Optical fiber has begun replacing copper in avionic networks. So far, however, it has been mainly restricted to non-critical applications (video transmission to the flight deck, IFE?). In order to take advantage of the high-bandwidth, low weight, no EMI properties of optical fibers in all data transmission networks, it will be necessary to improve the testing. One part of the puzzle, which is still missing, is the self-test button: the possibility to check the network and detect potential failures before they occur. The typical testing tool of a technician involved in optical fiber cables is the ?light source ? optical power meter? pair. With this tool, one can measure the insertion loss of the fiber link. A second important parameter, the return loss at each optical connector, is not analysed. In addition, this is only a global measurement, which does not allow the detection of possible weak points. Finally, there is no way to test the network once it is in use, except by opening both ends to insert the tools. This is not something that can be routinely executed on aircrafts. Therefore, once an optical network is certified, there are no regular checks or evolution analyses. In this work, we show that it is now possible to test and characterize all parameters of an avionic optical network, with a well-adapted Optical Time-Domain Reflectometer (OTDR). The OTDR does a single-ended measurement, which only requires the insertion of the instrument at any convenient point of the network. In addition, adding a tap-coupler to the network would enable an easy test, even after the network is in use. A tap-coupler is a small, inexpensive and entirely passive device, which removes a minute proportion of the light (for example 10%) and sends it to a different fiber. This is enough to test the network with an OTDR, without the need to disconnect any part of the network. Adding tap-couplers to optical networks is a first step towards on-board testing, which will be required for mission-critical applications of optical fibers.