Equipment Maintenance

Equipment Maintenance By Jim Sparks May 2000 Manufacturers of aircraft and of avionics equipment generally provide guidance to assist in keeping their equipment operating properly. This may include information for functional tests, bench tests...


Keep calibration of equipment current
The calibration of test equipment will vary with the type of equipment and possibly even the environment in which it is used. As a general rule, a one-year interval is acceptable; however, manufacturers' recommendations and even past calibration history may have an impact on recertification. The local airworthiness authorities also have some valuable input on determining test equipment calibration intervals.

Each aircraft, along with system manufacturer, will devise their own specific system testing particulars. There are, however, certain general rules that will apply to the majority of aircraft.

In addition to checking the adherence to bonding requirement, wires and cables should be checked and kept as short as possible except for those antenna cables that are set to a specific length. All components should be free of dirt or other contaminants, and ventilation systems and openings should allow proper airflow. Circuit breakers and electrical switches should be checked for proper operation. It should be considered that a spring-loaded switch might experience internal failures preventing it from tripping off or even failing to open the assigned electrical circuit. It is also important to note that improper switches or circuit breakers with improper ratings or properties have not been substituted. As an example, circuit breakers used in AC circuits are not designed to carry the same current as those used in DC systems.

Mounting racks
One often overlooked component when conducting inspections on avionics is the mounting rack. In addition to looking for the standard condition and structural integrity, particular attention should be paid to any mechanical shock suppression device. Vibrachoc makes one type of spring-loaded shock absorber, and like any spring-loaded device, these are apt to lose their tension with age. A simple test recommended by Vibrachoc is to measure the distance from the rack to the supporting aircraft structure, then compress the mount complete with component installed. Once compressed, re-measure. If the loaded measurement is within .039 inches (1 mm) of the at-rest measurement, the device should be replaced.

Antennas
Also in the inspection process, other often overlooked items are the antennas. In the case of single wire devices, they should always be checked for the condition and presence of insulators and springs. Other surface-mount antennas should have crack-free housings, and if the leading edge is protected, it should be with an anti-static coating. Proper bonding is also essential to correct operation. Some equipment useful in determining proper antenna operation is a Time Domain Reflectometer (TDR), which will pinpoint defects in the coax transmission line as well as internal defects in the antenna. A Wattmeter in line with a transmit- type antenna can also be useful to determine true system output.

Static wicks can also have adverse influence on aircraft electronic systems. In addition to checking security and mounting, wear and abrasion is also accountable for deterioration in the ability to discharge unwanted electric charges. The bonding of bases to aircraft structure should insure no voltage level difference between the two surfaces. If the aircraft has pitting on the surface under the metal base for the static wick, it is likely that it experienced a lightning strike. In the event of a confirmed lightning strike, removal and inspection of all static discharger mounting surfaces is a worthwhile precaution. Routine megohm checks on individual wicks may also be useful in heading off unwanted precip static conditions.

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Basic rules
Some basic rules pertaining to avionics systems in general include dealings with compass systems; a compass swing is considered a functional test. This should always be performed when a new compass or compass system component is replaced. Some other considerations on conducting a compass swing would be anytime the accuracy is in question and in the event of aircraft modification where ferrous metal is installed in the general area of the compass magnetic sensor. Should the aircraft experience a lightning strike, severe turbulence, or a hard landing, a functional test of the compass would be in order. Also, anytime a change is made in the electrical system that may cause a change in the aircraft electromagnetic field would require compass testing.

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