Monitoring aircraft to ensure long-term protection
When lightning strikes an aircraft, the result looks spectacular from the ground and it can also cause alarm for passengers. However, aircraft are designed to cope well with what is actually a fairly common event – on average a passenger jet will be hit by lightning once every 1,000 flight hours. Airbus monitors one of the different means of protection against lightning which also contributes to protecting aircraft against High Intensity Radiated Fields (HIRF).
What happens during a lightning strike?
Lightning strikes are most likely to occur during landing and takeoff flight phases when aircraft are passing through cumulonimbus clouds that create high electrical charges. Strikes are less likely during cruise phase, partly because most cruising altitudes are well above the clouds, but also because pilots can navigate around storms once they are well into their journey.
Lightning arrives at the extremities or protuberances of the aircraft, usually a wing tip or the nose; this is known as the entry point. It then leaves from another extremity, or exit point, such as the tail, as it continues on its path to earth. During this process the fuselage acts as a Faraday cage: an enclosure used to block electromagnetic fields and conduct the lightning current safely.
Those onboard might notice some noise, sense what feels like an impact and even experience brief disruption of the cabin lights; however there is usually no cause for concern. On traditionally designed aircraft the aluminium ‘skin’ does the conducting, while ultra-modern aircraft such as the A350 XWB have a copper layer embedded in their carbon fibre fuselage sections specifically to handle lightning.
The aircraft is designed to ensure aircraft structure electrical continuity. It is capable of distributing lightning current in a highly effective way, thus providing key protection. Nevertheless, there are other means of protection such as: systems architecture (ensuring redundancy and dissimilarity); devices/equipment protection (equipment qualification against electromagnetic hazards); and design and installation rules of the electrical installation.
Dealing with induced current
When an aircraft is struck by lightning, induced voltages inside the fuselage may occur. These induced voltages are produced between the external surface of the aircraft, where the current is high, and the internal part of the aircraft which acts as ground to the potential reference.
The induced voltages could be enough to disrupt the aircraft systems if harnesses lacked an additional layer of protection. This protection is provided through the use of braided metal conduits which conduct current safely to the aircraft’s voltage reference points, ensuring only a minor amount of current reaches the cables inside or metallic raceways protecting cable signals.
The conduits also function as a barrier reducing HIRFs* such as those generated by radar stations. These could interfere with the functioning of aircraft systems but the braided conduits, combined with Electro-Magnetic Hazards (EMH) qualification equipment, ensure attenuation of HIRF events. All aircraft equipment is qualified against EMH, based on Environmental EUROCAE RTCA-DO160 (environmental conditions and test procedures for airborne equipment).
*HIRF = High Intensity Radiated Fields
Checking for damage:
Lightning/HIRF Assurance Plan
Of course, all this protection relies on the good condition of the conduits and their correct functioning throughout the aircraft’s lifecycle. As well as satisfying airworthiness requirements of the conduits at the time of type certification, Airbus monitors them over time through a sampling campaign. This is known as the Lightning/HIRF Assurance Plan and through Airbus expertise, close collaboration with customers and systematic data gathering, it contributes to keeping the current fleet safe as well as to improving future aircraft design.
How aircraft are selected
For each campaign, Airbus works on a representative sample of aircraft considering a range of environmental factors such as humidity, heat levels, routes, airports anddistances covered. Having identified suitable aircraft for sampling, Airbus invites the corresponding operators to participate in the campaign with one aircraft from each fleet. Lightning/HIRF inspections are then organised to fit around the airline’s routine maintenance checks so that there is no disruption to normal MRO scheduled activities.
Sampling campaigns start six years after entry-into-service, with one aircraft per family inspected per year. This sampling is repeated after 12 years (2nd campaign), 18 years (3rd campaign) and so on.
The inspection: finding and fixing issues
Airlines are already obliged to carry out their own visual inspections. However, Airbus looks for signs which are not visible from the outside. These inspections can take place anywhere in the world and usually involve a small, multi-functional team of experts from Airbus with the full range of engineering and maintenance skills that could be required.
Potential sources of High Intensity Radiated Fields.
Most of the work involves a device known as a loop resistance tester (LRT). With all aircraft systems switched off, this is clamped onto a section of the braided conduit and a measured current is applied. The team measures the flow of charge through the conduits. The LRT shows how well the conduit is conducting the current, thus demonstrating the status of the harness protection. If any sign of conductivity reduction is revealed, the LRT pinpoints the cause. The team has a part 145 qualification allowing it to work on the aircraft and repair or rectify as required, free of charge.
This process is repeated on conduits all over the aircraft over a period of several days and all data about different forms of degradation is recorded. Causes of conductivity reduction have included mainly degradation of bonding areas linked to environmental conditions. The team can deal with many issues directly on-site. It tends to find fewer issues on older aircraft as modifications and service bulletins, made following earlier sampling campaigns, have guided constant refinements of the design.
After inspection: bringing long-term solutions
Once Airbus has inspected all targeted aircraft in a sampling campaign, it presents the findings to the aircraft certification authorities (EASA and FAA). After careful consideration, solutions are agreed for any issues that have been found. They could involve quality improvements, modifications, or changes to scheduled maintenance tasks. Finally, the outcomes may influence the scope and number of aircraft to involve for the following campaign.
The data gathered during campaigns is also extremely valuable in informing the design of aircraft and variants to remove common causes of degradation – an example of highly effective preventative maintenance. The work carried out on aircraft during inspections will influence current and next generations.
C O N C L U S I O N
Although aircraft are very well protected against lightning strike and HIRFs, the use of braided metal conduits to add an extra layer of protection to the aircraft’s systems is essential for airworthiness certification. Through regular sampling campaigns, the Lightning/HIRF Assurance Plan helps to ensure that these standards are maintained throughout the lifecycle. Findings from inspections are shared with the certification authorities and the Airbus design office also uses them to refine their future use of conduits.
Article by
Eduardo ORGAZ VILLEGAS
Maintenance Engineering – Systems
AIRBUS
