An ERJ145 with a deferred FMS lost both VOR receivers in flight which required ATC vectoring to their filed alternate because IMC weather prevented diversion to enroute airports.
Synopsis
An ERJ145 with a deferred FMS lost both VOR receivers in flight which required ATC vectoring to their filed alternate because IMC weather prevented diversion to enroute airports.
Narrative
At the gate in STL while boarding; Maintenance deferred the FMS; leaving us with short range (VOR) navigation only for our flight. We took off and proceeded on the BLUES 2 Departure in short range nav. After making the turn over HILTS; we noticed our CDI needle was acting erratic. Then ATC cleared us direct IIU. While navigating to IIU; both VOR 1 and 2 became completely unusable. Both CDI needles were going complete deflection over and over again in opposite directions. There were also numerous periods where the needle would lose the signal and a red X would appear. This whole time; we were getting a good ID when tuning the audio of the NAVAID. We were IMC for the entire climb and cruise; and alerted ATC we were having navigation issues and were going to troubleshoot. ATC gave us a heading and IIU when able. At this point; ATC asked an Air Carrier flight to attempt to tune in IIU; which they did and informed us and ATC they were getting a solid tune and course. During this entire time; we were getting closer to the IIU VOR and the situation did not get any better. As the flying pilot; I also attempted to tune the CVG VOR; and it gave the same completely unreliable guidance. At this point; we realized VOR 1 and 2 were unusable at all altitudes; multiple VORTAC; and any distance from the VOR. When the CDI began going full scale back and forth; the non-flying pilot called Maintenance on ARINC. Then the non-flying pilot was transferred to Dispatch on ARINC. We discussed going back to STL; but the field was IMC; with low ceilings; and we did not want to return to shoot an approach. After all the troubleshooting; communicating with ATC; and the company; we were past IIU and close to CVG. We checked the weather at CVG; which was 2 miles visibility; overcast skies at 1;400; and obviously shooting approaches. Dispatch relayed the destination weather; which was overcast at 8;000. Since there was an enormous weather pattern over the central U.S.; it was determined that our planned destination was the best option because of the VMC conditions and the ability to shoot a visual approach. The first cause of this event was the FMS deferral at the gate by maintenance; limiting us to one form of navigation. This stemmed from a problem updating the database. The second cause was the failure in flight of both our VOR 1 and 2 equipment on board the airplane; leaving us with no navigational equipment in IMC conditions. Once we identified that we had an unreliable VOR 1 and 2; we first alerted ATC and got a heading. Then we contacted Maintenance Control via ARINC to inform them. After that; we contacted Dispatch to check the weather to find a suitable airport. Since STL was IMC; we chose not to return. We checked the nearest airport only to find it was also IMC from the same weather system. Our destination was IMC and forecast IMC; so we declared an emergency and got vectors for a visual approach. Upon picking up the field; we were cleared for the visual and no further incident occurred. Extra care needs to be taken to try to avoid deferring a navigation system and leaving only one system left on board. The FMS deferral should be a last resort option; not something to defer to get a flight out because one of the disks was having trouble downloading the database. The VOR navigational equipment should be looked at since it is rarely used as anything more then a backup to the FMS.
Second reporter narrative
[Narrative #2 has no additional information]
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.