Is there any indication in the FMS if VOR/DME fails to operate/malfunction.?

Question

Is there any indication in the FMS if VOR/DME fails to operate/malfunction.?

Answer 1

The short answer: No



The VOR or DME is an external transmitter. The FMS will auto tune to the VOR/DME frequency, but if it finds that signal to be invalid or unreliable, it will just find another ground based signal to use instead. There will be no indication of a "failure" on the FMS.



If you were to "hard tune" the VOR/DME frequency in the Radio or in the FMS, and the VOR/DME was not transmitting, your indication would be on your Nav Display on the panel, not on the FMS.



The FMS is where a pilot inputs data for long term navigation calculations, it's not meant to be the source of aircraft system messages, those are generally displayed elsewhere.

Answer 2

It would depend on the FMS, but I'd expect some sort of message (I've never actually seen a failure so I'm not sure). Not all failures can be detected, so there might not be anything. However, a FMS will typically combine data from multiple sources to derive its position and heading commands, so it should fall back safely onto other sources, such as another VOR/DME receiver, IRS units, GPS, etc. This may also allow the FMS to figure out that one VOR/DME is bad.



Flight management systems differ a lot from one FMS to another. The best bet is to consult the documentation for your specific system (and revision!).

Answer 3

I don't work on the FMS, but the antiquated system I am familiar with with will show a fail message for any navigational aid that the system uses for information.



The DME and VOR systems have a validity signal that indicates if the system is working. This signal should be read by the FMS and displayed. This is probably over some sort of MUX bus.

Answer 4

FMS updating is done in a hierarchy, GPS, DME-DME, DME-VOR, VOR-VOR. If the FMS couldn't get an update from either GPS, and couldn't get updated by DMEs and/or VORs, I would expect a message such as unable RNP.

Answer 5

FMS only reports the failures reported on the MUX bus by the LRU (Line Repalcable Unit). If the T/R failed the FMS would report the R/T failure.

Answer 6

it will be noted in the NOTAMs...always check for an update before a flight.