Lightning Strikes

A look at how they affect aircraft and avionics.

Some positive strikes can occur within the parent thunderstorm and strike the ground beneath the cloud. However, many positive strikes occur near the edge of the cloud or strike more than 10 miles away, where you may not perceive any risk nor hear any thunder. Positive flashes are believed to be responsible for a large percentage of forest fires and power line damage. Thus, positive lightning is much more lethal and causes greater damage than negative lightning.

There are two principal sources of static electrification on aircraft. The Autogenous Field is caused by frictional charges resulting from contact between the aircraft and various particles such as dust as it moves through the atmosphere. Exogenous Fields are caused by the presence of the aircraft in a thunderstorm which can cause both positive and negative streamers.

Aircraft lightning strikes

In the United States, the Federal Aviation Administration (FAA) has a system in place to track lightning strikes on commercial aircraft. The reported statistical results indicate that lightning strike frequency is such that every commercial aircraft gets one and a half strikes per year and commercial pilots experience this phenomenon once every 3,000 flight hours. Although general aviation aircraft are not required to report these incidents, the U.S. Department of Transportation has conducted a research project which was completed in 2004 and is titled "General Aviation Lightning Strike Report and Protection Level Study."

This report analyzed 95 lightning strike reports from general aviation business jet aircraft that occurred over a five-year period. The analyses were conducted to determine which variables most affect the severity of indirect lightning effects damage of in-service aircraft and their systems and to assess the effect of the level of lightning and High-Intensity Radiated Fields (HIRF) protection design and implementation.

After validating the data, three variables were studied with respect to lightning damage: aircraft age, aircraft flight hours, and the level of lightning and HIRF protection. The level of protection for each aircraft model in the database was categorized as no protection, avionics protection, or full protection.

The study found that fully protected aircraft had a significantly lower percentage of electrical failure and interference due to lightning strikes when compared to aircraft with no protection or only avionics protection. The number of electrical failures reported did not increase over the age of the aircraft. Another result illustrates the areas where lightning tends to strike on various manufacturers' airframes.

This table shows that Zone 1, the radome and the wing tips, was the most frequent area of lightning attachment. Zone 2 includes areas on the bottom of the fuselage and wing tips, while Zone 3 includes large areas under the wings.

Protection and avoidance

Federal Air Regulation (FAR) 25.1316 along with Advisory Circular 20-136 provides guidance to manufacturers of Transport Category aircraft for protection against lightning strike and HIRF.

Combining new research on lightning with the lessons of the past we know it is true that some aircraft are less prone to lightning strikes. Size, shape, and speed are all aircraft-specific variables which determine susceptibility to a lightning strike. It is also true that aircraft damage varies with type. Airframe design can minimize lightning damage. However, all surfaces are susceptible to lightning strikes and all unprotected systems can be affected.

Some pilots are better at avoiding lightning strikes than others. The wider the berth given to thunderstorms, the better the chance of avoiding a lightning strike; however, the pilot who tries to pick his or her way between thunderstorm cells is asking for trouble.

Several theories proven false

The theory that if you avoid thunderstorms you will avoid all lightning strikes is false. Statistics show that many triggered strikes have occurred during flights that did not penetrate thunderstorms. Aircraft have triggered strikes in cirrus clouds downwind of previous thunderstorm activity, in cumulus clouds around the periphery of thunderstorms, and even in stratiform clouds and light rain showers not associated with thunderstorms.

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