42 PM ET 2009-06-01T16:20:42

Lightning hitting airplanes is commonplace and, though lightning has downed several commercial aircraft, it rarely leads to catastrophe thanks to modern protection systems.

Each commercial airplane is struck by lightning approximately once a year on average, according to Lightning Technologies, a Pittsfield, Mass., company that designs lightning-protection systems for aircraft. Not only are aircraft struck by lightning, but in fact they cause their own lightning, NASA research found, when they fly through heavily charged areas of clouds.

In 1963, a Pan Am Boeing 707, on a flight from Baltimore to Philadelphia, crashed near Elkton, Md., after being struck by lightning. All 81 people on board died. The probable cause was listed as "lightning-induced ignition of the fuel/air mixture in the no. 1 reserve fuel tank with resultant explosive disintegration of the left outer wing and loss of control."

In 1967, 23 people died when a Lockheed jet flown by the Imperial Iranian Air Force was brought down by lightning.

In 1971, 91 people died when a Lansa flight crashed in Peru after a lightning strike caused a fire and separation of the right wing.

In 1988, in Germany, a Swearingen Metro aircraft lost its wing after being struck by lightning, and 21 on board died.

A close call came just this March 13, when an American Airlines flight from Dallas to Houston was struck by lightning, putting a hole in the left side of the fuselage, according to FAA records. The plane landed safely.

In 1977, the National Transportation Safety Board sent an urgent message to the industry, seeking a concerted effort to reduce damage from lightning strikes. NASA scientists took up the challenge, conducting research through 1986.

In the past 30 years, much has been learned about avoiding damage from lightning strikes, according to information on the Lightning Technologies Web site. "Protection techniques have improved. Airplanes receive a rigorous set of lightning certification tests to verify the safety of their designs."

How are aircraft protected from lightning?

"Most aircraft skins are made primarily of aluminum, which is a very good conductor of electricity," according to Lightning Technologies. "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…. . 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."

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