Cessna Citation Sovereign (C680)
Situational Awareness; Human-Machine Interface; Distraction; Confusion; Workload
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Requested ATC Assistance / Clarification; General Declared Emergency
Aircraft; Weather
We were descending on a STAR into our destination. We were cleared direct to a fix and to cross it at flight level 240. Around FL300 the autopilot disengaged with an aural warning and we started to lose our primary flight and navigation instruments. The first instruments to fail were the ones on my side. So my partner took control of the aircraft from the right seat and I started to run the checklist. Shortly thereafter he lost his instruments as well and resorted to the standby instruments. We then realized that we were going to have trouble navigating and making our crossing restrictions. Added to the fact that we were not sure if we were picking up ice and that we were in airspace with a lot of air traffic we decided to declare an emergency. We requested a descent and vectors to the airport because we knew that the weather there would allow a landing in VMC and that seemed to be the safest course of action.The previous day the starter generator on the left engine had been replaced. When we flew our first leg the following day; as we were in our descent; we noticed that the avionics blinked for a split second but there was no CAS message and everything was operating normally. We flew two more legs and had no issues or problems until we flew this; our last; leg.
During descent we got MAU 1 and MAU 2 [Modular Avionics Unit] FAIL warnings along with multiple associated CAS messages. ATC did give us a quick descent to 4;000 FT where we were VMC and landed at our destination via a visual approach to Runway 34.
After suffering the loss of both MAUs (Modular Avionics Units) aboard their C-680 while in an IMC descent the flight crew declared an emergency to descent to VMC conditions and out of possible icing conditions.  Once clear they were vectored by ATC for a visual approach and landing at their planned destination.