The first step to troubleshooting an inoperable NAV is to identify the NAV source the pilot was using at the time of the failure. Is the aircraft flying with a GPS or conventional Very High Frequency (VHF) NAV?
When did the problem occur? When an aircraft is in NAV ARM mode, it is actually flying Heading Select (HDG SEL), at the same time it is computing the optimum point at which to capture the navigation source, based on the intercept angle, beam deviation and deviation rate.
Investigate the flight control system (FCS) capture logic and computations; including the course (CRS) error and deviation input signals. Most systems will require a valid flag from the navigation source in order to capture (or even ARM). There may be two valid flags out of the navigation source which exhibit identical logic, but differing voltage levels for valid. Most analog indicators will require a low-level flag, while most FCS require the high-level flag. Just because the NAV flag pulls in the HSI doesn’t necessarily mean the FCS has received its NAV flag valid. Loss of, or improper scaling of these signals, can result in premature or improper capture solutions and poor performance.
In this scenario, the flight control system track computation circuits may be at fault. Lack of capture can also be attributed to improper shielding of the wiring and induced noise on the lines which precludes the FCS from sensing the beam centering.
If the failure only occurs when VOR navigation source is selected, the fault could be attributed to the course resolver in the HSI, the navigation receiver or the wiring between them.
If the failure occurs only when performing ILS approaches (Instrument Landing System), there may be a lack of Localizer valid logic to the FCS. Some systems require a high level logic to signify Localizer mode, while some navigational systems only provide a low level logic. Proper logic inversion must be accomplished in the aircraft in order for the FCS to recognize the correct localizer condition.
In addition to the above criteria, there are other inputs that may affect the system operation and should be investigated if the above source signals have no exhibit failures. In many flight control systems, the localizer and glide slope signals are gain scheduled with radio altitude. This compensates for the localizer and glide slope beam narrowing as the aircraft nears the runway. Other systems utilize one of the Marker receiver outputs to initiate a degain strategy. With the loss of Markers at many airports, these systems will no longer function optimally.
There are two ways to wire up the GPS to the Control System
Roll Steer information feeds the Flight Data (FD) via a dedicated input. In this scenario, the flight control system follows instruction as given from the GPS receiver. If there are flight control issues, the GPS receiver or wiring would be the most likely suspect, with the FD Computer being a distant second choice.
This emulates the VHF navigation signals and scaling. The FCS presumes it is flying a VOR signal when actually it is flying GPS. This can be found on many older/smaller aircraft that have been retrofitted with GPS, but did not have FCS capable of accepting roll steer data from the GPS receiver. The appropriate deviation and flag logic signals to the FCS are generally switched between the GPS and VHF navigation sources through an external relay or relays.
If the aircraft is wired for Left/Right Deviation & Course Datum and functions improperly, check to see if the VOR is operating correctly. If yes, then check the external switching for proper operation.
Also keep in mind that many EFIS-equipped aircraft utilize the digital data buss to disseminate the navigational source information, with the EFIS processor providing the conversion from digital to the analog signals the flight control system may require. Therefore, your flight control system issue may be caused by the otherwise perfectly functioning EFIS system.