Electromagnetic protection for life
A wire bundle test article was developed by Cessna for mechanical and environmental degradation tests. |
Aircraft electromagnetic protection has always focused on the design, test, and analysis required for certifying aircraft. However, this effort is now being expanded by engineers at Cessna Aircraft Co. to include the continued airworthiness of electromagnetic protection over the entire life of the aircraft. To do so, engineers will require a better understanding of the scope and magnitude of maintaining assurance over the lifetime of the aircraft.
Wire bundle shielding of aircraft wiring harnesses, electrical bonding, and grounding are all used to provide electromagnetic protection in general aviation aircraft. The effectiveness of these protection methods is assumed to be maintained using periodic, detailed visual inspections and dc bond checks after any replacements or repairs. These inspections have been successful in ensuring protection considering the low number of catastrophic occurrences involving lightning strikes. However, the continued effectiveness of electromagnetic protection methods becomes more evident as the use and complexity of avionics increases along with the growing amount of composites used in the manufacture of the aircraft.
Becoming more aware of the importance of electromagnetic protection, the FAA is in the process of developing measurable standards for assuring its continued effectiveness. However, in developing measurable standards, regulators will require a better understanding of the electromagnetic situation and the necessary measurement tools. Therefore, engineers at Cessna have begun putting together research that will look at several different aspects of electromagnetic protection and some proposed measurement tools. The effort will entail:
- Subjecting sample shielded-wire bundles and their connectors to a variety of electrical, mechanical, and environmental laboratory tests to study the effects of shield degradation, corrosion, and the deterioration of bonding and grounding
- Conducting a field survey of bonding, corrosion, and shield degradation found on in-service aircraft to assess the impact of aircraft age, usage, in-service environmental conditions, and ground maintenance
- Reviewing historical lightning strike data to understand which variables most affect the severity of lightning damage of aircraft and their systems
- Evaluating the effectiveness and feasibility of single and swept-frequency loop impedance and transfer function measurement techniques for improving continued protection maintenance for general aviation in-service aircraft and systems.
Researchers fabricated six identical test articles to undergo mechanical and environmental degradation tests at low, medium, and high severity levels. Such tests included temperature and altitude, vibration, mechanical degradation, and salt spray and humidity evaluations. Electrical testing and visual inspections were performed after each severity level. No significant variations were observed from the temperature, altitude, and vibration tests, and no significant changes were measured in the bonding, grounding, and connection resistance for all degradation tests. Corrosion was observed on the ground plane, resistance boxes, and shields after the salt fog test. Variation in the loop impedance was still very small even after all the tests.
A wire bundle that has undergone mechanical degradation. |
Engineers also conducted a field survey of in-service aircraft to determine the amount of bonding, corrosion, and shield degradation. The Full Authority Digital Engine Control (FADEC) wiring was chosen as the focus of the investigation because it is a stable system that is currently in production, it contains both over-braided and "pigtail" wire bundle shielding, no engineering changes have been issued in last five years of service, and both newly manufactured and in-service aircraft will be available for measurement.
Data was collected from several production-line aircraft to establish a baseline reference. Measurements were taken using both the loop impedance tester and the network analyzer. Data was then collected on the same system wire bundles in the same manner for various in-service aircraft. The variation in the impedance of the FADEC wire bundles was found to be around 2 mohm.
According to Cessna, it has been the only manufacturer involved in the research. However, another manufacturer has completed the construction of test articles for laboratory testing of wire bundles. Researchers at Cessna believe that additional participation in the creation and analysis of a lightning strike database would greatly improve the confidence in the study. The tasks requiring loop impedance measurements have been delayed due to late deliveries of a production model loop impedance tester. This testing will evaluate the effectiveness and feasibility of single and swept frequency loop impedance and transfer function measurement techniques versus dc resistance measurements and visual inspection.
Information was provided by Michelle Cronkleton and Chuck Beuning of Cessna Aircraft Co.
