The statement that if you are greater than 20 miles from radar-indicated precipitation, you are not susceptible to a lightning strike is also false. Aircraft have been struck by the proverbial "bolt from the blue" on more than one occasion. In fact, aircraft have been struck at distances out to 50 nautical miles from thunderstorms, particularly when cirrus clouds existed above or at their altitude, or when there were other developing showers nearby that had not yet reached maturity. Flying through precipitation, volcanic ash, or heavily polluted air can cause an aircraft to experience electrostatic discharge or triggered lightning. Usually these discharges cause only minor aircraft damage; however, there is always the chance for catastrophic occurance if the discharge passes through the vaporized fuel-air mixture in the fuel tank.
Cirrus clouds are made up of ice particles, a large number of which will strike the aircraft and bounce away. At each impact electric charges are transferred leaving the aircraft with a significant potential. This can reach a value of 100,000 to 200,000 volts in a matter of seconds. A discharge back to atmosphere can be expected and usually occurs at the trailing edges of flying surfaces and most frequently at wing and stabilizer tips. As the discharge occurs, the surrounding air becomes ionized and a corona pulse will depart the aircraft.
How to ensure safety
It is initially up to the aircraft manufacturer to verify safety in the event of a lightning strike. In many cases special inspections have been created to look at designated areas on a particular airframe when a lightning strike has been reported. In addition to inspecting external surfaces for visual damage attention needs to be paid to the not so obvious areas such as bonding straps, hinges, and other surface connecting components. Should a lightning strike be observed on a propeller, arcing may have resulted on various engine bearings. If retractable landing gear is the target when extended then trunnion bearings or actuators may have incurred damage.
The United Kingdom Civil Aviation Authority (CAA) has issued Airworthiness Information Leaflet (AIL) 0014 which addresses enhanced lightning strike protection in light aircraft. The information given in this leaflet outlines the problems which may be encountered on aircraft fitted with metallic or nonmetallic tip tanks or other external nonmetallic components and gives guidance on methods of reducing their vulnerability to lightning strikes.
Direct and indirect effects
Aircraft damage from lightning can be caused as a direct or indirect effect. Direct effects result when the lightning current attaches to and flows through the aircraft skin. Locations on the aircraft where lightning strikes occur experience extreme heating which causes burning and melting damage. Current flowing through the aircraft structure can result in isolated arcing or sparking and heating. If this occurs in a fuel tank, explosion and fire can result.
Indirect effects are caused by transient electrical pulses produced by the changing electric and magnetic fields due to the lightning current. Unless avionics and other systems are properly shielded, they are easily damaged.
In some cases metal encased fuel tanks have had sealant boundaries reinforced to inhibit an electrical spark occurring within the tank in the event of the tank surface being the point of lightning entry.
It is interesting to note that 57 percent of the aircraft lightning mishaps occur during the months of March through July, and the state of Florida has the highest number of lightning events along with the most strike victims.
It would appear that participating in a kite-flying festival this spring in Florida may be an electrifying experience.
Jim Sparks has been in aviation for 30 years and is a licensed A&P.
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