Aircraft Lightning Protection - Chapter 13 – Full Vehicle Testing describes the methods available for measuring the transient voltages and currents induced by lightning in aircraft electrical wiring. These are known as "full vehicle" tests and are usually applied at reduced amplitudes so as not to damage the tested airplane.
When commercial airplanes are struck by lightning, the result can range from no damage to serious damage that requires extensive repairs that can take the airplane out of service for an extended period of time. Having an understanding of the typical effects of lightning strikes and proper damage inspection procedures can prepare operators to act quickly when a lightning strike is reported to apply the most effective maintenance actions.
Aircraft Lightning Protection
Chapter 1: An Introduction to High Voltage Phenomena, deals with the nature of high voltage electrical sparks and arcs and with related processes of electric charge formation, ionization, and spark propagation in air. All of these are factors that affect the way that lightning leaders attach to an aircraft and the way that the hot return stroke arc affects the surface to which it attaches.
Lightning-Strike Structural Inspection Procedures
The material introduces practices and terms used for many years in the electric power industry, but which are not commonly studied by those dealing with aircraft. These terms and practices have, however, affected the tests and practices used to evaluate the effects of lightning on aircraft.
The last confirmed commercial plane crash in the U.S. directly attributed to lightning occurred in 1967, when lightning caused a catastrophic fuel tank explosion. Since then, much has been learned about how lightning can affect airplanes.
As a result, protection techniques have improved. Today, airplanes receive a rigorous set of lightning certification tests to verify the safety of their designs. If the entrance and exit points are not found during the examination of Zones 1 and 2, the Zone 3 surface areas should be examined for signs of lightning-strike damage.
Inspections of Zone 3 are similar to Zones 1 and 2. Additional inspections for Zone 3 include: 22 Barnes Industrial Park Road Wallingford, CT 06492 Phone: (203) 294-4440 Fax: (203) 294-7899 A Zone 3 examination is highly recommended even if no damage is found during the Zone 1 and Zone 2 examinations.
Required Actions Following A Lightning Strike To An Airplane
In summary, any entrance and exit points must be identified in Zones 1, 2, or 3 so that the immediate areas around them can be thoroughly examined and repaired if necessary. Most aircraft skins consist primarily of aluminum, which conducts electricity very well.
By making sure that no gaps exist in this conductive path, the engineer can assure that most of the lightning current will remain on the exterior 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 contain an embedded layer of conductive fibers or screens designed to carry lightning currents. Lightning is initiated at the airplane's leading edges, which ionize, creating a strike opportunity. Lightning currents travel along the airplane and exit to the ground, forming a circuit with the airplane between the cloud energy and the ground.
Chapter 6: Protection Against Physical Effects and Chapter 7 – Fuel System Protection contain the basic elements of protection designed for the airframe, fuel tanks, and fuel system components. The methods presented here are basic approaches, and many variations on these, too numerous to describe in this book, have been successfully used.
Lightning-Strike Structural Repairs
The reader is cautioned that all candidate designs should be tested, especially those that do not have a successful history of prior use. Fuel vapor ignition remains one of the most serious lightning hazards, and should be given careful attention in any design and certification program.
It is not possible to verify adequacy of fuel system protection without lightning testing of fuel tanks and systems. Operators should be aware of the conditions that are conducive to lightning strikes on airplanes and avoid exposing airplanes unnecessarily to lightning-prone environments.
While Boeing airplanes incorporate extensive lightning-strike protection, lightning strikes can still affect airline operations and cause costly delays or service interruptions. A clear understanding of proper inspection and repair procedures can increase the effectiveness of maintenance personnel and ensure that all damage caused by lightning is identified and repaired.
For systems not operated by the flight crew in flight or systems where anomalies were found, additional operational test procedures, as specified in the respective AMM, may be required. In addition, even if a system was operated in flight after the lightning strike and no anomalies were found, but subsequent inspections showed lightning damage near that system antenna, additional checks of that system may be required.
How Microgrid® Protects Composite Aircraft And Wind Turbines
Because the airplane flies more than its own length during the time it takes a strike to begin and finish, the entry point will change as the flash reattaches to other spots after the initial entry point.
Evidence of this is seen in strike inspections where multiple burns are seen along the airplane's fuselage (see fig. 6). Areas of an airplane that are prone to lightning strikes are indicated by zone. Zone 1 indicates an area likely to be affected by the initial attachment of a strike.
Zone 2 indicates a swept, or moving, attachment. Zone 3 indicates areas that may experience conducted currents without the actual attachment of a lightning strike. If lightning strikes an airplane, a lightning-strike conditional inspection must be performed to locate the lightning-strike entrance and exit points.
When looking at the areas of entrance and exit, maintenance personnel should examine the structure carefully to find all of the damage that has occurred. Chapters 8 through 17 focus on protection of electrical and avionic systems against indirect effects and form the basis for our course, Lightning Protection of Avionics.
Commercial Airplane Lightning Protection
As with all aspects of electromagnetic interference and control, the prevention of damage and interference from lightning becomes more and more critical as aircraft evolve. Most of the navigation and control functions aboard modern aircraft place a computer between the pilot and the control surfaces or engines, often without mechanical backup.
This makes it essential that the computer and control equipment be designed to prevent damage or upsets by lightning. Control of these indirect effects requires coordination between those who design the air-frame and its interconnecting wiring, those who design avionic systems and those who oversee the certification process, Part of the overall control process requires the selection of transient design levels and application of suit
-able test standards and practices. After attachment, the airplane flies through the lightning event. As the strike pulses, the leader reattaches itself to the fuselage or other structure at other locations while the airplane is in the electric circuit between the cloud regions of opposite polarity.
Current travels through the airplane's conductive exterior skin and structure and exits out another extremity, such as the tail, seeking the opposite polarity or ground. Pilots may occasionally report temporary flickering of lights or short-lived interference with instruments.
Lightning Overview
The large amount of data gathered from airplanes in service constitutes an important source of lightning-strike protection information that Boeing uses to make improvements in lightning-strike damage control that will reduce significant lightning-strike damage if proper maintenance is performed.
Lightning strikes to airplanes may occur without indication to the flight crew. When an airplane is struck by lightning and the strike is evident to the pilot, the pilot must determine whether the flight will continue to its destination or be diverted to an alternate airport for inspection and possible repair.
In the next stage of the strike, a stepped leader may extend off the airplane from an ionized area seeking the large amount of lightning energy in a nearby cloud. Stepped leaders (also referred to as "leaders") refer to the path of ionized air containing a charge emanating from a charged airplane or cloud.
With the airplane flying through the charged atmosphere, leaders propagate from the airplane extremities where ionized areas have formed. Once the leader from the airplane meets a leader from the cloud, a strike to the ground can continue and the airplane becomes part of the event.
Identifying Lightning-Strike Damage On A Commercial Airplane
At this point, passengers and crew may see a flash and hear a loud noise when lightning strikes the airplane. Significant events are rare because of the lightning protection engineered into the airplane and its sensitive electronic components.
Detailed information and procedures for common lightning allowable damage limits and applicable rework or repairs can be found in the structural repair manual (SRM) for each airplane model. Maintenance personnel should restore the original structural integrity, ultimate load strength, protective finish, and materials after a lightning strike.
To learn more about the benefits of Dexmet's expanded MicroGrid® materials, its lightning protection performance, and how it can reduce your maintenance costs and downtime, download our guide. Let us show you how to incorporate our innovative expanded materials into your composite designs.
For example, if all the navigation and communications systems are operated by the flight crew in flight after the lightning strike and no anomalies are found, checks to the operated systems would not normally be required.
Chapter 16 – Design to Minimize Induced Effects discusses some of the policy matters relating to control of indirect effects, tasks that must be undertaken by those responsible for setting overall design practices. Principally these relate to shielding and grounding practices to be followed, and to transient design level specifications to be imposed on vendors.
Direct effects of a lightning strike can be identified by damage to the airplane's structure, such as melt through, resistive heating, pitting to structure, burn indications around fasteners, and even missing structure at the airplane's extremities, such as the vertical stabilizer, wing tips
, and horizontal stabilizer edges (see fig. 5). Airplane structure can also be crushed by the shock waves present during the lightning strike. Another indication of lightning strike is damage caused to bonding straps. These straps can become crushed during a lightning strike due to the high electromagnetic forces.
Most of the external parts of legacy airplanes are metal structure with sufficient thickness to be resistant to a lightning strike. This metal assembly is their basic protection. The thickness of the metal surface is sufficient to protect the airplane's internal spaces from a lightning strike.
The metal skin also protects against the entrance of electromagnetic energy into the electrical wires of the airplane. While the metal skin does not prevent all electromagnetic energy from entering the electrical wiring, it can keep the energy to a satisfactory level.
The frequency of lightning strikes that an airplane experiences is affected by several factors, including the geographic area where the airplane operates and how often the airplane passes through takeoff and landing altitudes, which is where lightning activity is most prevalent.
Lightning strikes to airplanes can affect structures at the entrance and exit points. In metal structures, lightning damage usually shows as pits, burn marks, or small circular holes. These holes can be grouped in one location or divided around a large area.
Burned or discolored skin also shows lightning-strike damage. Chapter 18 – Test Techniques for Evaluation of Induced Effects presents an overview of test methods used to verify the ability of equipment to tolerate lightning-induced transients and the ability of complete systems to tolerate those transients, particularly when applied in the multiple stroke and multiple
burst waveform sets. These test methods have recently been incorporated into new or updated lightning test standards. A few comments on personnel safety are also included, since lightning tests involve generating and applying very high voltages and currents – far exceeding the levels employed in most electrical test laboratories.
They also far exceed lethal levels and have proven fatal to inexperienced operators. Lightning tests to evaluate or verify either direct or indirect effects should be performed only by personnel experienced in this technology. Chapter 2: The Lightning Environment provides an elementary description of cloud electrification and lightning strike formation, and follows with statistics of cloud-to-earth lightning parameters from which the aircraft lightning design and test standards have been derived.
The user of this book is urged to study these two introductory chapters before proceeding with later sections of the book. The treatment of these topics is on an elementary level and is aided by simple illustrations, which should enable those with only a limited background in electricity to proceed to an adequate understanding of important principles.
Aluminum has been the main material used in aircraft and aerospace construction for the past 70 years. With the growing interest in constructing more efficient aircraft, manufacturers are designing more components out of light-weight composite materials.
The new generation of aircraft utilizes carbon fiber for major structures including the fuselage, wing, engine nacelles, flaps, wing tips, and even rotary blades and fuselage on helicopters. Composites, however, are poor conductors of electrical current.
Without proper protection, they are susceptible to severe damage in the event of a lightning strike. When Dexmet expanded aluminum and copper MicroGrid® materials are incorporated into the surface of these composite structures, the lightning strike energy is dissipated over the surface of the component, which prevents damage to the composite material below.
The other main area of concern is the fuel system, where even a tiny spark could be disastrous. Engineers thus take extreme precautions to ensure 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 of the structural joints and fasteners must be tightly designed to prevent sparks, because 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 protected against lightning.
In addition, new fuels that produce less explosive vapors are now widely used. MicroGrid® precision expanded metal foils from Dexmet are the materials of choice for lightning strike protection in composite aircraft structures. We've been developing the technology for the last 30 years with all major aircraft OEMs and we have the most configurations available in the industry.
Dexmet is the preferred supplier on many of the new generation designs for Airbus, Boeing, Bombardier, Embraer, and the majority of aircraft and rotorcraft manufacturers around the world. Aircraft manufacturers realize the benefits of using Dexmet's advanced expanded materials over the outdated technology of woven wire and come to Dexmet for technical knowledge when designing new components.
Seventy percent of all lightning strikes occur during the presence of rain. There is a strong relationship between temperatures around 32 degrees F (0 degrees C) and lightning strikes to airplanes. Most lightning strikes to airplanes occur at near freezing temperatures.
Control of lightning indirect effects by analysis can only be carried so far; proof of tolerance of indirect effects is most likely to come about by performing tests on individual items of equipment and on interconnected systems.
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