Avionics, No Faults Found, and the World of Fiber Optics

Avionics test methods are typically restricted to singular fashion testing. This defines the removal of a faulty avionic component from the aircraft, usually an avionics box, in order for it to be tested at a repair station. The main issue with this process is that it cannot accurately reflect the malfunctioning unit in tandem with its associated aircraft systems. Therefore, if a problem does not occur during testing and cannot be recreated by the service facility or an avionics technician, it is classified as a No Faults Found (NFF) case. 

Upon this occurrence, the said components have to be replaced. In commercial aviation, this process and potential replacement of avionics parts and systems can cost an airline up to 250,000 dollars a year, per aircraft. Paired with the gradual conversion of avionics from copper to fiber-optic technology, this predicament poses the demand for whole new methods of testing. 

Fiber optics are thin strands of plastic or glass, with a diameter averaging that of a human hair. Fiber optic wires are made up of bundles called optical cables that communicate light signals between systems. A fiber optic cable consists of three main parts. The core, which is made up of the thin fibers previously mentioned. Core fibers are contained within an outer optical material called cladding, whose purpose is to reflect any light leakage back into the core. These cables are coated with a plastic, called buffer coating, that surrounds the fiber optic bundles and protects them from corrosion and damage. 

Fiber optics require an optical transceiver in order to convert the optical signals into electrical signals, and vice versa. In the tumultuous environment of an aircraft, vibration and temperature changes can cause this device to degrade at a fast rate. This damage creates a predicament where cables may start to fail in ways that are difficult to detect, such as degraded output of power or degraded receiver sensitivity within a box. 

The integration of a new fiber optic system alone, much like that of a new electronic assembly, can cause failures in compatibility that are challenging to isolate. Bus functions and fiber optics systems interact with one another and function simultaneously within a network in real-time. Due to this exchange, it is imperative that both systems are tested together in the event of a failure. 

A possible solution for the complications of testing a fiber optics system, is a switch tester. This device is capable of checking optical power at sending and receiving points within a malfunctioning fiber optic assembly. Testing of this nature can isolate errors that originate from an optical transceiver using high-density switching. Some models of this device, in addition to this capability, have the capacity to transmit data to bus-test instruments while connected to the unit being tested. This allows the device to conduct a stress-test on Ethernet and Fiber Channels while they are in their typical interaction with the fiber optic system, in order to locate any errors within the communication networks. To add to its capabilities, this mechanism can also be utilized on the aircraft itself, or in a factory test, greatly reducing the risk of an NFF fiber optic component.


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