Plane & Pilot
Tuesday, July 31, 2012

Lightning Enlightenment

The NTSB wants the FAA to provide real-time lightning data

On April 13, 2012, United Airlines flight 930, a Boeing 777, took off from San Francisco International Airport en route to London. The airplane was struck by lightning, and the flight crew elected to return to San Francisco. On May 7, 2012, a Boeing 777 operated by Emirates Air as flight 407 was flying near Melbourne, Australia, on a flight from Auckland, New Zealand, to Dubai when it was struck by lightning. The flight crew made an emergency landing at Melbourne, failed to find any damage and resumed the flight. On May 15, 2012, a Dassault Falcon 7X jet carrying French President Francois Hollande from Paris to Berlin was struck by lightning and returned to Paris. President Hollande was transferred to another plane. Coincidentally, on May 18, 2012, the NTSB issued a Safety Recommendation calling for the FAA to install upgraded technology so that controllers see lightning strikes on their screens, thus providing additional information to help them keep aircraft away from storms.

The Safety Board wants the FAA to study the technical feasibility of presenting, through the use of existing weather and radar processor systems or other means, real-time total lightning data on controller displays in use now at both centers and TRACON facilities, and on the displays that will be used when the NextGen system is deployed. The Safety Board also points out that the new avionics the FAA wants aircraft operators to install would have enough capability to handle real-time lightning data, and it would make sense for the FAA to include lightning data in the weather information it plans to transmit for pilots to use via broadcast data link.

The NTSB notes that existing ground-based lightning detection technology can retrieve information for both cloud-to-ground and cloud-to-cloud lightning with a high degree of location accuracy. It's accepted that lightning activity correlates with areas of convective turbulence, and also can help identify vertical development of some thunderstorms and microbursts at the ground. Because lightning detection networks and weather radar operate independently, lightning information may help detect thunderstorms in regions of the National Airspace System where weather radar coverage doesn't exist or is spotty. Lightning information supplements weather radar because it can reflect possible convective activity in airspace where precipitation returns from weather radar don't appear to be significant.

According to the National Weather Service, nearly 16 million thunderstorms occur around the world each year, with 1,800 occurring at any given moment. In the U.S., there are an estimated 100,000 thunderstorms each year. A study by Texas A&M University indicated there were 16.9 million lightning flashes in the U.S. during calendar 1991.

The value of lightning activity as a means to identify and avoid hazardous weather started to become obvious in the aviation community on June 10, 1976, when Paul A. Ryan and Nicholas Spitzer filed their patent application for a device called the Stormscope. This was a storm mapping system using electrical activity as the basis for making a visual display of significant weather. The distance of the weather was inversely related to signal strength, and the azimuth could be determined by the relationship of signal strength to directional antennas. Their invention went on to become a significant presence in aircraft lacking weather radar, and a popular addition to the equipment on aircraft having radar capability. Now, 36 years later, the NTSB says it's time for FAA controllers to have that same type of tool available.

Given the huge number of thunderstorms, lightning strikes and aircraft, it's noteworthy that only one or two lightning strikes a year may rise to a level requiring an NTSB investigation. A common rule of thumb is that each aircraft in the U.S. air carrier fleet is going to be struck by lightning once a year, with strikes affecting general aviation aircraft being relatively rare. Even though electrical systems and avionics have become more complex over the years, the vulnerability of aircraft to damage from lightning has decreased due to better design and airframe bonding to safely dissipate the high current and voltage. This is true of both aluminum and composite structures.

The first aircraft to be destroyed due to a lightning strike is believed to have been a German Zeppelin that was venting hydrogen gas on September 3, 1915. A lightning strike had the predictable effect on the hydrogen. The crash of a Ford Tri-Motor in New Mexico on September 3, 1929, is believed to be the first fixed-wing fatal accident caused by a lightning strike. All eight people on board were killed. In its Safety Recommendation, the NTSB made reference to a number of more recent incidents.


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